Study On The Influences Of DCT On The Stress And Mechanical Properties Of TC4Joint Welded By Electron Beam

Many advantages of titanium alloy meet the requests of aerospace importantbearing structural parts. Electron beam welding(EBW) is one of the best methodsemployed in the welding of titanium alloy because of high energy density, smallheat-affected zone, and low pollution etc.Deep cryogenic treatment(DCT), as a process to change the microstructure andimprove the properties of materials, has been widely used in tool and die steel, highspeed steel, cemented carbide etc. However, the research in non-ferrous metals,especially about Titanium alloy is less. It is a new method to adopt DCT in the field ofTitanium alloys. Therefore, the research of the effect of DCT on Titanium alloys is ofsignificance to both theory and practice aspects.In this paper, TC4titanium alloy is chosen as the research object. First theElectron beam welding process parameters of9mm TC4titanium alloy was found bythe welding experiments. And then the finite element software ABAQUS is adopt tosimulate the process of welding, and got the correct finite element model throughverifying the simulating results and the experimental results, which both of themhave a high degree of consistency. And then high residual stress was found, especiallythree tensile stress inside the TC4flat. Therefore Deep Cryogenic Treatment wasproposed to reduce the residual stress and improve the microstructure and mechanicalproperties of TC4titanium alloy EBW joint. Blind hole method was used to residualstress testing, mechanical properties of the TC4joints which experienced DCT weregotten by experiments. Methods like metallography, SEM and XRD were used tostudied the mechanism of DCT on the joints. The results are as follows:Direct put the TC4joints into the liquid nitrogen for0h,2h,15h,24h and48h,and the residual stress in the joints is reduced with increasing the time for cryogenictreatment. What’s more, when the treating time gets24hours, the surface residualstress in the weld line is significantly reduced. So it can be concluded that the optimalcryogenic treatment process is directly keeping the samples in the liquid nitrogentemperature for24hours or more.After DCT, the metallographic and SEM images shows there are a large numberof second phase grains near the fusion line which arranged along a certain direction. Small and fine grains also precipitate in the weld line. Phase transition occurs insidethe weld grains, making bulk of α phase increasing. XRD result also shows new phaseprecipitation occurring, and with DCT time increasing, the relative intensity of themain diffraction planes have changed. Which can learn, the mechanism of cryogenicon reducing residual stress of the TC4EWB joint is the second phase precipitates, βphase shift, the grain to rotate, so those improve the organization of the joint.Hardness test results show that it has little effect of cryogenic on TC4EWBjoints. But the hardness will slightly decrease with the DCT time. This is mainlybecause the hardness of the organization after DCT is almost equal to the original’s,while the organization will be more stable, so there is a slight decrease. Tensile testresults show: the tensile strength, yield strength have no significance change, but theelongation and percentage reduction of area are varied with the DCT time. Both ofthem will drop when the DCT time is about2hours, and then go up rapidly when theDCT time gets15hours. Thus the plastic of the joints will improve largely when theDCT time is more than24hours. The impact test results show: with the DCT timeincreases, the impact toughness decreases firstly, and then increase. Only when theDCT time gets24hours, can the impact toughness restore to the level of jointswithout DCT.