Investigation On The Microstructure Of TiAl Intermetallic Compound Electron Beam Welded Joints And The Crack Control Technology

Electron beam welding of TiAl/TiAl and TiAl/TC4 with thickness 2.5mm have been studied in this paper. The electron beam weldability of TiAl intermetallic compound is poor, and cold cracks easily occurred in the joints after welding. At present, there are few reports about crack formation mechanism and methods of avoiding cracks. Therefore, based on the analysis of the joints structure transition and heat stress distribution during weld thermal cycle, a compound control welding method of TiAl electron beam welding has been developed. Finally, macro crack-free joints of TiAl/TiAl and TiAl/TC4 have been obtained by electron beam welding.There are mainly two reasons why cold cracks easily occured during electron beam welding of TiAl. One is structural embrittlement of the welded joints, the other is heat stress. First of all, influence of joints structure on cracks formation was analyzed.α(α2) phase dissociation was restrained during rapid cooling, and the transition product mainly composed ofα2 phase in this experiment. This kind of phase structure obviously decreased the plastic property and malleability of joints. The joints tensile strength was lower, and fracture occurred in the weld and HAZ, which was transcrystalline fracture. The temperature of the first crack of TiAl electron beam welding was calculated, which defined the lower limit of the high temperature range that was perfect at the structure transition. It can be used to evaluate the cold crack sensibility; moreover, it is a basis of the compound control welding method theory.Cold cracks just occured near TiAl side after electron beam welding of TiAl/TC4. Because of the differences of chemical composition and thermal physical properties between TiAl and TC4, Al quantity was lower in the weld, which led to the joints tensile strength was higher than TiAl/TiAl. According to these characteristics, non-centered electron beam welding was applied to weld TiAl/TC4. If the lateral deviation quantity hs was in a specific range, the joints tensile strength was increasing. The melting quantity ratio of two kinds of alloys was calculated. The reason of the difference about electron beam weldability between Ti3Al and TiAl was analyzed. Heat stress, which generated due to rapid heating and cooling during electron beam welding, was the mechanical factor of cracks formation. There was residual tensile stress along weld in electron beam welding of TiAl/TiAl. In the middle of the weld the residual stress was higher, so cold cracks were normal to weld. In electron beam welding of TiAl/TC4, all of the three dimensional stress were residual tensile stress along welding direction, and the value was larger. Moreover, due to the material difference, there was higher residual tensile stress in the thickness direction. When the tensile stress reached to 590MPa in the crack location, the joint was still in the high temperature plastic stage. So part of the stress could be released due to plastic deformation. Therefore, the cold cracks of the joints didn't occur in the maximum stress condition. The residual stress was 153MPa in the room temperature, which added to the external applied load, so that the joint fractured at lower stress.According to above results, a compound control welding method was developed, which could obtain crack-free joints of TiAl intermetallic compound by eletron beam welding. The procedure as follows: application of heat insulation plate, weld preheating step-by-step and heat treatment associated with welding, which were used to control weld thermal crycle and increase the high temperature range for structure improvment; change the mixed way of the pieces to decrease constrain, and release the heat stress. Finally, macro crack-free joints of TiAl/TiAl and TiAl/TC4 with thickness 2.5mm have been obtained by electron beam welding. The joints obtained by compound control welding method were different from joints by general method such as phase content, grains size, and structure styles and so on. X-ray diffraction was applied to measure the residual stress distribution by compound control welding method, which approved the practicability of this method through quantitative analysis.Besides the crack-free flat plate butted joints by compound control welding method, tubes of Ti-43Al-9V-0.3Y, Ti-45Al-5Nb-0.3Y and so on with different thickness were jointed by this method. It was indicated that this method had signification of industry application.