MINISTRY OF EDUCATION AND TRAININGHO CHI MINH CITY UNIVERSITY OF TECHNOLOGY AND EDUCATION GRADUATION THESIS MAJOR: MECHANICAL ENGINEERING TECHNOLOGY INSTRUCTOR: NGUYEN THANH TAN NGUYEN T
INTRODUCTION
Urgency of the study
Metal welding is essential in the metal industry, particularly through the MIG (Metal Inert Gas) method, known for its flexibility and high performance This process utilizes an electric current with a continuous welding wire and an inert shielding gas, creating a contamination-free welding environment that ensures high-quality results MIG welding is versatile, effectively joining materials like carbon steels, stainless steel, aluminum, and various alloys, making it invaluable in sectors such as automobile manufacturing, shipbuilding, medical equipment production, and aerospace The automation and adaptability of MIG welding significantly enhance labor efficiency while ensuring precision and uniformity, allowing the industry to meet the growing and diverse demands for metal fabrication.
The quality of a weld is primarily influenced by its geometrical characteristics, which directly impact its mechanical properties To ensure optimal weld quality and performance, it is crucial to select and control specific welding process parameters, including welding current, voltage, speed, electrode length, gas flow rate, wire feed speed, position, protective gas, and welding angle.
Accurate adjustment and maintenance of welding parameters are crucial for successful outcomes, relying not only on technical factors like equipment and welding technology but also significantly on the operator's skills and experience A deep understanding of the welding process and the ability to manage fluctuations can further enhance or diminish the accuracy of the welding operation.
Manufacturers are actively researching and implementing advancements in the MIG welding process to address current challenges Recent studies by various authors have explored multiple perspectives to optimize the welding process and improve its overall efficiency.
2 ability to control the quality of welds This is an important area of research and contributes to the continuous development of welding technology in modern industry
In a study by P Sakthivel et al., the MIG welding process was explored for joining various materials, specifically aluminum alloys AA6063, AA7075, and AA2014 with mild steel This innovative approach to MIG welding dissimilar materials aims to achieve weight reduction in industries such as aerospace and automotive, where aluminum alloys are preferred for their lightweight properties The mechanical properties were assessed using tensile tests, Rockwell hardness, Vickers hardness, and both macro and micro examinations, demonstrating good weld quality Notably, the combination of AA6063 alloy and mild steel showed the highest Rockwell and Vickers hardness, as well as tensile strength.
Aysha Sh Hasan et al conducted a study on the effects of welding current during MIG and TIG welding on the mechanical properties of medium carbon steel (0.58% C) and 304L stainless steel welds using E309L welding wire They optimized welding current for both butt and lap joints to enhance tensile strength and analyze heat generation The results indicated that increasing welding current raised the welding temperature, with TIG welding exhibiting higher temperatures than MIG Additionally, tensile strength was greater in butt joints compared to lap joints in TIG welding, and TIG welding consistently generated more heat than MIG for both joint types.
Bhuvan Bhardwaj et al conducted a study on welding 8mm thick AISI 202 stainless steel plates using SS-304L welding wire, noting that AISI 202, while similar to AISI 304 in mechanical properties, is less corrosion-resistant in chloride environments but more cost-effective, making it suitable for applications in furniture, kitchen utensils, the food industry, oil and gas, and automobiles The research focused on optimizing welding parameters, including current, CO2 gas flow rate, and speed, to improve weld quality Findings indicated that lower gas flow rates led to higher tensile strength but reduced ductility, while increased welding speeds resulted in more defects and improper weld penetration The lowest hardness was recorded at a speed of 4mm/min and a current of 200A, whereas the highest hardness occurred at the same speed with a current of 300A The lowest tensile strength was observed at 5mm/min, 100A, and 1.5 liters/min gas flow, while the highest tensile strength was achieved at 3mm/min, 300A, and 1 liter/min gas flow, with a 5% elongation Additionally, Saurabh Gandhe et al employed the Taguchi method with an L9 orthogonal array to optimize welding parameters for AISI 1040 steel plates, focusing on enhancing output quality parameters such as tensile strength and microhardness in both the base metal and heat-affected zone.
In their study, Vijaya Sankar B et al [39] used AISI 310, known for its high- temperature resistance, good surface finish, corrosion resistance, ductility, and weldability
AISI 310 steel is widely utilized in applications such as plates, tubes, furnaces, and pressure vessels To optimize the welding parameters for AISI 310 steel plates, the Gray-Taguchi method was employed, highlighting the significant impact of factors like welding current, voltage, speed, gas flow, and electrode material on penetration depth This optimization enhances tensile strength, microstructure, and microhardness of the welds The L9 orthogonal array was utilized for efficient test planning in this process.
R Sakthivel et al [35] also used the L9 Taguchi orthogonal array to study the effect of welding current, voltage, and gas flow on penetration, bead height, and bead width when welding AA2014 plates (150 x 50 x 6 mm) with the MIG method Results indicated that increasing voltage and current boosts penetration, bead width, and height, whereas increasing gas flow increases penetration depth but decreases bead height and width
SD Ambekar et al investigated the impact of process parameters on weld penetration depth in gas metal arc welding of Martensitic stainless steel AISI 410, utilizing the Taguchi method with 16 experiments based on an L16 orthogonal array Their analysis focused on welding speed, current, and wire diameter, employing analysis of variance and signal-to-noise ratio to pinpoint critical factors and optimize parameters Validation tests confirmed the efficiency and reliability of the method, as results closely aligned with predictions Similarly, Tadele Tesfaw et al optimized welding parameters for mild carbon steel plates using a semi-automatic welding machine By adjusting current, voltage, gas flow rate, and wire feed speed through the Taguchi method, they enhanced weld hardness and provided valuable insights into material variation and plate thickness, particularly benefiting Ethiopia's automobile industry.
Kapil B Pipavat et al [24] used the Taguchi technique to design an experiment with
This study utilized an orthogonal array and analysis of variance to optimize the welding properties of AISI 316 material through nine samples It focused on identifying optimal process parameters from experimental data to achieve superior weld quality and enhanced tensile strength Key factors affecting weld durability were examined, including the effects of welding angle and shielding gas ratio on overall weld quality The ultimate aim was to determine the best parameters to improve ultimate tensile strength, yield strength, and elongation in welds.
Mehmet Şükrü Adin et al [10] varied the welding angle (60°, 75°, 90°), along with voltage, current, speed, and gas flow rate, when welding cylindrical AISI 1040 medium
Carbon steel exhibits notable strength, abrasion resistance, and ductility The study revealed that the optimal tensile strength of 597.963 MPa and elongation of 11.551% occurred at a groove angle of 90°, with a current of 120A and voltage of 30V Conversely, the lowest recorded tensile strength was 395.125 MPa, accompanied by an elongation of 8.354% Variations in current and voltage had a significant impact on both tensile strength and elongation ANOVA analysis identified the groove angle as the most influential factor, contributing 62.75% to average tensile strength and 75.58% to elongation The optimal parameters for achieving maximum tensile strength and elongation were established using the S/N ratio.
Diganta Kalita et al [23] conducted an experiment utilizing Taguchi’s L9 orthogonal array to improve weld quality by optimizing tensile strength They focused on welding parameters such as current, voltage, and shielding gas flow rates while welding C20 steel with a diameter of 16mm using ER70S-4 material The findings revealed that welding voltage had a significant impact on both the mean value and variation of weld tensile strength, whereas welding current primarily influenced the mean value In contrast, the shielding gas flow rate was found to have a minimal effect on tensile strength.
Nabendu Ghosh et al utilized the Gray-Taguchi method to optimize welding parameters, including electric current, gas flow rate, and nozzle-to-plate distance, for welding 316L stainless steel with an ER 316L filler rod Their research revealed that electric current significantly affects joint quality, whereas gas flow rate and nozzle-to-plate distance have a lesser impact.
Weld quality was evaluated using tensile tests, visual inspections, and microscopic analysis The assessments revealed defects in some samples, including dents, holes, blotches, poor penetration, low porosity, and lack of fusion.
Scientific and practical significance of the study
1.2.1 Scientific significance of the study
- Research to investigate the basic factors affecting MIG welding mode on two different materials: C20 steel and SUS201
- Evaluate the strength, hardness, microstructure, etc of the weld, thereby drawing conclusions and applying it to production
1.2.2 Practical significance of the study
MIG welding technology has gained popularity for its effectiveness in joining materials, yet many, particularly novices, face challenges when welding dissimilar materials This research aims to enhance the selection of MIG welding parameters for effectively welding C20 steel and SUS201 stainless steel Additionally, it seeks to deepen the understanding of the welding process between different materials, ultimately minimizing errors in parameter selection.
Aims of the study
- Investigate the parameters affecting MIG welding mode with two different materials using the Taguchi method
- Compare experimental results with previous results, then make comments
- Provide effective solutions to save costs and time as well as simplify processes
- Understand the influence of parameters in the MIG welding process for research to improve the process more effectively.
Object and range of the study
- Flexural strength, tensile strength, elongation and elastic modulus of the material
- Learn the operating principles of welding robots, tensile strength tester, bending tester, microstructure
- Learn about welding mode, MIG welding slag
- Calculate experimental layout for the project
- Analyze, evaluate and propose improvement plans after obtaining experimental results.
Research methods
- Based on the growing need to connect steel and stainless steel materials
- Based on MIG welding technology, which is being used very commonly today
- Inherit and expand previous experimental results
- Read books and newspapers, collect data, determine sample size, and arrange experiments
- Research the experimental results of previous experiments
- Calculate and select welding parameters appropriate to laboratory conditions
- Evaluate the results of trial experiments then conduct mass experiments
- Compare experimental results with previous studies, thereby making comments and improving experimental procedures to optimize.
Structure of the study
- Chapter 5: Conclusion and Development Direction
THEORETICAL BACKGROUND
Introduction to alloy steel material C20
C20 steel is a type of alloy steel with a carbon content ranging from 0.18% to 0.23%
C20 steel, conforming to the JIS G4051 – S20C standard, is extensively utilized in mechanical engineering, construction, and the manufacturing of seamless steel tubes and machinery components Its production involves multiple methods, including forging, blast furnace processing, heat treatment (Basic Oxygen Process), hot rolling, cold rolling, and casting Known for its high strength, good ductility, and excellent machinability, C20 steel is a favored material in numerous industrial applications.
Yield Strength: > 245 MPA The maximum stress S20C steel can endure without permanently deforming Beyond this point, the steel will deform rapidly without an increase in stress [50][17][8]
Tensile Strength: > 400 MPa The tensile stress C20 steel can withstand before cracking or breaking, crucial for understanding its load-bearing capacity in practical applications [50][17][8]
Elongation: > 28 % Measures the steel’s ability to stretch before breaking Important for applications requiring flexibility and deformation, such as construction and mechanical engineering [8]
Hardness: 116 – 174 HB Indicates resistance to impacts and scratches Determined by measuring the indentation from a steel ball under a fixed force, crucial for evaluating the steel’s durability in various applications [8]
Table 2.1: Composition of C20 Steel according to JIS G4051 Standard [50][8]
Chemical Composition Carbon (C) Silicon (Si) Manganese (Mn) Phosphorus (P) Sulfur (S)
Copper (Cu) Chromium (Cr) Nickel (Ni) Iron (Fe) Ni + Cr
C20 alloy steel, also known as C20 steel, finds widespread applications in various fields due to its flexibility and excellent load-bearing capabilities Here are some common applications of C20 alloy steel:
Mechanical Engineering: Used in the production of machine components such as shafts, gears, bearings, and load-bearing and wear-resistant parts in machinery and mechanical equipment
The construction and building industry heavily relies on structural elements such as columns, beams, and frameworks, while also utilizing materials for construction accessories like pipes, steel plates, and various structural components.
Hot Forging: Employed in the manufacturing and processing of hot-forged products such as shafts and wear-resistant tool bits
Seamless Steel Tubes: Used to produce seamless steel tubes widely employed in the oil and gas, energy, and chemical industries
Plastic Molding Dies: Used for making molds in the plastic production process
Manufacturing Machine Components: Produced for a diverse range of parts and components in various industries
These applications are just a few examples and not an exhaustive list The versatility and flexibility of C20 alloy steel make it a preferred material in many industrial sectors
Figure 2.1: Some Examples of Applications of C20 Steel [51]
2.1.5 Distinguishing C20 steel from other types of steel
To differentiate C20 steel from other types of steel, various criteria can be considered
Chemical Composition: C20 steel is classified in the low carbon steel group, with a carbon content higher than medium carbon steel (C: 0.23% – 0.6%) and high carbon steel (C
> 0.6%) Additionally, the content of alloying elements in C20 steel is generally lower than
10 in other alloy steels, such as chrome (Cr), nickel (Ni), manganese (Mn), molybdenum (Mo), titanium (Ti), copper (Cu), etc., which often have values exceeding 2.5%
Mechanical Properties: C20 steel typically exhibits a tensile strength greater than 402
With a hardness ranging from 116 to 174 HB and impressive toughness, this steel exhibits higher strength and hardness than low carbon steel, yet remains inferior to high carbon and alloy steels Additionally, it offers superior machinability when compared to high carbon and alloy steels, making it a versatile choice for various applications.
C20 steel is characterized by its metallic gray color and corrosion resistance, distinguishing it from stainless steel Typically produced through the hot rolling process, C20 steel is available in solid round bars or cast tubes.
Introduction to stainless steel material SUS201
SUS201, also known as AISI 201, is an Austenitic stainless steel that primarily consists of Manganese (Mn) and Nitrogen (N), featuring a lower Nickel (Ni) content than many other stainless steel types This unique composition provides enhanced stability and cost-effectiveness, as Manganese can effectively substitute Nickel in the alloy.
SUS201, while having lower corrosion resistance than SUS304, is favored for its versatility across various applications Its notable ductility and excellent machinability enhance its appeal in numerous industries, particularly in the manufacturing of household appliances, automatic doors, and mechanical components The diverse properties and flexibility of SUS201 make it a cost-effective material with outstanding processability, suitable for a wide range of uses.
2.2.2 Characteristics of stainless steel SUS201
The minimum tensile strength of stainless steel SUS201 is 515 MPa, which varies based on the manufacturing process and heat treatment This value represents the maximum stress the material can withstand before experiencing cracking or fracturing.
SUS201 exhibits an elongation of 40%, showcasing its flexibility and resistance to cracking under tensile forces This property makes it highly suitable for mechanical engineering, household appliances, and machinery manufacturing, where ductility and flexibility are essential.
Hardness: The maximum hardness of SUS201 is 95 HRB or 217 HB, balancing strength and scratch resistance This makes it suitable for various industrial and daily applications [12][20]
Thermal Conductivity: SUS201 has moderate thermal conductivity, effectively transmitting heat from high-temperature to low-temperature areas [12]
Machinability: SUS201 is easily processed into products like tubes, sheets, and wires through methods such as cutting, welding, and grinding [12]
2.2.3 Composition of stainless steel SUS201
The primary chemical composition of stainless steel SUS201 includes:
Carbon (C): ≤ 0.15% , Silicon (Si): ≤ 1.00%, Manganese (Mn): From 5.50% to 7.50%, Phosphorus (P): ≤ 0.060%, Sulfur (S): ≤ 0.030%, Nickel (Ni): From 3.50% to 5.50%, Chromium (Cr): From 16.00% to 18.00%, Nitrogen (N): ≤ 0.25% [12][20]
SUS201 is characterized by its impressive strength, corrosion resistance, and ductility, making it an ideal choice for a wide range of applications in both industrial and everyday settings.
2.2.4 Applications of stainless steel SUS201
Due to its combination of mechanical and chemical properties, stainless steel SUS201 has found widespread applications in various fields
Mechanical engineering utilizes SUS201 for the production of essential machine components, including bearings, shafts, and load-bearing parts Additionally, this versatile material is favored for creating visually appealing mechanical products, serving decorative purposes across a range of applications.
SUS201 stainless steel is widely used in household appliances and furniture, featuring in products such as pots, pans, sinks, and stove control panels Its application extends to furniture items, where it is employed in door handles, decorative pipes, and various accessories, ensuring the creation of high-quality and visually appealing products.
SUS201 stainless steel is widely utilized in construction and architecture, particularly for gates, railings, and signage Its versatility allows for seamless integration into both interior and exterior designs, enhancing the overall quality and aesthetic appeal of structures.
SUS201 is widely utilized in the food industry for equipment like ice cream machines and steam tables, as well as for materials that come into contact with food Additionally, it plays a crucial role in the production and installation of medical equipment and tools.
SUS201 plays a vital role in the chemical and oil & gas industries, as it is essential for the production of pipes and equipment This material is specifically designed to manufacture components that withstand abrasion and corrosion, making it ideal for demanding industrial conditions.
The electronics and electrical industry frequently utilizes SUS201 for manufacturing electronic components, including water conduits, machine casings, and various technical parts Additionally, it plays a significant role in the production of electrical components, such as LED lights and compact electronic devices.
Figure 2.2: Some Examples of Applications of SUS201 [52]
2.2.5 Distinguishing stainless steel SUS201 from other steel types
Distinguishing SUS201 from other stainless steels can be based on the following factors:
Chemical Composition: Stainless steel SUS201 is primarily composed of Manganese
(Mn) and Nitrogen (Ni), with a lower Nickel (Ni) content compared to many other stainless steel
Color and Surface Shine: SUS201 stainless steel typically has a shiny metallic gray color This can be used as a relative indicator, although this method is not entirely accurate
Magnetic Testing: SUS201 stainless steel exhibits non-magnetic properties A magnet can be used to test whether it adheres to the material
SUS201 stainless steel exhibits good resistance to weak acids, particularly acetic acid To perform an acid test, apply a small amount of acetic acid, commonly found in vinegar, to the surface of the steel and monitor the reaction for assessment.
Density Testing: SUS201 has a relatively light density compared to some other steel types.
MIG welding technology
2.3.1 Overview of MIG welding technology in mechanical engineering
Gas Metal Arc Welding (GMAW), commonly known as MIG welding when using inert gases like argon, and MAG welding when using active gases such as CO2, utilizes an electric arc to melt and join metal wires In Europe, this process is often referred to as MIG/MAG welding or simply MIG welding.
MIG welding is ideal for a range of metal sheet thicknesses, especially thin sheets, thanks to its simplicity in starting and stopping, which boosts productivity Unlike Shielded Metal Arc Welding (SMAW), MIG welding eliminates the need for frequent electrode changes and produces no slag.
MIG welding operates by continuously feeding a metal wire into the welding area from the welding gun, where the wire serves as both a current-carrying electrode and a filler material that matches the welded metal's composition An inert gas, such as Argon with 2% O2 for stainless steel or an Argon-CO2 mixture for low alloy steel, protects the molten metal from reactions, ensuring a stable weld with deep penetration and minimal splashing However, windy conditions can pose challenges, as they disrupt the protective gas shield essential for quality MIG welding.
2.3.2 Principles of MIG welding process
Figure 2.4: The principle of MIG welding [44]
1 Electric arc; 2 Electrode; 3 Reel or drum; 4 Drive rollers;
5 Flexible conduit; 6 Hose package; 7 Welding gun; 8 Power source;
9 Contact tip; 10 Shielding gas; 11 Shielding gas nozzle; 12 Weld pool
Figure 2.4 depicts the essential process of MIG welding, where molten welding wire is continuously fed into the workpiece Inert gases are simultaneously supplied around the electrode, providing protection and shielding for the metal and heat-affected zone from environmental exposure.
The ionized supply gas generates an arc between the welding wire and the workpiece, while the wire is continuously fed from a feeder at speeds of several meters per minute through driven rollers to the welding gun.
The power source delivers current to the electrode through the contact tip of the gun, where the wire serves as the positive pole and the workpiece acts as the negative Usually, a DC electrode with a constant voltage facilitates arc length adjustment by modifying the voltage settings.
Welding robots are now widely used, offering automated, flexible welding for any position and material They enhance productivity with continuous operation and diverse movements, improving welding efficiency [1][44]
The welding arc is composed of an anode, cathode, and arc column, created by electric discharge between electrodes with current flowing through ionized plasma The power released, which melts the electrode and joint surfaces, is determined by the arc's voltage drop and current The maintenance of plasma ionization relies on the shielding gas used, as different gases necessitate varying temperatures Notably, helium results in higher voltage drops and increased heat input Additionally, heat loss occurs near the electrodes and workpiece surfaces due to conduction, as illustrated by significant voltage drops in the arc regions.
Figure 2.5: The voltage distribution in the arc [44]
A shielding gas is essential for protecting molten metal from harmful air exposure, as even minimal oxygen levels can cause oxidation of welding alloys, resulting in welding slag formation Additionally, as nitrogen solidifies, its solubility decreases and gas evaporation can create pores, while nitrogen may also increase brittleness Furthermore, the choice of shielding gas greatly affects welding properties, impacting penetration and the overall geometry of the weld bead.
Argon is widely used as a protective gas in welding due to its inert characteristics, making it ideal for materials such as stainless steel and aluminum When combined with CO2 or O2, argon enhances the welding process, particularly in short arc welding applications It is also recommended for various alloys, including nickel, copper, aluminum, titanium, and magnesium, as it improves the transfer efficiency of the welding wire into the weld puddle.
Helium, like argon, is utilized in protective gas mixtures for welding steel and aluminum, enhancing thermal conductivity for increased heat input and producing wider, shallower welds When combined with argon, helium improves arc stability and prevents corrosion, making it ideal for welding thick-walled materials over 25mm, such as aluminum and copper Despite its benefits, helium is expensive and has a low density.
Pure CO2 is widely used in short arc welding, particularly for galvanized steel, due to its effective protection and cost efficiency However, it has limitations, including the potential for increased sparks and its unsuitability for spray arc welding.
Table 2.2: Application of Shielding Gas to Weld Metal [6]
Welding metal Low alloy steel
Aluminum (Alloy) Nickel Titanium Copper alloy
MIG welding is often used for carbon and low alloy steel
Welding wire with diameter from 0.6 to 1.6 mm: 0.6; 0.8; 0.9; 1.1; 1.2; 1.4; 1.6 According to AWS standards, welding wire is denoted as follows:
R – Rod adds metal to the weld
70 minimum tensile strength of the weld metal, measured in ksi –
S – Solid solid welding wire, to distinguish it from Flux Cored Arc Welding
3 alloy composition and deoxidizing elements contained in the welding wire
• Some popular types of welding wire [4][6]
ER70-S2 is a welding wire characterized by its high silicon and manganese content, along with additional deoxidizing elements like aluminum, zirconium, and titanium This versatile welding wire is suitable for both single-layer and multi-layer welding applications.
ER70–S3: has average Si and Mn content, used for single-layer or multi-layer welds This is the most commonly used type of welding wire
ER70–S4: has higher Si and Mn content than ER70-S3 Can be used for single–layer and multi–layer welds
ER70–S6: has high Si and Mn content, used for materials with high oxidation potential Can be used for single-layer and multi-layer welds
ER70–S7: has higher Mn content but lower Si content than ER70–S6, good for use with a mixture of Ar and CO2 protective gases
The size of the welding wire significantly influences weld quality and the associated welding parameters Typically, the appropriate wire size is determined by the welding current; however, unlike stick electrodes, each type of wire can function effectively across a broad and overlapping range of currents For detailed guidance on selecting the right welding wire based on current, refer to Table 2.3.
Table 2.3: Welding wire size with corresponding current [4]
Wire diameter (mm) Welding current range (Amperes)
Using thinner solder wire generally facilitates smoother material transfer; however, it can lead to feeding issues during the soldering process To mitigate these challenges, opting for solder wires with larger diameters is often a more effective solution.
Welding voltage plays a vital role in MIG welding, influencing the metal transfer process The ideal voltage is determined by several factors, including the thickness of the material, the type of weld, electrode size and composition, protective gas, and welding position Higher arc voltage leads to a longer arc and flatter welds, whereas lower voltage results in a shorter arc and more pronounced welds Additionally, voltage impacts arc stability and the amount of metal splatter produced.
Experimental methods and planning
In the field of welding parameter experimentation, various methods are available, including level 1 and level 2 orthogonal planning and the Taguchi experimental design method Traditional orthogonal planning often requires a substantial number of experiments; for instance, a project with three welding parameters, each having three levels, necessitates up to 27 experiments (3^3) In contrast, the Taguchi method significantly reduces the number of required experiments, streamlining the research process.
9 experiments need to be performed, helping to save time and research costs Therefore, choosing the Taguchi method as a means to design experiments in this project is a reasonable choice [3][38]
An orthogonal array, specifically a fixed-element orthogonal array, is represented as OAN(s, m) and consists of an N × m matrix This matrix is characterized by its columns, where every pair of columns contains all possible ordered pairs of elements appearing an equal number of times The elements within an orthogonal array can be represented by arbitrary symbols.
Taguchi represents orthogonal arrays, denoted as OAN (s m), using the notation LN (s m), where 'L' signifies a Latin square This indicates that orthogonal arrays can be viewed as generalized Latin squares In this context, Taguchi employs symbols or numbers to represent the elements within an orthogonal array.
An orthogonal array, specifically a fixed-element orthogonal array, is defined as an N × m matrix where each possible ordered pair of elements appears an equal number of times This type of array is denoted as OAN(s, m) and its elements can be represented by any symbols.
Taguchi refers to orthogonal arrays as OAN (s m) using the notation LN (s m), where the letter L signifies Latin squares He denotes the elements of an orthogonal array with symbols or integers.
The signal-to-noise ratio (S/N ratio) is a crucial metric for assessing system quality and performance, typically calculated using standard deviation In the Taguchi method, the S/N ratio reflects system performance, with its optimal value aligning with ideal parameter adjustments This ratio serves as an effective tool for evaluating system quality during optimization, where the maximum S/N ratio indicates the best level of parameter changes.
Analysis of Variance (ANOVA) is a widely used statistical technique for comparing the means of three or more groups It effectively highlights the differences between groups and evaluates the significance of various factors related to research objectives By dissecting the total variance of observations into two key components—variance between groups and variance within groups—ANOVA provides insights into the classification of data variation, focusing on both inter-group differences and intra-group variability.
The Taguchi method is a combination of factors affecting the objective function implemented through OAs OAs are generally denoted LN (s m ), in which:
The array consists of N rows, representing the number of test cases generated, and m columns, indicating the maximum number of variables that can be managed Each column contains s levels, which denotes the maximum number of values that any single factor can assume.
An orthogonal array organizes experiments by representing each experiment in a row and each experimental factor, along with its influence levels, in columns This structure ensures that all experimental configurations are balanced and statistically independent, which minimizes the number of experiments required while maintaining the reliability of the results.
The characteristics of orthogonal arrays are expressed as [3][5][30][38]:
In an array, the total number of test conditions is represented by the formula Lk, where L denotes the number of levels for each factor and k signifies the number of factors involved.
Each row in the array signifies a distinct test condition, with each cell holding a value that corresponds to a specific factor, thereby outlining the assumptions and conditions relevant to each test.
Each column in the array represents a specific element under investigation, with the values indicating the various levels of that factor Importantly, every level of the factor is included at least once within the array, ensuring comprehensive representation.
The columns of the array are orthogonal, meaning that every pair of element values must be represented together at least once throughout the array This design enables the independent and synergistic evaluation of each factor's impact.
Orthogonal arrays are versatile tools that can be tailored for various experimental scenarios By modifying the values of the elements in each row, researchers can adjust the specific test conditions to suit their needs.
Results analysis: After collecting data from the test conditions, results analysis is performed to determine the influence of each factor and their interactions
The array's columns are structured to maintain orthogonality and balance, ensuring that each element's levels are equivalent within a column Additionally, there is a necessary balance between any two columns, guaranteeing that each combination level appears in equal quantities.
In Taguchi, an orthogonal array N × m is formed from p (number of parameters) and l (number of levels)
Methods of checking and evaluating welding quality
2.5.1 Research on structural components of welded joints
• Evaluation of appearance – weld shape
Utilizing the naked eye, magnifying glasses, and measuring tools allows for a thorough evaluation of weld coverage after each work advancement This method enables the inspection of electrode marks, analysis of surface cracks, and precise measurement of the welded area, ensuring quality and uniformity throughout the welding process.
• Evaluate the rough structure of the weld
To assess the deformation, weld zone size, and heat-affected zone, as well as to identify defects like cracks, pits, and uneven adhesion between the weld layer and base layer or adjacent layers of auxiliary welding wire, rough inspection samples are collected from the welded structure after each feed step These samples are then cut, ground, and polished to remove heat effects and comply with technical standards for testing.
This test sample processing process not only helps determine the quality of the welded structure but also ensures full compliance with technical standards [1]
Research examines the microstructure of the base metal, weld metal, and the interface between them, along with the heat-affected zone This study aims to analyze the microscopic structure and assess the composition, size, and distribution of metal phases in both the weld and the heat-affected zone.
Analyzing the microstructure of welds allows researchers to assess the bonding strength and hardness of the weld layer, while also identifying defects like cracks and pits within the weld and heat-affected zone This analysis is crucial for ensuring the quality and consistency of the welding process, as well as for enhancing repair techniques and the overall performance of welded structures.
2.5.2 Method for determining the durability of the weld bond with materials
Steel tensile testing is a crucial method for assessing the mechanical properties and durability of steel This guide outlines the essential steps involved in conducting tensile tests to evaluate the material's strength and resilience.
Sample selection: Select a steel sample that is representative of the test material
Samples are usually cylindrical or bar shaped to facilitate bending or breaking
Figure 2.8: Test Samples Include the Unprocessed Part of the Product [13]
Sample fabrication: Cut or bend steel samples to size and shape according to ASTM
E8/E8M standards Prepare the sample surface to ensure uniformity and accuracy of test results
Figure 2.9: Dog-bone Sample Parameters according to ASTM E8/E8M Standards [14]:
From the tensile test, the weld quality can be evaluated through the following criteria:
The yield point marks the transition of a material from the linear elastic stage to the non-linear elastic stage during deformation In the linear stage, the material exhibits elastic deformation, where the amount of deformation is directly proportional to the applied tensile force However, once the yield point is surpassed, the relationship changes, and the material enters a non-linear elastic stage, resulting in deformation that is no longer proportional to the tensile force.
At the yield point, there is a significant alteration in the mechanical properties of the material, indicating the onset of deformation beyond its elastic limit Once the pressure is released, the material does not fully revert to its original shape, resulting in permanent deformation The extent of this change can vary, being either abrupt or gradual, depending on the material's specific characteristics.
The breaking point, or tensile strength, is a critical parameter in steel tensile testing, representing the maximum tensile force a material can endure before failure Illustrated on the tensile stress-strain graph, this point indicates the material's maximum load capacity, making it essential for evaluating its performance and durability.
Figure 2.10: Stress-Strain Curves For Mild Steel and Low And High Alloy Steel
During this phase, tensile stress rises, leading the sample into a non-linear elasticity stage that precedes fracture As loading persists, the tensile stress peaks before starting to decline, typically indicated by a flattening or reduction in the stress-strain curve.
The maximum stress of steel calculated with the formula [2]:
With ∙ 𝑃 𝑚𝑎𝑥 : maximum tensile force that steel can withstand (N)
∙ 𝐴 0 : cross-sectional area of the test specimen (mm 2 )
The Modulus of Elasticity (E) is a crucial mechanical property that quantifies a material's elasticity, indicating its ability to stretch and revert to its original form under applied pressure or traction without undergoing permanent deformation.
Elongation: is an important index to evaluate the elongation of steel in a tensile test
Elongation is usually measured as a percentage and represents the percentage of deformation that a steel sample can withstand before breaking during a tensile test
EXPERIMENTAL DESIGN
Experimental setup
The experimental setup is built based on the process inputs and the output target after recovery welding The detailed structure of this model is illustrated in Figure 3.1
Figure 3.2: Flowchart of The Experimental Work.
Experimental equipment
MIG welding robot G2, model: PANASONIC TA-1400G2 (Figure 3.3) is used for butt welds
Figure 3.3: PANASONIC TA-1400G2 MIG Welding Robot
Figure 3.4: MIG Welding process by G2 Welding Robot
3.2.2 Hydraulic press machine – PROFI PRESS
In this study, a Hydraulic Press machine is used to stamp welded samples into rectangular tension test specimens
Figure 3.5: Hydraulic Press Machine – PPCM 50
Using a Profi hydraulic press to stamp out dog bone samples according to the ASTM E8/E8M [14] standard for tensile testing through pre-designed and fabricated stamping molds
Figure 3.6: Stamping die for specimen testing
In this study, the Universal Testing machine is used to test the tensile strength of the welded test specimens and related parameters such as elongation, elasticity,
Figure 3.7: Universal Testing Machine – WE-1000B
Figure 3.8: Tensile - Bending test using WE-1000B tensile machine
MIG welding technology relies heavily on the use of effective jigs to weld steel sheet metal parts These jigs are essential for ensuring a secure clamp, which minimizes movement and maintains balance during the welding process at the chosen speed.
The design and manufacturing of welding fixtures, essential for the experimental process, are based on the principle of tightly clamping and restricting the movement of components As illustrated in Figure 3.9, the jig consists of three main clusters: Cluster I, which includes the working plane part; Cluster II, featuring the clamping mechanism that secures the workpiece; and Cluster III, which incorporates a shielding edge to accurately position the workpiece during welding.
Figure 3.9: Model of MIG Welding Jig for Robots
The jig is processed and assembled as shown in Figure 3.10
Figure 3.10: Jigs for Automatic Welding Robots
Prepare test sample and welding process
A total of 64 welded test specimens were produced by cutting 2mm thick SUS201 and C20 stainless steel plates using a laser cutting machine, with each plate measuring 110 mm in length and 105 mm in width (see Figure 3.11).
Figure 3.11: Laser Cutting Profile of Welded Workpiece (a) SUS – 201, (b) C20
The samples undergo edge deburring before being prepared for the welding process, as illustrated in Figure 3.12 Following this, the welding is performed using specified parameters with steel and 1mm diameter filler materials.
Figure 3.12: The Welding Sample Has Been Prepared
This study employs a one-sided butt welding method for 2mm thick materials, utilizing a square preparation type for effective implementation To ensure precise positioning and uniform welding of the samples, a gap of approximately 0mm is maintained, with the two plates clamped closely together.
Figure 3.13: Joint square preparation for butt welds, welded from one side [49]
The mechanical properties of tensile strength, elongation, yield strength, and microstructure were evaluated using a sample welded with the PANASONIC TA-1400G2 Welding Robot, following the installation method illustrated in Figure 3.14.
Figure 3.14: Fix The Welding Workpiece on The Jig
The sample after welding is stamped with a mold and cut according to the profile according to ASTM E8 and ISO 15614-1 standard dimensions (shown in Figure 3.15 and Figure 3.16)
Figure 3.15: Profile for Tensile Test Specimen According to ASTM E8 Standard
Figure 3.16: Profile for Bending Test Specimen According to ISO 15614-1 Standard
The chemical composition and the mechanical properties of the steel C20 and inox SUS201 is given in Table 3.1 and Table 3.2
Table 3.1: Chemical Composition and Mechanical Components of SUS 201 Stainless Steel according to ASTM 240/240M Standard [12]
Table 3.2: Chemical Composition and Mechanical Components of C20 Steel according to
Select welding parameters for MIG welding robots
Optimizing input parameters is essential for enhancing weld quality in MIG welding Research by Nabendu Ghosh et al demonstrated that welding AISI 409 stainless steel with currents ranging from 100A to 124A on 3mm thick steel produced high-quality welds Similarly, a study on AISI 316L using comparable current levels for 3mm thick steel yielded strong, defect-free welds.
- 10149 - 2 steel, Kedir Beyene Behredin et al [15] identified optimal parameters as 60-70A, 16-18V, and 2-6mm/s Raja Subramanian et al [42] conducted experiments on C276, finding 90-110A and 14.5-16.5V to produce well-penetrated, defect-free welds
Selecting parameters through exploratory experiments is crucial, as each material type and working condition possesses distinct characteristics These experimental trials are instrumental in pinpointing the optimal parameter set for specific scenarios, guaranteeing the best alignment with technical requirements and real-world conditions Each parameter plays a significant role in influencing arc stability.
37 penetration depth, and the weld bead’s appearance Incorrect adjustments can lead to defects like cracks, porosity, or lack of fusion, compromising the weld’s strength and aesthetics
This study investigates the impact of MIG welding parameters—specifically welding voltage, current, speed, and electrical stick-out—on weld quality Additional input parameters are detailed in Table 3.3.
Table 3.3: Fixed MIG welding parameters with GM70S filler materials
Parameters Travel angle Working angle Gas flow rate Shielding gas Welding wire diameter
Table 3.4 presents the selected parameters using GM70S filler materials This study features a wider range of welding parameters than previous MIG welding research on 2mm thick steel, particularly in terms of welding current and voltage Additionally, the investigation will assess the impact of electrode extension (d).
Table 3.4: MIG Welding Parameters Performed with GM70S filler materials
Welding Parameters Level 1 Level 2 Level 3 Level 4 Welding Current, I (A) 80 90 100 110
The welding process involves delivering shielding gas to the welding area through a cylindrical nozzle with a diameter of 20.3 mm During the experiment, welded samples were securely fixed using fixtures, with opposing distances varying from 0 to 1.6 mm, while the welding parameters were adjusted accordingly.
16 matrix-based experiments are performed sequentially in combination with parameters throughout the welding process.
Statistical analysis using the taguchi method to optimize process parameters
With the goal of simultaneously studying 4 factors (U, I, v, d) with each factor having
4 levels, the suitable orthogonal array is the L16 array shown in Table 3.5
Table 3.5: Taguchi Experimental Plan for Array L16 with GM70S filler material
No Voltage (V) Current (A) Stick-out (mm) Speed (m/min)
Following the welding of samples using parameters from the Taguchi table, they will undergo processing as outlined in the flowchart in Figure 3.2 These processed samples will then be subjected to tensile tests, bending tests, and microstructural examinations to gather data on their strength characteristics.
Figure 3.17: Experimental and Sample Processing Procedure
Figure 3.17 depicts the welding and sample processing procedures, where the sample is securely fixed onto the welding jig and welded according to the parameters outlined in the Taguchi table (Table 3.5) A total of 32 samples are welded, with 16 allocated for tensile testing and 16 for bending testing Post-welding, the samples are processed to meet the standard profiles illustrated in Figures 3.15 and 3.16, including one sample designated for microstructure examination of the weld.
RESULTS AND DISCUSSION
Effects of welding parameters on weld quality
4.1.1 Effects of welding parameters on weld quality by tensile test
In this study, the objective was to establish optimal parameters to maximize weld quality Therefore, it was decided to choose the S/N ratio related to the quality characteristic
"Once the experimental design is established and tests are conducted, the performance characteristics obtained can be analyzed to understand the relative impact of the four parameters, emphasizing the principle that 'larger is better'."
Table 4.1: Table L16 of Taguchi and tensile test results
Input Parameters Output Parameters S/N Ratios
Table 4.1 displays the findings from experiments conducted to assess how welding input parameters affect weld quality The study focused on the correlation between variables, including welding current (I), welding voltage (U), stick-out length (d), and welding speed (v), and their impact on key mechanical properties of the weld.
The quality of the weld was evaluated based on key parameters including ultimate tensile strength (UTS), yield strength (YS), elongation (%), and modulus of elasticity (E)
Figure 4.1: Average graph of output results during tensile testing
Figure 4.1 Illustrates the average results of the values for tensile strength (UTS), yield strength (YS), elongation (Lf), and the modulus of elasticity of the weld when performing tensile tests
• Effect of welding parameters on Tensile Strength
Utilizing the tensile strength parameters from the tensile experiment, as detailed in Table 4.1, and applying the Taguchi optimization method with Minitab 17 software, we derived the Signal-to-Noise ratio for tensile strength The results, shown in Table 4.2, illustrate the tensile strength values, while Figure 4.2 displays the S/N ratio effect chart for weld tensile strength.
Table 4.2: Response table for S/N ratios (dB)
The tensile strength of the weld is most significantly influenced by the welding current (I), which has a value of 4.97, followed by welding speed (d) at 2.53 The ESO (d) ranks third with a value of 1.61, while the welding voltage (U) has the least impact, ranking last with a value of 1.6.
Figure 4.2: Main effects plot for ratios
The S/N (Signal-to-Noise) graph reveals that welding current (I) is the most significant factor affecting the tensile strength of the weld joint, followed by welding speed (v) as the second most influential parameter In contrast, welding voltage (U) is the least impactful on tensile strength Additionally, the main effects plot of the S/N ratio indicates that adjustments in these parameters can enhance the tensile strength of the weld joint.
Analysis of Variance for Tensile Strength
ANOVA tables for mean and signal-to-noise ratio are computed and listed in Tables
4.3 The F- test is conducted to assess the significance of process parameters A high F-value indicates that the factor significantly affects the process response In this study, welding current is the most important factor and contributes the most to the weld’s tensile strength
Table 4.3: Analysis of variance for tensile strength
Source DF Adj SS Adj MS F-Value P-Value Contribution (%)
From Table 4.3, the the comment that:
The welding current (I) significantly influences the tensile strength of the weld, with a P-value of less than 0.05 at a 95% confidence level In contrast, the P-values for welding speed (v), electrical stick-out (d), and welding voltage (U) exceed 0.05, indicating that these parameters have minimal impact on the tensile strength.
In a tensile test, welding current (I) significantly impacts the tensile strength of the weld, accounting for 63.47% of the influence Following this, welding speed (v) contributes 6.9% to the tensile strength, while welding voltage (U) and electrode stick-out (ESO, d) have lesser effects at 1.46% and 4.84%, respectively.
Increasing the welding current (I) leads to enhanced tensile strength due to greater heat input, which improves the weld's penetration capability and strengthens the bond with the base material However, excessive heat input can cause dilution and damage to the base material According to ISO/TR 18491:2015(E), the formula for calculating heat input is essential for maintaining optimal welding conditions.
The relationship between welding current (I) and heat generation (Q) is directly proportional, meaning that as the welding current increases, so does the heat produced in the welding arc Additionally, a slower welding speed (v) results in higher heat input during the MIG welding process, as it is inversely related to the heat generated Elevated temperatures during welding promote faster weld formation and enhance penetration capability, particularly when the welding gun operates at a slower speed Therefore, increasing the welding current and decreasing the speed can optimize the welding process.
Increasing the welding current and reducing the welding speed enhances penetration depth However, a longer electrode stick-out increases arc length, causing energy to disperse over a larger area and decreasing heat density at the contact point, which lowers arc temperature Additionally, a longer stick-out raises electrode resistance, reducing the efficiency of converting electric current into thermal energy at the welding point This diminishes the melting capability of both the base metal and filler metal Furthermore, an increased arc length results in lower arc pressure, which leads to reduced penetration of the molten metal into the base metal, ultimately decreasing weld penetration.
I = 80A, U = 18V, d = 16mm, v = 600mm/min I = 80A, U = 17V, d = 14mm, v = 550mm/min
I = 110A, U = 16V, d = 14mm, v = 450mm/min I = 110A, U = 18V, d = 10mm, v = 550mm/min
Figure 4.3: The penetration depth of the sample was measured using ImageJ software; a)
Figure 4.3 shows the influence of parameters on the quality of MIG welding In figures
Welding at high speeds of 550 mm/min and 600 mm/min with a low current of 80A leads to insufficient heat generation, resulting in inadequate penetration and weakened samples during tensile testing In contrast, welding speeds of 450 mm/min and 550 mm/min paired with a current of 110A significantly enhance penetration and improve weld quality However, using the highest voltage setting of 18V negatively impacts the integrity of the welds.
16 has a much greater penetration depth compared to sample 4 This indicates that the welding voltage (U) does not significantly impact the heat generated during the welding process
Figure 4.4: Penetration depends on the microstructure of the weld; a) Specimen 4; b) Specimen 3; c) Specimen 14; d) Specimen 16
Figure 4.4 shows the penetration and the weld penetration of specimens: L3; L4; L14;
L16 when these samples are welded with high welding current (I): 110A; Electrical stick – out (d):10mm (Figure a) and 14mm (Figure b) shows higher tensile strength based on the
Sample No 16 demonstrated superior weld penetration and tensile strength compared to samples No 3 and 4 when welded at a low current of 80A with electrical stick-out measurements of 14mm and 16mm Although samples 3 and 4 were welded at higher voltage levels, sample No 16's tensile strength indicates that voltage has a minimal impact on weld durability To enhance weld durability during tensile tests, it is essential to adjust the welding voltage while considering other factors such as welding speed to achieve optimal results.
In the same study, when optimizing parameters for the tensile strength of weld joints, Kumar, L Suresh et al [28] investigated the factors affecting the mechanical properties of
The study investigates the welding of AISI 304 stainless steel using Gas Metal Arc Welding (GMAW) through the Taguchi method, focusing on current, gas flow rate, and voltage with a sample thickness of 3mm Results indicate that welding current significantly influences tensile strength, contributing 41%, followed by arc voltage at 20% and gas flow rate at 16% Consistent findings were reported by K.R Madavi et al., who identified welding current as the primary factor affecting ultimate tensile strength (UTS) in their MIG welding research on 8mm thick steel plates Similarly, Raja Subramanian's team found that increasing welding current enhances the tensile strength of MIG welds in 2mm thick C276 material, with their optimal parameters (90-110A current and 14.5-16.5V voltage) yielding significant results The optimal parameters identified in the current study were 102A current, 16.5V voltage, and a travel speed of 19.8 cm/min, achieving a maximum tensile strength of 741MPa Additionally, research by Régis Henrique Gonỗalves Silva et al demonstrated that increasing electrode extension while reducing welding current decreases heat generation during MIG welding Their findings revealed that a 7mm extension achieved the highest penetration depth and width of 2.74mm and 8.41mm, respectively, while a 20mm extension resulted in the lowest penetration of 1.92mm depth and 8.38mm width.
In addition, to find the relationship between factors affecting weld tensile strength, use Minitab software to produce a linear regression function equation:
The regression analysis reveals that welding current (I) is directly proportional to tensile strength (𝜎 Tensile), indicating that an increase in welding current leads to higher weld tensile strength Conversely, welding voltage (U), electrical stick-out (d), and welding speed (v) are inversely related to tensile strength, meaning that lower values of these parameters result in greater tensile strength.
Experiment validation compared to prediction, base metal, and optimal results
This study aims to compare the tensile strength, yield strength, elongation, and flexural strength of welds against the base metal Tensile and bending tests were performed on three samples each of C20 steel and SUS201, adhering to ASTM E8/E8M standards, to ensure consistency with the weld sample evaluations.
To validate the conclusions drawn from the optimal parameters identified through the Taguchi method and ANOVA, exploratory experiments were performed These experiments aimed to provide an objective perspective on the optimal results and to improve the reliability of the identified parameters The samples were newly welded utilizing these optimal parameters.
For filler material GM70S: I = 110A; U = 15V; d = 10mm; v = 450 mm/min for tensile tests and I = 110A; U = 15V; d = 12mm; v = 450 mm/min for bending tests
Table 4.21: Comparison table of optimal parameters and experimental parameters results
Predicted output parameters Experimental optimal parameters
Table 4.22: Tensile Test Results of Base Metal
Figure 4.14: Stress-Strain graph from the tensile test
Figure 4.15: Stress-Strain graph from the bending test
Figures 4.14 and 4.15 present the stress-strain graphs from the validation welding process During this experiment, welding samples were created using GM70s with optimal parameters established for the tensile test Furthermore, base metal samples C20 and SUS201 were welded to assess and compare the mechanical properties of the weld metal regions utilizing different filler materials.
SUS201, an austenitic stainless steel, exhibits exceptional mechanical properties When welded with optimal parameters using GM70S filler materials, the resulting samples show superior mechanical performance compared to the tensile test samples of the base metal C20.
C20 steel is a low-carbon alloy primarily composed of ferrite and a small percentage of pearlite, resulting in high ductility but limited tensile strength While ferrite offers good ductility, it fails to endure high stress levels, causing the material to fracture at a tensile limit of 443.09 MPa.
For the weld sample between C20 steel and SUS201 using GM70S filler wire:
During the welding process, the molten metal from the GM70S filler wire merges with the C20 base metal, creating an alloy with distinct characteristics The cooling rate post-welding plays a crucial role in determining the microstructure of the weld zone Rapid cooling can lead to the formation of martensite, a hard and brittle structure commonly found in high-carbon steel alloys Alternatively, bainite can also develop, offering superior mechanical properties, including enhanced tensile strength and ductility compared to martensite.
The Heat Affected Zone (HAZ) is a region that, while not melting during the welding process, experiences significant thermal effects that alter its microstructure High temperatures within the HAZ can lead to hardening, producing structures like martensite or bainite depending on the cooling rate, which enhances strength but may decrease ductility Near the weld, temperatures can reach levels that cause recrystallization, resulting in the formation of smaller grains with altered mechanical properties Conversely, areas further from the weld may experience sufficient heat to relieve stress and soften the material, ultimately reducing its mechanical strength.
• Transition Zone: Due to the chemical composition differences between C20 steel and
The transition zone of SUS201 exhibits a complex microstructure due to the diffusion of alloying elements from the GM70S filler wire and the stainless steel itself This interaction can lead to the formation of various structures, including martensite, bainite, and other distinct phases.
Martensite is a hard and brittle phase that develops in high-carbon steel when it is rapidly cooled from elevated temperatures Its presence in the weld and heat-affected zone (HAZ) enhances tensile strength due to its tightly packed crystal structure, which effectively resists stretching However, this increased strength comes at the cost of increased brittleness in the material.
Martensite exhibits high hardness but is notably brittle, limiting its ability to deform plastically In contrast, while bainite is less brittle than martensite, it still possesses lower ductility than ferrite or pearlite structures Consequently, welds rich in martensite and bainite experience a reduction in ductility C20 steel, characterized by a predominance of ferrite and pearlite, demonstrates significantly higher ductility than martensite and bainite This distinction highlights why C20 steel maintains greater ductility compared to the weld and heat-affected zone (HAZ) when unaffected by the welding process.
The rapid cooling during welding leads to the formation of martensite and bainite in the weld and heat-affected zone (HAZ), enhancing the material's tensile strength and hardness while decreasing ductility Consequently, the weld and HAZ exhibit higher tensile strength than C20 steel, preventing fracture during tensile tests despite their reduced ductility This indicates that the welding process and the use of GM70S filler wire resulted in a region with superior mechanical properties compared to the base metal C20, albeit with some limitations in ductility.
The tensile testing of samples welded with optimal parameters using GM70S filler wire revealed tensile strengths of 449 MPa, 453.96 MPa, and 455.37 MPa, closely matching the tensile strength range of the base metal C20 This is demonstrated in Figure 4.16, which shows that during the tensile test, fractures occurred either in the steel portion of the sample or in the heat-affected zone (HAZ) for two of the samples.
The danger zone indicates a strong bond in the weld, demonstrating tensile strength that surpasses that of the base metal.
Figure 4.16: Fracture position of the tensile tests sample with optimal parameters
Figure 4.17: Tensile Test Bar and Force - Displacement Diagram
When tension is applied to bar AB, it experiences a uniform tensile force F along its length, resulting in consistent stress at every point The calculation of stress at each point is determined based on this uniform force distribution.
𝜎 𝑧 : is the tensile stress (uniform at every point)
A is the cross-sectional area
Thus, when the sample is subjected to tension, the tensile strength for stainless steel
The tensile strength of the materials indicates that the steel component is the weakest link, with a tensile strength of 443 MPa compared to the weld's strength of ≥480 MPa and the overall material strength of 601 MPa If the applied force exceeds the tensile strength of the steel, it risks breaking Some samples show that the heat-affected zone (HAZ) of the steel is weaker than the base steel, likely due to insufficient weld metal coverage during the welding process, leading to potential breakage in this area However, most of the HAZ demonstrates a higher tolerance when welded under optimal parameters, as evidenced by the fact that 2/3 of the samples experienced breakage in the steel part This highlights that utilizing optimal welding parameters, as per the Taguchi method, significantly enhances the quality of the weld zone.
Summary of the MIG welding process
Key welding parameters include welding current (I), voltage (U), electrode extension length (d), and welding speed (v) The quality of the weld is evaluated through mechanical indices such as ultimate tensile strength (UTS), flexural strength (FT), yield strength (YS), elongation (Lf), and modulus of elasticity (E).
• Level of influence and optimization
In MIG welding with GM70S material, the primary factors affecting weld quality include welding current, voltage, electrode stick-out, and welding speed Among these, welding current is the most crucial, with optimal settings being 110A for current, 18V for voltage, a 10mm electrode stick-out, and a welding speed of 450.
73 mm/min speed These settings balance heat input and penetration, enhancing tensile strength and weld quality, and provide valuable guidance for industrial MIG welding optimization
• Table of input parameter rules
Table 4.23: Rules of the Studied Parameters
Optimizing output parameters such as Ultimate Tensile Strength (UTS), Yield Strength (YS), Length of Fusion (Lf), and Elongation (E) in bending and tensile tests is positively influenced by increased welding current (I), as indicated in Table 4.23 This trend aligns with formula (5.1) and reference [45], demonstrating that higher current generates more heat, facilitating the melting of both filler and base metals, which enhances penetration during arc formation and results in stronger weld joints Conversely, low welding currents produce inadequate heat, leading to weak weld bonds susceptible to failure in tensile and bending tests Notably, experiments with GM70S filler material reveal that welding current (I) significantly affects weld quality.
In optimization experiments with two types of filler materials, welding voltage (U) is found to be one of the least influential factors on weld quality While it primarily affects arc length and stability, a stable arc is essential for a smooth welding process but does not significantly impact penetration depth or weld strength Although voltage influences arc temperature, its effect is minimal compared to welding current (I), and the relationship between voltage and heat transfer to the weld material is not linear This is exemplified by the comparison of samples 16 and 4, welded with GM70S wire at the same voltage, where sample 16 exhibits greater strength than sample 4.
In welding experiments with GM70S, key parameters such as electrode stick-out (ESO) and welding speed significantly impact weld quality, with lower values generally producing superior results An increase in ESO leads to higher resistance in the welding wire, generating excess heat before the arc reaches the weld material, which in turn diminishes heat transfer to the weld area and reduces penetration While a larger electrode stick-out can enhance the melting rate of the electrode, it simultaneously decreases the concentrated heat in the weld area, adversely affecting penetration depth and overall weld quality Additionally, welding speed influences the duration of heat application from the arc to the material, further affecting the weld's integrity.
Faster welding speeds minimize heat exposure time, leading to reduced penetration as the material lacks adequate time to melt deeply The welding speed significantly influences heat distribution both on the surface and within the weld material; excessively fast speeds can result in shallow welds with inadequate penetration, while slower speeds may cause burn-through and defects such as undercuts Moreover, welding speed affects thermal stresses in the weld and surrounding areas; high speeds can induce substantial thermal stresses, resulting in weld distortion and diminished strength.
• Conclusions on Taguchi predictions, weld joints, and base metals
The experimental results indicate that samples welded with GM70S filler material exhibit greater strength than the C20 base metal, yet lower strength compared to the SUS201 base metal Additionally, the findings from Tables 4.21 and 4.22, along with Figures 4.14 and 4.15, reveal that the outcomes achieved using the optimal parameter set closely align with the predictions made by the Taguchi method.
CONCLUSION AND DEVELOPMENT DIRECTIONS
Conclusion
After a period of research on the topic “Research on The Effects of MIG Welding
In our investigation of the welding quality parameters for C20 steel and SUS201 stainless steel, we conducted tests on 96 specimens and performed exploratory experiments to determine optimal welding conditions By reviewing various studies focused on optimizing MIG welding parameters, we identified the best settings for achieving superior tensile strength, flexural strength, yield strength, elongation, and elastic modulus in welds using GM70S filler material.
Tensile Strength – Yield Strength – Elongation – Elastic Modulus
Welding current (I) significantly impacts the tensile strength, yield strength, elongation, and elastic modulus of a weld, accounting for 63.61%, 50.37%, 83.41%, and 33.45% of their respective influences Following closely, welding speed (v) contributes 48.98%, 6.91%, 7.98%, and 2.32% to weld quality In contrast, electrode stick-out (d) and welding voltage (U) exert a lesser influence, with d affecting the quality by 4.85%, 7.41%, 2.25%, and 0.08%, while U has the lowest impact at 1.54%, 0.002%, 0.98%, and 0.01%.
Welding current (I) significantly impacts the flexural strength and elastic modulus of welds, accounting for 68.47% and 28.38% of their quality, respectively The electrode stick-out (d) follows as the second most influential factor, contributing 6.39% to 16.52% of weld quality In contrast, welding voltage (U) and welding speed (v) have a lesser effect, with influences of 2.62% to 1.21% and 6.77% to 1.41%, respectively, making welding speed the least influential factor in determining weld quality.
The application of Taguchi and ANOVA methods in experiments significantly decreases the number of trials needed while simultaneously improving the reliability of the results and values obtained in this study.
Future Development of the Research
Our research group encourages future students to explore additional parameters, such as environmental factors like shielding gas, humidity, and ambient temperature, in their studies on the welding process between C20 steel and SUS201, as well as between carbon steel and various stainless steels This comprehensive investigation could be compiled into a reference document to support further research in the field.
Choosing the right welding parameters for various materials and filler wires is crucial The parameters examined by the team can also be utilized in welding processes for thinner materials, helping to prevent defects in the welds.
This project investigated how welding parameters affect the quality of welds between C20 and SUS201 materials Due to constraints in research experience and experimental time, further analysis on basic bend testing, weld hardness, and detailed examinations of fractures and microstructures was not performed Future research should focus on these areas to enhance the study's reliability Additionally, this research can be expanded to include various materials, workpiece sizes, and welding processes, aiming to identify cost-effective procedures that maintain research efficiency and mitigate potential welding issues.
PENETRATION DEPTH OF THE WELD
MICROSTRUCTURE OF THE WELD
Steel Side Microstructure Inox Side Microstructure
WELD STRENGTH GRAPH
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