Influence of Heat Treatment on the Surface Structure of 6082 Al Alloys
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NTRODUCTION
6XXX Al alloys are widely used, mainly in extruded form, because of properties such as strengths, ductility, and corrosion resistance. Especially the 6082 alloy, with the highest strength combined with acceptable extrudability, is growing in use in the automotive industry. The intermetallic particles in the as-cast billets of a 6xxx alloys consist mainly of b-AlFeSi phase and coarse Mg2Si particles.[1–4] The b-AlFeSi phase is an undesirable phase due to formation of hot cracking and surface defects during subsequent procedures, e.g., extrusion.[5,6] To obtain high surface-quality extruded products, at reasonable extrusion rates, the as-cast billets need to be homogenized before extrusion.[1,2,6] The homogenization temperature, holding time, and cooling rate influence the heat-treated microstructure.[6] Increasing the heat treatment temperature increases both the solubility and the diffusion coefficient,[7] and it is known that the percentage of a-phase will be increased by time during homogenization.[8] In addition, different as-cast Al alloys require varying heat treatment procedures depending on the alloying elements and microstructures.[9]
N. BAYAT and T. CARLBERG are with the Department of Natural Sciences, Mid Sweden University, Holmgatan 10, 851 70 Sundsvall, Sweden. Contact e-mail: [email protected] Manuscript submitted March 23, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
A thick shell zone can affect the surface quality of the extruded products.[10] An as-cast billet should have minimum surface segregation and shell zone in addition to fine secondary phases, fine grains, and cells which are uniformly distributed.[6] If the surface zone is of unacceptable quality scalping of the outer layer is necessary, however, this is a costly treatment and needs to be avoided unless especially strong demands are set on the billet quality. The surface segregation zone with a thickness of about 100 lm in the as-cast billets has higher alloy content compared with the bulk. The concentration of alloy elements at the surface is about five to six times higher than the bulk concentration at smooth surfaces without lapping.[11,12] In previous studies,[12–14] the surface segregation was investigated, and possible mechanisms for surface layer formation during solidification were proposed. However, the changes of the surface zone structure during the homogenization, and the effect of heat treatment on the outer part of the surface layer, have not been thoroughly studied. Local melting may occur during heat treatment of the billets. In industry, when the cooling rate is high, the last liquid to solidify may contain non-equilibrium eutectic composition.[15] The local melting will take place at the surface layer rather than in the bulk due to higher alloy content and fraction of the eutectic phase. The surface layer thickness, remelting, and deformation of the billet surface during homogenization as well as the fraction of phase transformation will affect the amount of scrap at the end of the extrusion procedure.
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