Research On Numerical Analysis And Characteristics Of Titanium Alloy Sheet Back-reflection Laser Welding With Low Energy Threshold

Aiming at the problems existing in the research of titanium alloy sheet 1.5mm thick TC4 titanium alloy sheet was taken as an object to develop research on numerical analysis and characteristics of titanium alloy sheet back-reflection laser welding with low energy.The numerical simulation and analysis of temperature field and stress field were studied in the process of back reflection laser welding,which expound the variation and distribution of temperature field and stress field and reveal the mechanism of back-reflection laser welding;At the same time,through research on the process characteristics and microstructure and properties,the new method of high efficiency and low threshold laser welding is used to verify the adaptability of thick plate of 1.5mm so as to provide further technical support and application foundation.The method of "combining theoretical analysis with experimental research","numerical simulation and experimental optimization" are adopted in the course of the study,the exploratory work done and the results obtained are as follows:(1)Numerical simulation and analysis of the temperature field in the laser welding process of titanium alloy sheet.Based on the experimental verification of the "double heat source" temperature field model of 0.8mm thick titanium alloy sheet,we set up new parameters to establish temperature field model,the variation of temperature field is simulated and analyzed during welding.The experiment result indicated the model can be used to simulate the temperature distribution,the stable temperature field is similar to that of the teardrop shape formed both on the front and the back face of the plate in a short time,the temperature of the front of the heat source is dense,the temperature gradient is large,the end of the heat source is relatively sparse,the temperature gradient is relatively small.The physical process of the two heating is well described by the variation of temperature field.With the increasing of power or the decreasing of welding speed,The width of molten pool and penetration depth increases gradually,the temperature gradient is proportional to the power and inversely proportional to velocity.(2)Numerical simulation and analysis of the stress field in the laser welding process of titanium alloy sheet.Based on temperature field,the numerical simulation and analysis of the stress field is adopted by indirect coupling method.The experiment result indicated the thermal stress increases with the increasing of welding time during the laser welding of titanium alloy sheet;the stress distribution of the titanium alloy sheet is the largest at both ends which show a clear boundary effect during the cooling process.In the conventional laser welding,the residual stress on the surface of the weld cross section is larger than that of the lower surface and the residual stress distribution is wider;the overlap degree of the residual stress curve on the upper and lower surface of the back-reflection laser welding cross section is higher,the distribution of the residual stress on the upper and lower surfaces is very similar.In addition,different line energy have a little effect on longitudinal residual stress.(3)Study on the characteristics of titanium alloy sheet by laser welding.The weld formability is qualitatively and quantitatively represented by the back width ratio,the height and the deformation of the weld,which reveal the influence of laser processing parameters on the formability of back-reflection laser welding with low threshold.The experiment result indicated the effect of laser processing parameters on the back width ratio is V>D>P,the laser processing parameters under the optimal back width ratio of the weld are 1400 W power,scanning speed of 800mm/min,defocusing amount of 1mm;The influence of laser processing parameters on the stack height is P>V>D,the laser processing parameters of the minimum welding height data of the welding seam are 1400 W power,scanning speed of 900mm/min,defocusing amount of 0mm;Influence of laser processing parameters on weld deformation is P>D>V,the laser processing parameters under the minimum deformation data of the welding seam are 1400 W power,scanning speed of 1000mm/min,defocusing amount of-1mm.(4)Study on microstructure and properties of titanium alloy sheet by laser welding.The laser welding joint of titanium alloy sheet was optimized by the process parameters,the macro morphology of the cross section,the microstructure of weld zone,heat affected zone,the distribution of hardness along the cross section of the weld,the tensile strength and the fracture morphology are studied systematically,which is compared with the titanium alloy sheet without reflecting plate.The experiment result indicated the weld obtained by the back reflection laser welding of titanium alloy sheet presents a more standard "X" shape",and the front and back seams are relatively smooth.Compared with conventional laser welding,thegrain size of weld center region of back reflection laser welding is slightly larger,the heat affected zone,the overheated zone and the partial phase transition zone are small,the normalized area is large;The hardness distribution of the two welding methods showed similar distribution law,but the hardness of the weld area is lower than that of conventional laser welding,heat affected zone is slightly higher than conventional laser welding;the fracture positions of the tensile specimens are both the vicinity of the weld matrix under the two welding methods,which are plastic fracture.