基于激光毛化的不锈钢与GFRP单面单点电阻焊接工艺及性能研究

Single Side Resistance Spot Welding Process and Performance of Stainless Steel and Glass Fiber Reinforced Plastics Based on Laser Texturing

不锈钢与玻璃纤维增强树脂基复合材料(GFRP)的异质材料复合结构在汽车轻量化领域有着广阔的应用前景。采用激光毛化技术在不锈钢表面制备网格微织构,开展了微织构的引入对不锈钢与GFRP单面单点电阻连接界面的强化机理研究。研究结果表明:表面微织构的引入显著改善了不锈钢表面的润湿性,熔化的GFRP在不锈钢表面由不润湿性转变为润湿性;随着微织构间距的增大,接触角先减小后增大,当微织构间距为0.2 mm时,接触角达到最小值62.4°,此时表面微织构的引入对界面机械嵌合的促进效果最佳;此外,微织构的引入促进了界面化学扩散,使得接头拉剪力达到最大值3548 N,界面失效形式由界面断裂转变为界面断裂与内聚断裂混合的失效形式。

Objective Hybrid material structures have various applications in the automotive industry owing to their light weight.Stainless steel,which exhibits good corrosion resistance and remarkable mechanical properties,is widely used in automotive applications.Glass fiber reinforced plastics(GFRPs)that exhibit high specific strength and cost performance have replaced existing materials in applications requiring lightweight materials.Single-side resistance spot welding of stainless steel and GFRP can help combine the advantages of the two materials.However,owing to the difference in the thermal physical properties and chemical structures of these two materials,the combined strength cannot meet industrial requirements.Improving mechanical interlocking and chemical bonding is an effective approach for enhancing the joint performance.The laser joining process can be used to fabricate micro-textures and change the surface chemical state.Thus,micro-textures on the surface of stainless steel are prepared using a nanosecond laser,and the strengthening mechanism of the interface under the influence of the micro-textures is studied.Methods Initially,304 stainless steel and GFRP are selected as base materials.The 304 stainless-steel sheets are subjected to laser texturing.The cruciform mesh micro-texture is selected as the basic morphology of the stainless-steel surface.The grid line uses contained multiple equally spaced scan lines,and a laser processing system supporting software is used to preset different micro-texture widths.The number of laser scanning times is set as 10,and the micro-texture width is set as 0.1-0.5 mm.An optical digital microscope and a field-emission scanning electron microscope are used to detect the laser texture,surface morphology,and fracture surface of the joint.A constant-temperature heating platform and a high-temperature wetting angle measurement system are used to measure the GFRP contact angle on the stainless-steel surface to characterize its wettability.A universal material testing machine is used to conduct tensile-shear tests on the 304 stainless steel/GFRP single-side resistance spot welding joints.Results and Discussions The introduction of micro-textures on the surface of stainless steel significantly improves the wettability of the surface.The surface of stainless steel changes from an untreated non-wetting state to a wet state after laser treatment.As the width of the micro-texture increases,the wettability initially increases and then decreases(Fig.5).When the micro-texture width is 0.2 mm,the wettability reaches the optimum value.The interior of the micro-textures is completely filled with molten GFRP.When the micro-texture width is too large,the molten GFRP cannot completely fill the interior of the micro-textures(Fig.7).C and Fe diffuse at the interface,and an element diffusion layer is formed(Figs.8 and 9).When the micro-texture width is 0.2 mm,the tensile-shear force reaches the maximum value of 3548 N,which is 385%higher than that of the untreated stainless steel/GFRP single-side resistance spot welding joint.The tensile-shear force first increases and then decreases as the micro-texture width increases.Compared with the case of the joint without micro-textures,after laser treatment of the stainless-steel surface,a large amount of the resin-glass fiber mixture is observed in the center area of the fracture of the joint(Fig.11).The fracture mode changes from an interfacial fracture to a mixed form of interfacial and cohesive fractures.Corresponding to the wettability and joint tensile-shear force,the bonded-area ratio first increases and then decreases,indicating an improvement in mechanical properties.Conclusions Laser texturing is used to improve the performance of stainless steel/GFRP single-sided resistance spot welding joints.After the nanosecond laser treatment,the wettability of the molten GFRP on the stainless-steel surface is significantly improved,and the state changes from non-wetting to wetting.The introduction of the micro-textures improves the mechanical properties of the stainless steel/GFRP resistance spot welding joint.When the micro-texture width is 0.2 mm,the tensile-shear force of the stainless steel/GFRP single-side resistance spot welding joint reaches the maximum value of 3548 N.Compared to the case wherein the micro-textures are not introduced,the tensile-shear force of the textured joint is 731 N.The introduction of the micro-textures increases the contact area between the stainless steel and GFRP,thereby significantly enhancing mechanical interlocking.When the micro-texture width is suitable,the GFRP completely fills the inside of the micro-textures.When the micro-texture width is too small or too large,the GFRP does not completely fill the inside of the micro-textures owing to the influence of wettability.In addition to mechanical interlocking,Fe and C chemically diffuse at the interface to form a compound layer,which further improves joint strength.