dissimilar welding hardness high temperature titanium alloy laser beam welding laser welding mechanical properties microstructure penetration tensile properties tensile strength AA2024-T4/AA7075-T6Al alloys AA5456 ASTM A553-1 (9% nickel steel) Al/steel joints BTi-6431S Bead on Plate (BOP) CMT DP1000 steel EBSD EBSD phase mapping H-shaped fusion zone HLAW History of engineering & technology IMC layers Incomplete penetration K-TIG L-joint LKW MAG MIG