Influence of energy ratio on dual-spot laser welded-brazed Al/steel butt joint

Al/steel butt joints were successfully obtained using a dual-spot laser welding-brazing process with different R (R = E-Fe:E-Al = 4:6, 5:5, and 6:4; E-Fe and E-Al represented the energy ratios of the laser near steel and Al, respectively). The molten filler metals wet and spread well along the front and back surfaces of the steel when R was 5:5 and 6:4, whereas an undercut occurred when R was 4:6. The interfacial intermetallic compound (IMC) along the brazing interface consisted of only thin-layered tau(5)-Fe1.8Al7.2Si when R was 5:5 and 4:6. The IMC had a greater thickness when R was 5:5 owing to its higher peak temperatures than when R was 4:6. A mixed IMC (theta-Fe(Al,Si)(3)+tau(5)-Fe1.8Al7.2Si) was newly formed in the top region of the brazing interface when R was 6:4. TEM observations revealed that two types of grains existed in theta-Fe(Al,Si)(3): normal and twinned. Numerical simulations for the thermal cycles along the brazing interface indicated that the IMC thickness was determined by the interfacial peak temperature, while the IMC homogeneity was determined by the temperature gradient. The results of an experiment involving the tensile properties indicated that the highest average joint strength (193 MPa) and largest average fracture displacement (0.64 mm) when R was 5:5, owing to its satisfactory weld formation and reasonable IMC distribution.