Local Plastic Deformation and Failure Behavior of Nd:YAG Laser Welds in AA5182-O and AA6111-T4
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DEMAND for improved fuel efficiency has led to the increased usage of aluminum alloys in automobiles. According to Kelkar et al.,[1] the total mass of aluminum increased from 39 kg (3 pct of vehicle weight) in 1976 to 89 kg (7 pct of vehicle weight) in the 1990s. During 2003 alone, aluminum shipments to the automotive industry reached 5.4 billion pounds (mostly as castings, forgings, and extrusions).[2] Despite this increased cost, formability and joining issues continue to impede wider-scale use of aluminum in auto body components. Aluminum has a far lower tolerance for damage and strain localization than does steel, which is clearly seen through a comparison of the stress-strain curves of both materials.[3] Aluminum joint materials, particularly those manufactured with a focused energy beam, are even more problematic, due to the loss of volatile alloying agents and reduced ductility resulting from internal damage.[4,5] Tailor-welded blanks made of the aluminum alloys AA5182-O and AA6111-T4, in which two or more sheets are butt welded prior to forming, are of particular WEI TONG, Professor, is with the Department of Mechanical Engineering, Southern Methodist University, Dallas, TX 75275-0337, USA. Contact e-mail: [email protected] LOUIS G. HECTOR, Jr., and CAMERON DASCH, Staff Researchers, are with the Materials and Processes Lab, General Motors R&D Center, Warren, MI 48090-9055, USA. HONG TAO, Graduate Student, and XIQUAN JIANG, Postgraduate Fellow, are with the Department of Mechanical Engineering, Yale University, New Haven, CT 06520-8284, USA. Manuscript submitted September 15, 2006. Article published online November 6, 2007 METALLURGICAL AND MATERIALS TRANSACTIONS A
interest. The AA5182-O is a solid-solution strengthened alloy that has found application as inner-body panel material, due to its strength, ductility, and cost.[6] The AA6111-T4 is a higher-strength (although lower-ductility) material, produced by solution heat treatment and natural aging; it has found application as an outer-body panel material due to its superior dent resistance.[7] Aside from formability issues, one of the most challenging problems for automotive assembly is joining.[7,8] In fusion welding, much of the decrease in joint strength (relative to the base material) can be attributed to the loss of key alloying agents during welding (e.g., Mg is a high-vapor-pressure alloying element with a vaporization temperature of 1090 C, cf. with aluminum at 2519 C[9]), and porosity formation.[10] Pore nucleation and growth is thought to occur via hydrogen absorption and release from the melt upon solidification and via entrapped shielding gas in the turbulent (and often unstable) weld pool.[11] In high-power laser beam welding, the stability of the weld pool is strongly affected by the keyhole. In aluminum alloys, keyhole instabilities are accentuated by the low interfacial tension (~1.0 N/m) and the viscosity of molten aluminum.[12] The generation of definitive tensile stress-strain relations is a necessary first step for materials characterizat
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