Machining and welding process notes for TC4 titanium alloy parts

Common welding methods for titanium and titanium alloys are as follows: Argon arc welding, submerged arc welding, vacuum electron beam welding, etc.

Tungsten argon arc welding is used for thicknesses of 3 mm or less, and argon arc welding is done for 3 mm or more. The purity of argon is over 99.99%, and the content of air and water vapor in argon is strictly controlled.

Currently, for TC4 titanium alloys, argon arc welding, plasma arc welding are often used for welding, but both methods need to be filled with welding material.

Limitations on shield gas, purity and effectiveness increase the oxygen content of the joint, reduce its strength and increase its post-weld deformation.

Electron beam welding and laser beam welding are used. The following is a description of precision welding and precautions for TC4 titanium alloy.

1. Weld hole problem

Weld pores are the most common defect in welded titanium alloys, and the main causes of pores are hydrogen and oxygen present in the arc region of the metal to be welded. In electron beam welding of TC4 titanium alloy, there are almost no vent defects in the weld. To this end, the study focuses on process factors for the formation of pores in laser welds.
Test results show that weld pores are closely related to the energy of the weld line during laser welding. If the weld line has moderate energy, there are very few or even no pores in the weld. If the line energy is too high or too low, it will cause serious pore defects in the weld. In addition, the presence of poor porosity in the weld is also related to the wall thickness of the weld. Comparing the test results of the samples, we can see that as the wall thickness of the weld increases, the probability of pores in the weld increases.

2, internal quality of welded part

Sample of titanium plate butt welding by electron beam welding and laser welding. Examine the internal quality of the weld. After testing, the internal quality of the weld has X-ray defects up to GB3233-87 Level II requirements. There are no cracks on the surface and inside of the weld, the appearance of the weld is well formed and the color is normal.

3. Weld depth and its variation

Titanium alloys are used as engineering components with specific requirements for weld depth. Otherwise, the component strength requirements cannot be met.

In addition, in order to realize precision welding, it is necessary to control fluctuations in welding depth. For this reason, two sets of butt test rings were welded by electron beam welding and laser welding, respectively. After welding, the test ring was analyzed in the vertical and horizontal directions to examine the variation in weld depth and weld depth. The results show that the average welding depth of electron beam welding reaches 2.70 mm or more. The range of variation in welding depth is -5.2 to + 6.0% and does not exceed ± 10%. The average welding depth of laser welding is about 2.70mm, and the welding depth varies from -3.8 to + 5.9% and does not exceed ± 10%.

4, joint deformation analysis

Butt test rings are used to inspect weld deformations at joints and detect radial and axial deformations of the butch test rings. The results show that the deformation of electron beam welding and laser welding is very small. Radial shrinkage deformation of electron beam welding is f 0.05 to f 0.09 mm, and axial shrinkage is 0.06 to 0.14 mm. The radial shrinkage deformation of laser welding is f 0.03 to f 0.10 mm, and the axial shrinkage deformation is 0.02 to 0.03 mm.

5, Titanium welding seam analysis

After chemical detection, the weld structure is a + b and the fine structure is columnar crystal + equiaxed crystal. A small amount of rasmartensite appears, the particle size is close to the matrix, the heat-affected zone is narrow, and the morphology and properties are ideal.
After investigation, we can conclude that: For TC4 titanium alloys, whether laser welding or electron beam welding, the internal quality of the weld is a national standard as long as the process parameters are properly matched. GB3233-87II Welding requirements can be met. Achieves precision welding of TC4 titanium alloy. The appearance of the weld is well-formed and the color is normal. The welding height is low, and defects such as undercuts, dents, and surface cracks do not occur.