Effects of heat treatment on the mechanical properties of 6201 aluminum alloy wire

Type 6201 aluminum alloy wires are produced by drawing 4.7 mm diameter billet-onbillet extruded redraw rod down to 2.7 mm diameter wires. Before drawing, the first group of redraw rod coils was annealed at 480 oC for 4 hours to reduce the hardness of the redraw rod.

Vietnam Journal of Science and Technology 57 (3A) (2019) 11-20 doi:10.15625/2525-2518/57/3A/13936 EFFECTS OF HEAT TREATMENT ON THE MECHANICAL PROPERTIES OF 6201 ALUMINUM ALLOY WIRE Huynh Cong Khanh Faculty of Materials Technology, Ho Chi Minh City University of Technology, VNUHCM 268 Ly Thuong Kiet Street, Ward 14, Dist 10, Ho Chi Minh City * Email: hckhanh@hcmut.edu.vn Received: 13 July 2019, Accepted for publication: 18 September 2019 Abstract Type 6201 aluminum alloy wires are produced by drawing 4.7 mm diameter billet-onbillet extruded redraw rod down to 2.7 mm diameter wires Before drawing, the first group of redraw rod coils was annealed at 480 oC for hours to reduce the hardness of the redraw rod The second group of redraw rod coils was drawn without annealing With each group of redraw rod, after drawing, some wire coils were solution heat treated, then artificially aged or naturally aged The other wire coils were artificially aged or naturally aged without solution heat treatment Mechanical properties of the wires were assessed by a tensile testing machine (model UTM-1000) With suitable aging temperature and aging time, wires produced from each group of redraw rod coils with or without solution heat treatment attain tensile requirements of ASTM B398, but wires produced with solution heat treatment attain higher elongation than wires produced without solution heat treatment Keywords: 6201 aluminum alloy, Aluminum alloy wire, Aluminum Alloy Conductor, Aluminum alloy cable Classification numbers: 2.9.1, 2.10.2 INTRODUCTION Type 6201 aluminum alloy is a heat treatable one with high electrical conductivity and good mechanical strength It is widely used for the transmission and distribution of electricity [1] Type 6201 wires are used in production of AAAC (All Aluminum Alloy Conductor) A comparison of the performance of the AAAC and conventional ACSR (Aluminum Conductor Steel Reinforced) conductors, AAAC has lower power losses, excellent corrosion resistance, and better resistance to abrasion, all without compromising either electrical conductivity or strength and sag properties [2] Type 6201 aluminum alloy wire is drawn from 6201 aluminum alloy redraw rod The 6201 aluminum alloy redraw rod has been manufactured by various procedures consisting of separate steps which include: DC casting, continuous casting and rolling rod on a Properzi or Secim mill Additional steps may include: Huynh Cong Khanh a) Immediate quenching after rolling, but before winding into or 3-tonne coils The rod is then cold drawn to form wire, and the wire is artificially aged at a temperature between 250 F (121.11 oC) and 450 F (232.22 oC) [3] b) The 3/8 inch (9.65 mm) rod was drawn to 0.210÷0.211 inches (5.33÷5.35 mm) and solution-treated in molten salt at 965 F (518 oC) for 20 min, quenched in cold water, and followed by a rinse in tepid water The T4 wire was drawn to finish gage and artificially aged for 10 hours at 320 F (160 oC) to the T81 condition [4] According to G Davies [5], Chia supposes the continuous quenching and solution treatment method (method a) saves cost, reduces in handling damage, and attains more uniform mechanical properties of aluminum alloy redraw rod Nicoud et al think that more reliable product which is easier to process can be achieved with separate solution treatment (method b) Both methods (a) and (b) produce rod capable of being processed into wire to meet UK specification BS 3242:1970 Type 6201 aluminum alloy rod is also manufactured by extrusion press In practice for manufacturing of semi-finished products of low-ductile and low-tech alloys that are difficult to deform in casting and rolling plants, one has to use discrete methods of pressing, making products of limited length [6] The reaction of the billet with the container and die results in high compressive stress that is effective in reducing the cracking of the billet material during primary breakdown from the billet Extrusion is the best method for breaking down the casting structure of the billet, since the billet is subjected only to compression [7] In addition, when producing rods by shape-rolling methods including production-on-casting and rolling mills the degrees of reduction at one stage are so low in comparison to pressing that the number of mill stand and passes during rolling (even on a continuous mill) is 15-20 or more [6] For the production of coiled semifinished products for further processing, such as rod drawing production, billet-on-billet is also a viable process Billet-on-billet extrusion is a special method for aluminum alloys that are easily welded together, at the extrusion temperature and pressure, as 6000-series alloys Using this process, the continuous length of semifinished products can be produced by different methods [7] Correct heat treatment of soft and medium grade aluminum alloy is important to obtain the required mechanical properties For all heat treatable aluminum alloys, heat treatment is a twostage process: solution heat treatment followed by precipitation hardening (aging) For 6000series alloys, the extrusion may be quenched directly from the extrusion temperature Usually, a forced-fan cooling system is sufficient, but water quenching is sometimes needed [7] Therefore, billet-on-billet extrusion permits producing press-quenched redraw rod From that redraw rod, 6201 alloy wire is drawn and then subjected to artificial aging to meet required properties without a separate solution quenching operation The aim of this work is to study the process of manufacturing 6201 aluminum alloy wire from billet-on-billet extruded redraw rod of about 4.7 mm in diameter EXPERIMENTAL METHOD Type 6201 aluminum alloy wire was manufactured from 4.7 mm diameter 6201 aluminum alloy redraw rod coils Redraw rod coils were manufactured by billet-on-billet extrusion on a 300 ton Cincinnati Milacron hydraulic extrusion press The working parameter of billet extrusion for production of redraw rod was as follows: pressing capacity: 300 tons; type of die: feeder plate die with 14 exit holes of 4.8 mm each; extrusion speed: 15 m/min; billet 12 Effects of heat treatment on the mechanical properties of 6201 aluminum alloy wire temperature: 470 oC The chemical composition of used 6201 alloy was analyzed by optical emission spectrometer SPECTROLAB M12 and the result shown in Table For metallographic examination of as-cast used 6201 alloy, samples were prepared by using standard grinding papers and were polished Samples were then etched with 20% H3PO4 solution to study microstructure using the optical microscope (OLYMPUS MPE3) As shown in Fig.1, the microstructure of used 6201 alloy in the as-cast state consists of α-Al matrix and intermetallic phases, as Mg2Si, Si, -Al8Fe2Si, -Al5FeSi in grain boundaries Table Chemical composition (in wt%) of 6201 aluminum alloy Composition, % Alloy 6201 Cu Fe Si Mn Mg Zn Cr B Ti Al 0.048 0.314 0.832 0.010 0.601 0.005 0.002 0.0025 0.0183 Balance Figure The microstructure of used 6201 alloy in as-cast state (×100) The redraw rod coils were divided into two groups: 1) Group A: The redraw rod coils were annealed at 480 oC for hours in a chamber-type electric resistance furnace, then redraw rod coils were taken out and air-cooled After annealing, the redraw rod coils were drawn from 4.7 mm to 2.7 mm in diameter Some samples of 2.7 mm wire were artificially aged at 170 oC with aging duration of 5, or hours or naturally aged (not artificially aged) for days without solution heat treatment Other samples of 2.7 mm wire were solution heat treated at a temperature of 520 oC, followed by water quenching The soaking time for solution heat treatment was hour Then the wire samples were artificially aged at 150, 160, and 170 oC for 5, 6, and hours, respectively 2) Group B: The redraw rod coils were drawn from 4.7 mm to 2.7 mm in diameter without annealing before drawing Some samples of 2.7 mm wire were artificially aged at 170 oC with aging times of 5, 6, 7, hours and naturally aged for days without 13 Huynh Cong Khanh solution heat treatment Other samples of 2.7 mm wire were solution heat treated at 520 o C, followed by water quenching The soaking time for the solution heat treatment was hour, then the wire samples were artificially aged at 150 oC for 5, 6, hours Finally, the samples of both groups were assessed for their mechanical properties by the tensile testing machine (model UTM-1000) Tensile tests were conducted using 250 mm gauge length samples according to ASTM B398 The number of samples used for tensile tests for each state of Aluminum alloy wires was 2-3 RESULTS AND DISCUSSION Table and Figure show the mechanical properties of aluminum alloy wires drawn from the annealed redraw rod Wire samples were subjected to artificial aging at 170 oC for hours without solution heat treatment to achieve tensile strength and elongation according to the requirements of ASTM B398 (≥ 315 MPa and ≥ 3.0 %) But, when keeping aging temperature at 170 oC and increasing aging time to or hours, the tensile strength of those wire samples decreases to the lower requirements of ASTM B398 Used alloy in this work has 0.601 % Mg, 0.832 % Si and 0.314 % Fe, so under equilibrium solidification, the following equilibrium phases are possible: Mg2Si, Si, -Al8Fe2Si, -Al5FeSi [8, 9] Assuming that all Fe and Mg combine with Si and -Al5FeSi is the predominant phase solution formed, the amounts of Mg2Si and excess Si can be calculated as follows [1]: Mg (%) = %Mg + 1/1.73(%Mg) = 0.948 % Excess Si (%) = %Si – [1/2(%Fe) + 1/1.73(%Mg)] = 0.328 % Therefore, according to Al-Mg2Si pseudobinary phase diagram [10], most Mg2Si phases dissolve in the solid solution when redrawing rod coils were annealed at 480 oC for hours While redrawing rod coils were cooled in air, some part of Mg2Si precipitated but nearly all Mg2Si remained in the solid solution because, with a small diameter of redraw rod and a small amount of magnesium and silicon, the alloy may be quenched in the air Furthermore, excess Si promotes an additional response to age hardening by both refining the size of the Mg2Si particles and precipitating as Si The addition of a small amount of Cu may lessen the adverse effect of delay at room temperature by reducing the onset and rate of natural aging and promoting an increased response to artificial aging The addition of iron can refine grain structure and increase the mechanical properties of Al-Mg-Si alloy [11, 12] Deformation of supersaturated solid solution causes partial precipitation and considerable effects on the precipitation during subsequent heating Cold deformation increases the precipitation process and changes distribution characteristics of the second phase and its dispersion during aging [12] As a result, after annealing, drawing, and artificial aging at 170 oC for hours, 6201 alloy samples (sample A1) have high tensile strength and suitable elongation The age hardenable precipitation formation sequence in 6xxx alloy is as follows: α supersaturated solid solution (SSS) → GP zones → β’’ → hexagonal β’ (Mg2Si) → fcc β(Mg2Si) Precipitation of the alloying as coherent GP zones and coherent β’’ and hexagonal coherent β’ (Mg2Si), during aging treatment, provides strengthening due to the presence of coherency strain field around the precipitates, which interacts with the moving dislocation The equilibrium fcc β(Mg2Si) is incoherent and formation of incoherent equilibrium precipitates culminates in a slight loss in hardness and strength due to the absence of lattice mismatch strain, that is, coherency strain around the equilibrium precipitates in 6xxx alloys Overaging of the fcc β(Mg2Si) precipitates leads to coarsening of precipitates and results in a further loss in strength 14 Effects of heat treatment on the mechanical properties of 6201 aluminum alloy wire [13, 14] Therefore, with an increase of aging time to or hours at 170 oC, more incoherent equilibrium fcc β(Mg2Si) phase is formed and coarsened due to overaging Tensile strength is also decreased With solution heat treatment at 520 oC followed by water quenching, Mg2Si phase dissolves in the supersaturated solid solution The high content of Mg and Si in supersaturated solid solution results in a large number of precipitates during aging A large amount of small and uniformly distributed precipitates will appear, thus the mechanical properties of the materials can be improved [15] Siddiqui et al showed that as the aging temperature and time increase, the density of GP zone also increases [16] Table Mechanical properties of 2.7 mm diameter 6201 aluminum alloy wires drawn from annealed redraw rod (The group A) Sample designation Sample state Average tensile strength ± SD,  (MPa) Average elongation in 250 mm ± SD,  (%) A1 Annealed – Drawn - Artificially aged at 170 oC for hours 340.46 ± 38.97 3.6 ± A2 Annealed – Drawn - Artificially aged at 170 oC for hours 250.57 ± 19.69 2.59 ± 0.20 A3 Annealed – Drawn - Artificially aged at 170 oC for hours 263.53 ± 2.25 3.27 ± 0.31 A4 Annealed – Drawn - Naturally aged for days 302.73 ± 10.10 3.39 ± 0.93 A5 Annealed – Drawn - Solution heat treated Artificially aged at 150 oC for hours 282.13 ± 19.07 11.53 ± 3.06 A6 Annealed – Drawn - Solution heat treated Artificially aged at 150 oC for hours 297.70 ± 17.15 9.47 ± 2.81 A7 Annealed – Drawn - Solution heat treated Artificially aged at 150 oC for hours 298.17 ± 9.76 15.00 ± 2.25 A8 Annealed – Drawn - Solution heat treated Artificially aged at 160 oC for hours 304.47 ± 16.95 12.80 ± 2.43 A9 Annealed – Drawn - Solution heat treated Artificially aged at 160 oC for hours 326.8 ± 6.05 9.47 ± 1.40 A10 Annealed – Drawn - Solution heat treated Artificially aged at 160 oC for hours 360.55 ± 22.70 8.80 ± 0.57 A11 Annealed – Drawn - Solution heat treated Artificially aged at 170 oC for hours 243.15 ± 6.29 11.4 ± 1.41 A12 Annealed – Drawn - Solution heat treated Artificially aged at 170 oC for 15 hours 213.55 ± 12.09 11.00 ± 1.41 15 Tensile strength, MPa Huynh Cong Khanh 400 350 300 250 200 10 12 14 16 Ageing times, h Artificial ageing at 170oC (without solution heat treatment) Artificial ageing at 150oC (with solution heat treatment) Artificial ageing at 160oC (With solution heat treatment) Artificial ageing at 170oC (With solution heat treatment) Figure Effect of aging temperature and aging time on the tensile strength of 6201 aluminum alloy wires drawn from annealed redraw rod Figure The force-displacement curves from tensile tests of samples A10 Hence, the degree of irregularity in the lattices causes an increase in the mechanical properties of Al-Mg-Si alloy [17] As seen in Fig 2, the tensile strength of wire samples subjected to artificial aging at 160 oC is higher than the tensile strength of wire samples subjected to artificial aging at 150 oC Also, the tensile strength of samples subjected at 150 and 160 oC increases as aging time increases However, the strength of samples subjected artificial aging at 170 oC is lower than the strength of samples subjected to artificial aging at 150 and 160 oC due to overaging, thus precipitates agglomerate and coarsen into larger particles and bigger grain size, causing fewer obstacles to the movement of dislocation [17] Figure shows the force-displacement curves from tensile tests of samples A10 which attain the highest tensile strength in group A After artificial aging at 170 oC and natural aging for days, wire samples drawn from a second redraw rod coil group, without annealing before drawing and without solution heat treatment, had high tensile strength With an extruded billet temperature of 470 oC, nearly all Mg2Si phase alloy dissolved in the solid solution, as seen in the Al-Mg2Si pseudobinary phase diagram [10] Type 6000-series alloys may be quenched at the extrusion press when the product emerges hot from the die, thereby eliminating the need to solution treat 16 Effects of heat treatment on the mechanical properties of 6201 aluminum alloy wire as a separate operation [7] With a small diameter redraw rod (4.7 mm), an air quenching in the press may be sufficient Therefore, after extrusion, following drawing, and artificial aging at 170 oC or natural aging for days, wire samples attain high tensile strength, as shown in Tab and Fig The elongation of those samples is low because, after cold drawing, tensile strength, yield strength and hardness of redraw rod increase at the expense of ductility and formability [11] However, as aging time increases, tensile strength and elongation increase Mechanical properties of samples aged at 170 oC for hours meet the requirements of ASTM B398 (tensile strength  315 MPa, elongation of 250 mm  3.0 %) When artificial aging time increases to hours, the mechanical properties of samples decrease due to overaging Table Mechanical properties of 2.7 mm diameter 6201 aluminum alloy wires drawn from 4.7 mm diameter non-annealed redraw rod (The group B) Sample designation Sample state Average tensile strength ± SD,  (MPa) Average elongation in 250 mm ± SD, (%) B1 Non-annealed –Drawn - Artificially aged at 170 oC for hours 324.6 ± 45.24 2.67 ± 0.23 B2 Non-annealed - Drawn – Artificially aged at 170 oC for hours 364.47 ± 9.81 2.8 ± B3 Non-annealed –Drawn - Artificially aged at 170 oC for hours 383.9 ± 12.68 3.47 ± 1.01 B4 Non-annealed - Drawn - Artificially aged at 170 oC for hours 316.05 ± 12.09 1.80 ± 0.28 B5 Non-annealed - Drawn – Naturally aged for days 360.97 ± 10.10 - B6 Non-annealed - Drawn - Solution heat treated - Artificially aged at 150 oC for hours 307.00 ± 31.25 10.60 ± 0.28 B7 Non-annealed - Drawn - Solution heat treated - Artificially aged at 150 oC for hours 305.45 ± 9.69 8.00 ± 0.57 B8 Non-annealed - Drawn - Solution heat treated - Artificially aged at 150 oC for hours 342.35 ± 10.39 12.00 ± 0.57 Wire samples are drawn from non-annealed redraw rod, then solution heat treated and artificially aged at 150 oC, have tensile strength lower than the tensile strength of wire samples aged without solution heat treatment, but elongation of those samples is greater than the elongation of samples aged without solution heat treatment The reason for this is that the recrystallization of deformed aluminum alloy appears at solution heat treatment temperature During recrystallization, the dislocation density will be reduced down to an equilibrium value A new microstructure with relative equilibrium defect density is formed and the alloy loses the additional strength acquired in cold drawing at the increased ductility and formability As artificial aging time increase to hours, the mechanical properties of those wire samples achieve 17 Huynh Cong Khanh Tensile strength, MPa the requirements of ASTM B398 Figure shows the force-displacement curves from tensile tests of samples B3 which attain the highest tensile strength in group B 450 400 350 300 250 200 Ageing times, h Artificial ageing at 170oC (without solution heat treatment) Artificial ageing at 150oC (with solution heat treatment) Figure Effect of aging temperature and aging time on the tensile strength of 6201 aluminum alloy wires drawn from non-annealed redraw rod Figure The force-displacement curves from tensile tests of samples B3 18 Effects of heat treatment on the mechanical properties of 6201 aluminum alloy wire CONCLUSION With suitable aging temperature and aging time, wires drawn from annealed and nonannealed redraw rod, then artificially aged without solution heat treatment, can attain the tensile requirements of ASTM B398 with high tensile strength, such as samples A1, B3 This is also the advantage of the billet-on-billet extrusion method for production of small (4.7 mm diameter) quenched redraw rod, and from that redraw rod, quality 6201 aluminum alloy wire can be produced without a separate quenching solution operation With suitable aging temperature and aging time, wires drawn from annealed and nonannealed redraw rod, then solution heat 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