Research On The Factors Of Influencing TIG Welded Joints’ Mechanical Property For TC4Titanium Alloy

Titanium alloys due to its high strength, low density, corrosion resistance, hightemperature oxidation, high-temperature creep resistance and other properties, It becomethe most attractive materials in the field of aerospace aviation industry, electric powerindustry, navigation, marine engineering, chemical industry, automobile manufacturing,and other field. Titanium alloy usually have poor weld ability, it will produce Blowholes,cracks, embrittlement, coarse grains and other defects in welding process, and lead to thedecline of comprehensive mechanical properties. Therefore, it is necessary to carry outresearch on these issues, to explore factors of affecting TC4welding mechanical propertiesfor its wider application in industry.By changing the TIG welding process parameters: welding current, welding speed, TC4titanium alloy of3mm thick was welded under the different heat input conditions. Bymeasuring the hardness, tensile, bending performance and comprehensive morphology oftensile fracture characteristics. Author has analysised the welded joint mechanicalproperties and fracture. By observing the metallurgical test, it gave the microstructure ofwelded joints and regularity of organizational transformation. Further more, bydetermining the grain size and martensite needle size at joint fracture，regularity of themechanical properties can be studied.And then we draw the following conclusions:(1)Analyzing the distribution curve of TIG welding joint’s micro hardness can be seen,the hardness values of the center at the weld site was significantly higher than theheat-affected zone, slightly higher than the base metal. And there is a softening in the heataffected zone of coarse grain zone parts, about4~5mm from the center of the weld.Indicating that the weld will have more higher strength and deformation resistance,softening area may be relatively weak area of the welded joint strength.(2)It is derived by analyzing the fracture and deformation of TC4titanium alloy weldedjoints, most of the sample’s cleavage site at the boundary of the weld and tend to the heataffected zone, plasticity and toughness of the weld’s heat affected zone significantly lowerthan the base metal. But Its excellent tensile properties of the welded joints: when the TIGwelding head for welding heat input is637.63J/mm, the average tensile strength reached97.9%to the tensile strength of the base material. The welding joint has completed performance in this condition.(3) Analysis of the fracture morphology of the sample found that the specimen under4set of process parameters have common characteristics, fracture is flush, cross-sectioncontraction is small near the fracture, and can be observed that there are different shades ofdimples at the torn edges. So TC4titanium alloy TIG welded joints‘s fracture is mainlyobjective brittle fracture.(4) Analysis the X-ray diffraction of4simples weld area material shows that the weldregional organizations is almost basic α′martensite phase, and has not found β phase orother new phases. The Welded joint’s phase structure under different process in theparameters composed is basically no difference.(5) By EDS analysis shows that1#3#4#sample elements main components Al, Ti, V,2#sample in addition to the three main elements, also appeared the C element, and thecontent is up to6.78%, may be the weld gap in the air not removed. The C-containinginclusions dissociation will be obstacles for crack continued to expand, leading todissociation step formation, thus changing the form of the fracture, thereby affecting themechanical properties of the material.(6) After experiencing TIG welding thermal cycle, the base material equiaxed grainstructure disappeared, and coarse grain in heat affected zone is serious rough. Grain sizehaving a mutation trends between the coarse grain zone and the transition zone. By thedetermination of the grain size in the fracture, it is found that with decreasing grain size of500μm or less, the grains in the tensile strength of the titanium alloy increases, about grainsize in the sample more than500μm, the tensile strength values increases with the increaseof the grain. Conclusions that the impact of the fracture is within a certain grain size range.For the fracture exceeds this range, the material morphology (widmanstaten, basketorganizations, equiaxed, bimodal organization) has played a important role.(7) Welded morphology of TC4titanium alloy is basically needle-like or woven basketmesh α’ martensite, and needle-like distribution a certain misorientation. By determinationof the fracture α′needle size,, it is found that the length of martensite needles for the TC4tensile strength is not obvious. The tensile strength values of the sample, more than900MPa martensitic needle size the same within a large range of (27~134μm) changes. The size of the grain size and the needle does not exist the change of an increment ordecrement.