Fundamental Research On Double-sided Laser Welding Of TC4-BTi6431S Titanium Alloys With Variable Thickness

A new method, double-sided welding with continuous increasing laser power, wasproposed for welding of TC4-BTi6431S with linear variable thickness to obtain weldbeads with small deformation, few defects and excellent high temperature strength.Proper welding parameters were obtain through fiber laser welding TC4andBTi6431S with uniform thickness plates. Relation between the welding parameters andpenetration depth were obtained by single-sided laser welding of TC4with thickness of10mm. Then, double-sided welding with continuous increasing laser power was applied toachieve sound weld beads of TC4-BTi6431S dissimilar titanium alloys with variablethickness. The effect of welding speed, laser power and overlap rate on the quality ofdissimilar titanium alloys joint achieved by double-sided laser welding was studied.TEM was taken for analysing precipitations on weld bead of TC4-BTi6431Sdissimilar titanium alloys. Metallurgical microscopy, XRD and SEM was utilized toanalysing microstructure of TC4-BTi6431S joints.Microhardness distribution on TC4-BTi6431S joints was obtained through LECOHardness Mapping. Tensile tests of weld beams at room temperature and high temperature(500℃) were taken for studying tensile properties of dissimilar joints, and fracture surfacewas analysed by SEM.The important results was giving as follow:The fitting equation y1348582xfor relationship between penetration and laserpower at welding speed equal2.0m/min was achieved by continuous increasing laserpower single pass welding, offered guidance on double-sided welding through controllingof penetration and overlap rate. Finally double-sided laser welding joints with appropriateoverlap rate were obtained. With the controlling of overlap rate, welding deformation ofvariable thickness weld beam could be very small. And welding deformation could be further reduced by pulsed laser double-sided welding.A serial testing methods were used for phase analysis of joints of dissimilar titaniumalloys, precipitations ranged from50nm to100nm were found at HAZ of BTi6431S side.The weld bead microstructure appearance of TC4-BTi6431S and BTi6431S-BTi6431Smake no difference, while that of the HAZs were a little different. High hardnessWidmannstatten microstructure was observed in HAZ of BTi6431S side.Harden phenomenon was observed in the HAZ of BTi6431S side. High-hardnessWidmanstatten structure and fine precipitations were discovered with the testing methodsof XRD/SEM and TEM. According to the results of tensile testing, the tensile strength ofdissimilar joints was higher than the parament metals during proper parameters ranges.Facture positions of dissimilar joints in tensile testing was at TC4base metal. HAZ wasnot the weaken zone of the dissimilar joints. The tensile strength of dissimilar titaniumalloys reached1100MPa at room temperature. High temperature tensile strength was over600Mpa, satisfying the expected tensile strength of320MPa.