Parameters Controlling the Performance of Al-Si-Mg Automotive Alloys Containing Fe-Intermetallics: Part II -- Work Hardening Behavior

摘要

In this part, the formability of experimental and industrial 6XXX type alloys was investigated, in terms of the variation in the alloy hardness (Brinell hardness) as a function of the amount of cold rolling applied (0.5 percent per pass). Slabs were prepared from rectangular metallic mold castings and homogenized at 520 deg C/10 h. The microstructures of the homogenized and cold rolled samples were examined using optical microscopy. Results showed that the base experimental alloy (Al-0.61 percent Si-0.38 percent Fe-0.64 percent Mg), containing mainly the beta-Al_5FeSi intermetallic, displays low formability, due to the brittle, plate-like nature of the beta-phase which hinders deformation of the grains during cold rolling, leading to premature cracking. In the high-Si (1.01 percent) containing alloy, the larger size and volume fraction of the beta-phase results in only 43 percent total reduction (compared to 85 percent for the base alloy). Strontium addition does not affect the microstructure and, hence, the formability. With Mn addition (0.2 percent Mn, Mn/Fe: approx 0.7), precipitation of the Fe-intermetallics in the form of much smaller alpha-Al_(15)(Fe,Mn)_3Si_2 script phase particles improves the formability (77 percent total reduction in thickness). In this case also, Sr addition does not improve the alloy formability (67 percent total reduction). Addition of Be (0.02 percent) leads to precipitation of the Fe-intermetallics in the form of small, globular alpha-Al_8Fe_2SiBe script particles in interdendritic regions. Addition of Sr in this case, however, or increasing the cooling rate, produces very fine broken particles of alpha-Al_8Fe_2SiBe, which enhances the alloy formability (cf. 97 percent total reduction with 77 percent in the non-Sr containing alloy). The industrial alloys also show the same response to cold rolling.