Prediction Of Solidification Crack In Laser Oscillating Lap Welding Of Aluminum Alloy Sheets

The car body which made by aluminum alloy(Al)can effectively reduce vehicle weight and energy consumption.As a new welding technology,laser oscillating welding of aluminum alloy has been more and more applied in the automobile manufacturing industry.However,the solidification cracks were found in welding seam during the production process,which greatly reduces the safety and reliability of the welding structure.The coupled thermo-mechanical model of laser oscillating welding for thin aluminum alloy plate in lap joints is established to compare the distribution characteristics of temperature field and strain field when solidification crack appears or not,and establish the criterion of solidification crack corresponding to the process conditions which is of great significance to realize the optimization of laser oscillating welding process for aluminum alloy plate in lap joints.Based on the finite element analysis software ANSYS,this paper fully considered the physical process of the laser oscillating welding,an exponentially increasing gaussian columnar heat source was adopted,the weld morphology under different process parameters was preset,and the loading position and load step of the heat source load were set according to the power modulation and other related process parameters.Meanwhile,a series of problems such as thermal properties and mechanical properties of the material at high temperature,stress relaxation state,phase change latent heat,solidification shrinkage and so on were considered to establish the coupled thermo-mechanical model of laser oscillating welding for thin aluminum alloy plate in lap joints.The fusion lines in the numerical calculation results under different process parameters and the actual test results were compared to verify the accuracy of the numerical model,and reveal the distribution rules of the temperature field and strain field of laser oscillating welding under power modulation.The numerical results of temperature field and strain field were displayed.In the temperature field,the shape of molten pool changes when the laser beam moves.As the heat source moves,the heat gradually accumulates at the free end of the welding piece for the position of the weld is closer to the free end of the lower sheet,so the temperature field distribution shifts to the lower sheet.The temperature of the point from lower sheet is higher than the temperature of the Isometric point from the upper sheet.The strain field calculation result shows that the maximum transverse mechanical strain in the weld appears in the position where the center line of the weld is inclined to the lower sheet side which means the sensitivity of solidification crack is the greatest,while the transverse mechanical strain on both sides of the weld is lower.The numerical simulation results of laser oscillating welding of aluminum alloy lap joints under different technological parameters were compared and analyzed.The temperature field is similar under different laser offset.With the increase of the laser offset,the maximum transverse mechanical strain increases,the sensitivity of solidification crack increases.Under different laser oscillating frequencies,as the frequency increases,the volume of weld pool increases,and the sensitivity of solidification crack increases,the maximum mechanical strain value at 250Hz is the highest.The numerical simulation results under different laser deflection angle shows that as the laser deflection angle decreases,the range of laser oscillating increase,so the volume of weld pool increases,the peak temperature on the transverse line increases and the maximum mechanical strain increases,so the sensitivity of solidification crack increases.The experimental groups with and without solidification cracks were calculated and compared.The results of numerical calculation show that the temperature distribution in the whole weldment of crack computing group is higher than that of no crack,meanwhile,the volume of molten pool is bigger,the molten metal is more,so the strain accumulation is more which leads the transverse mechanical strain distribution of the weldment is higher than that of no solidification cracks.The maximum transverse mechanical strain of the three transverse lines at the same position behind the weld pool on all groups was extracted,and the average values were calculated for comparison.The results showed that when the average value of the maximum transverse mechanical strain exceeded 1.69%,the longitudinal solidification crack would appear where the center line on the weld inclined to the side of the lower sheet.