Research On Mechanical Behavior Of Fe-Mn-Si Memory Alloy Welding Seam Formed In Laser Welding With Filler Powder

With the development tendency of vehicles to lightweight,green and high reliability,the new technology of laser powder filling welding is widely applied to connect various bodies and parts of vehicles.In this dissertation,the frequently-used 304 stainless steel of vehicles is choosen as the substrate,and the Fe-Mn-Si shape memory alloy(SMA)welding seam is prepared in the laser powder filling welding process.The mechanical behaviour of Fe-Mn-Si SMA welding seam is studied,and its improvement mechanism of mechanical properties is revealed.The microstructure and mechanical properties of welding seam are systematically analysed,and the temperature field and stress field of laser powder filling welding are numerically simulated.In the single factor experiments of laser powder filling welding,it can be seen that the width of welding seam and burn out rate of Mn/Si oxyphile elements are rising with the increment of linear energy(divide laser power by welding speed).Under the conditions of excessive laser linear energy,the coarse welding grain will reduce the microhardness and tensile strength of welded joints.Meanwhile the excessive welding speed will weaken the metallurgical bond of substrate and powder,which also lowers the tensile strength of specimens.Based on the orthogonal test,the optimum technique parameters are obtained as follows:laser power of 2700 W,welding speed of 160 mm/min,and defocusing amount of+5 mm.Under the optimum process parameters,the welding seam possesses well macroscopic morphology what transits smoothly and has no obvious welded reinforcement.From the edge to the center of the welding seam,the microstructure is planar crystal,cellular crystal,dendritic crystal and isometric crystal successively.With the restrained mixing uniform design method and DPS data processing system,the composition design of Fe-Mn-Si SMA welding seam is obtained.The empirical expressions between proportion of Fe/Mn/Si/Cr/Ni mixed powder and mass fraction of Fe-Mn-Si-Cr-Ni welding seam are shown as follows:yMn=0.00958+0.273xMn,ySi=0.013+0.191xsi,yCr=0.163-0.1 77xs,yNi=0.0881-0.0666xMn,yFe=1-yMn-ySi-yCr-yNi.Under the above expressions,the Fe-Mn-Si SMA welding seam can be prepared in laser welding with filler powder.To verify the accuracy of equations,the Fel5Mn5Sil2Cr6Ni SMA welding seam is formed,and it possesses obvious properties of shape memory alloy with 6%pre-strain treatment.Firstly,the micro-structure of welding seam is ?-austenite after solid solution treatment,and the s-martensite appeares.Secondly,the shape memory effect and recovery strain of Fe-Mn-Si SMA welding seam is 39.5%and 2.37%,respectively.By mechanical property measurements of the Fe-Mn-Si SMA welded joint,the residual stress value is maximal in the middle zone of welding seam along the direction of welding seam.Perpendicular to the direction of welding seam,the peak value of residual stress locates in the narrow zone of welding seam.When the measuring points goes far away from welding zone,the residual stress sharply decreases and gradually closes to zero.With the cyclic bending test,the final fatigue rupture zone of Fe-Mn-Si SMA welding seam joint is consist of numbers of dimple,and its cycles are 249(90.9%of the substrate).Those phenomena indicate the Fe-Mn-Si SMA welding seam joint possesses excellent bending fatigue characteristic.Compared with the 304 stainless steel welding seam joint,the Fe-Mn-Si welding seam can reduce the maximal residual stress by 29.1%and enhance the bending fatigue strength by 83.1%.The simulation results indicate that the temperature only rises large in the welding seam,and temperature variation of the substrate is small because of the concentrated laser source energy.In the simulated stress filed,the residual stress mainly locates in the welding seam and matches the distribution of experimental results.That is,the residual stress of simulation points is highest in the middle zone of welding seam,and its value reduces with the growth of the distance between the point and welding seam.Based on the macro-strain expression of stress-induced ??? martensite phase transformation,in the laser powder filling welding,the ??? martensite phase transformation and phase transformation strain ??? are induced by the welding residual stress ?p inside the Fe-Mn-Si SMA welding seam.The ?-martensite is firstly formed along the maximum work direction of stress-induced ??? transformation,and the residual stress is released in the phase transformation.With the thermoplastic model and residual stress expression of welded joint,the residual thermal contraction is resisted by the phase transformation strain ????,and the residual stress is relaxed.During the cyclic bending process of Fe-Mn-Si SMA welding seam,the variation of external stress accommodates by the cyclic stress-induced martensitic and reverse transformation is the primary cause to enhance the fatigue strength.