激光功率对双激光束双侧同步焊接TC4钛合金T型接头断裂性能的影响机理

Influence of Laser Power on Fracture Properties of TC4 Titanium Alloy T-Joint Manufactured Using Dual-Laser-Beam Bilateral Synchronous Welding

TC4钛合金具有良好的综合力学性能,其激光焊接结构现已广泛应用于航空航天等领域。通过开展TC4钛合金双激光束双侧同步焊接试验,对不同激光功率下钛合金T型接头进行微观组织观察、Z向拉伸试验以及断口观察。结果表明,激光功率的提高可显著增大接头的熔深、熔宽,激光功率为2.1 kW时,接头热影响区多为细小的等轴晶粒,当激光功率提高至2.3 kW时,下熔合线下部出现大面积粗晶区,部分晶粒尺寸超过了100μm。此外,热输入的提高导致了接头焊缝区强度的降低,采用激光功率为2.3 kW的拉伸试样,其断裂面贯穿焊缝区域,断裂强度远低于采用较低功率的1号试样。断面微观形貌显示,激光功率较高时,试样断裂模式为韧性断裂与解离断裂组成的混合断裂。

Objective In recent years,the aerospace manufacturing industry has proposed the requirements for large-scale and lightweight aircraft,and weight reduction has become the first priority of the aircraft manufacturing industry.Duallaser-beam bilateral synchronous welding(DLBSW)is an innovative manufacturing process for producing the skinstringer T-shaped structure,which was first proposed by Airbus.Currently,the study of DLBSW has mainly focused on the skin-stringer T-joints of aluminum alloy,with only a few references to the Ti6Al4V alloy T-joints and their properties.This T-joint forms a molten pool under the action of bilateral laser beams during the process of DLBSW,and the thermal effect is superimposed at the bottom of the molten pool.Because the Ti6Al4V alloy is sensitive to thermal action,the equiaxial grain in the heat-affected zone(HAZ)is easily coarsened during welding,which weakens the mechanical properties and service performance of the T-joint.Therefore,a rational selection of heat input is necessary for the laser welding of the titanium alloy.This study aims to reveal the influence of laser power on the microstructure of the Ti6Al4V alloy T-joint and explore the influence of microstructure on the fracture performance of the joint.Methods In this study,the DLBSW experiment of the Ti6Al4V alloy T-joints was performed using two KUKA robots and a TruDisk 12003laser produced by TRUMPF.The laser and the laser welding head are connected by two optical fibers.During the welding process,the laser beam and shielding gas were delivered symmetrically along the center line of the stringer(Fig.1).Following the welding experiments,two groups of well-formed T-joints were selected for metallographic and tensile specimen preparation.The specimens were cut along the cross-section of T-joints,and the uneven area is avoided.The microstructure of the joint after polishing and corrosion was observed under an MR-5000 metallographic microscope.The tensile experiment was performed using an electromechanical universal testing machine at 25℃.The loading rate was set to 0.8 mm/min,and the fracture morphology was observed using a scanning electron microscope.Results and Discussions When the laser power is 2.1kW,the HAZ above the upper fusion zone of the joint can be divided into the obvious coarse grain region(CGR),fine grain region(FGR),and transition region(TR)owing to the thermal effect of the molten metal on both sides of specimen.There is a large area of TR below the lower fusion zone without the obvious CGR and FGR(Fig.6).The fracture surface of the joint is near the HAZ,and the tensile strength exceeds 800MPa(Fig.8).When the laser power is 2.3kW,theα′phase that precipitated inside the columnar crystal in the weld bead zone coarsenes significantly because of the increase in heat input.Additionally,different from the joint adopting laser power of 2.1kW,the HAZ below the lower fusion zone of joint No.2can be divided into the CGR,FGR,and TR owing to the increase in laser power.A wide CGR appears below the lower fusion zone,where the grains have severely coarsened and some of the equiaxed grains have increased to 100μm in size(Fig.7).The joint is invalid in the weld bead zone,and the tensile strength is only 478MPa(Fig.8).The fractographs of the joint are mixed,except for the equiaxed dimples,and the exfoliated lamellarα′phase interface can be found on the fracture surface.It can be concluded that the fracture mode of specimen No.3is a hybrid fracture,comprising both the cleavage and ductile fractures(Fig.11).Conclusions Experimental results indicate that the Ti6Al4V alloy T-joint manufactured by DLBSW is more sensitive to changes in the laser power.When the laser power increases,a large area of CGR is observed below the lower fusion zone,with some grain size reaching 100μm.However,the region below the lower fusion zone of the joint adopting the laser power of 2.1kW is a wider TR,while those of the CGR and FGR are not obvious.The joint under the laser power of 2.1kW failed in the HAZ on the skin when a higher tensile strength exceeded 90%of the strength of the base material.When the laser power is increased to 2.3kW,the joint is fractured in the weld bead zone,and its strength is significantly reduced compared with the specimen adopting a laser power of 2.1kW.According to the fractographs of the joint,the fracture mode of the joint under lower laser power is a microporous aggregation fracture,which is classified as a ductile fracture.When the laser power is increased to 2.3kW,the fracture mode of the joint is a combination of the ductile and cleavage fractures,with a mixed morphology comprising equiaxed dimples and exfoliated lamellarα′phase interface.