The Processes And Mechanism For The Enhancement Of Laser-welded Joint Strength Of Dissimilar Ti6Al4V-304 Stainless Steel

The laser welding method of the Ti6Al4V alloy to 304 stainless steel by using a1 mm thickness of pure Cu interlayer was developed by changing the laser power,interlayer thickness,composite interlayer,and laser offsetting settings.The fracture characteristics of the joints were analyzed by scanning electron microscopy?SEM?,energy-dispersive spectroscopy?EDS?,X-ray diffraction?XRD?and Electron backscatter diffraction?EBSD?.The fracture examinations indicated that both the joint strength and the fracture occurrence location in the joints depend on the laser power settings.The optimal value for the maximized tensile strength of the joints,up to 300 MPa,was obtained at a power of 4 kW.Increasing the laser power by 4 kW increased the tensile strength;it decreased as the laser power exceeded 4 kW.The increased distance between the fracture surface and the Ti alloy,as well as the higher remaining Cu deposited on the Ti sheet,in the 4 kW joint compared with those welded at 3 kW or 6 kW power illustrated that the Cu atoms can diffuse more inside the Ti surface and that thus the bond strength between the Ti alloy and Cu interlayer was higher than that of the other samples.The fractography investigations illustrated that the fracture surface of a 4 kW joint had a greater rough area than that of either the 3kW or the 6 kW joints.XRD results demonstrated that most of the solid solution phases,such as?Ti,?V?,and?Ni?were formed on the fracture surface of a 4 kW weldment.A 4 kW sample with a maximum thickness of the intermetallic compounds?IMCs?layer that contained discontinuous brittle compounds and also some soft Ti-Cu IMCs resulted in mechanical strength that was higher than other samples.EBSD observations indicated that a straight relation existed between the IMCs layer thickness and the joint strength.EBSD observations also showed that in the 4 kW sample,Cu₃Ti₂ GBs,which had the most connectivity of the high angle grain boundaries,fractured as the ductile behavior.The influence of different Cu interlayer thicknesses on the strength and microstructural characterization of SS-Ti joints was investigated with single and double interlayers.In the single interlayer experiments using 0.5 mm,1 mm,and 1.5mm thick Cu,the greatest strength was reached at a laser power of 4 kW using 1 mm thick Cu.As the Cu thickness increased from 0.5 mm towards 1 mm,the maximum joint strength also increased.However,as Cu thickness increased to more than 1 mm,maximum strength started to decrease.XRD characteristics showed that FeTi was formed along the fracture propagation path and also on the fracture surface of 0.5 mm thick Cu;however,no Fe-Ti IMCs or Cr₂Ti could be found in the 1 mm thick Cu sample.Meanwhile,?Ti+FeTi and CuTi+CuTi₂ compounds fractured after tensile testing in 0.5 mm and 1 mm thick Cu joints,respectively.The EBSD observation showed that the fracture happened intergranularly through grain boundaries of CuTi and Cu₃Ti₂ compounds at 0.5 mm thick Cu joint;however,that happened roughly along the grain boundaries of Cu₃Ti₂ for the 1 mm thick Cu joint.For the double interlayer experiments,XRD result of SS/Cu0.5-V/Ti sample showed the formation of Fe₂Ti and also FeTi at the fracture surface,while the fracture surface of the SS/Cu1-V/Ti sample primarily contained some Cu-Ti IMCs.The microstructures of the fracture edges after tensile testing indicated that eutectic FeTi+Fe₂Ti and eutectoid?Ti+Ti₂Cu fractured after tensile testing in the SS/Cu0.5-V/Ti and SS/Cu1-V/Ti samples,respectively.EBSD observation indicated that the fracture happened primarily along the low angle grain boundaries of Cu₃Ti₂ and?Ti compounds at the SS/Cu1-V/Ti joint;however,the fracture occurred intergranularly along the high angle grain boundaries of Fe₂Ti in the SS/Cu0.5-V/Ti joint.Hence,the joint strength of the SS/Cu1-V/Ti was about 179%greater,and the elongation was roughly 10 times greater than that of the SS/Cu0.5-V/Ti joint.XRD characterization based on a 0.5 mm thick Cu interlayer showed that the SS/Cu/Ti joint contained considerable brittle IMCs,such as Fe₂Ti and Cr₂Ti;however,the fracture surface of the composite interlayer sample did not contain any Fe-Ti IMCs or Cr₂Ti.EBSD observation indicated that the fracture propagated transgranularly from CuTi solidified in interdendritic regions of the FeTi compounds in the composite interlayer sample;however,the fracture propagated intergranularly along Fe₂Ti grain boundaries in the single interlayer sample.Hence,the tensile strength and elongation of the SS/Cu-V/Ti joint were 32%and 61%more than that of the SS/Cu/Ti joint,respectively.The investigation based on a 1 mm thick Cu interlayer showed that the SS/Ni-Cu1/Ti sample was stronger than that of the SS/Cu1/Ti sample,due to a lack of CuTi acting as a brittle IMC,at 3.5 kW.According to the XRD results,the tensile strength of the SS/Cu1-V/Ti joint was more than that of the SS/Ni-Cu1/Ti joint,due to the formation of ductile NiTi instead of brittle NiTi₂.The microstructures of the fracture edges after tensile testing illustrated that the eutectic CuTi+CuTi₂ at SS/Cu1/Ti sample fractured after tensile testing,while CuTi₂ and eutectoid?Ti+CuTi₂compounds fractured during the tensile experiment on the SS/Ni-Cu1/Ti and SS/Cu1-V/Ti joints,respectively.The existence of aforementioned phases as the predominant compounds created at the fracture edges was consistent with that predicted using the CALPHAD model for the same compositions.Cracking happened along the low-angle grain boundaries between Cu₃Ti₂ and?Ti,in the SS/Cu1-V/Ti joint were the soft compounds compared to the low-strength joints that cracked along the grain boundaries of more brittle compounds.Hence,the tensile strength of the SS/Cu1-V/Ti joint at a laser power of 3.5 kW was 49%more than that of the SS/Cu1/Ti joint and its elongation was 317%more than that of the SS/Cu1/Ti joint.In the single interlayer SS/Cu/Ti joints,the tensile strength and elongation of the sample with laser focusing at the SS-Cu interface are 131%and 181%more than that of the sample with the laser focusing on the Cu interlayer center,respectively.The fracture surface of the SS/Cu/Ti joint with laser focusing on Cu center contains some brittle IMCs such as Fe₂Ti and Cr₂Ti;however,the fracture surface of SS/Cu/Ti joint with the laser focused at the SS-Cu interface contains some solid solution compounds,some Cu-Ti IMCs,and also some weak peaks of FeTi.FeTi+Fe₂Ti+?Cu?fractured after tensile testing at the sample with the laser focusing on Cu center;however,?Ti+FeTi fractured after tensile testing at the sample with laser focusing at the SS-Cu interface.The fracture propagation path of the sample with the laser focusing on the Cu center roughly passes through the grain boundaries of Fe₂Ti;however,the cracking occurs intergranularly through the grain boundaries of CuTi and Cu₃Ti₂ in the sample with laser focusing at SS-Cu interface.In the double interlayer SS/Cu-V/Ti joints,with changing the laser position from the Cu-V interface toward the Cu center,the joint strength increased from 123 MPa to 166.34 MPa when the welding performed at the 3.5 kW.The XRD result of the sample with the laser focusing at the Cu-V interface showed the formation of Fe₂Ti and FeTi at the fracture surface.However,the fracture surface of the joint with the laser focusing on Cu center shows no Fe₂Ti or Cr₂Ti in the XRD result.The microstructure characterizations of the fracture edges illustrate that the eutectic FeTi+Fe₂Ti,fractured in the sample with the laser focusing at the Cu-V interface;the FeTi+CuTi₂ fractured in the sample with focusing on Cu center.EBSD observation indicated that the fracture propagated transgranularly from CuTi compound solidified at the interdendritic regions of FeTi compounds in the sample with the laser focusing on the Cu center;however,the fracture happened intergranularly along the Fe₂Ti compound in the sample with the laser focusing at Cu-V interface.