1. Introduction
Our company, along with other companies, frequently performs welding and fabrication tasks involving titanium alloys and other non-ferrous metals. Initially, to successfully complete production tasks, I sought out information, consulted experts and experienced welders, and learned through a combination of study and practice.
Eventually, we successfully fulfilled orders for welding and manufacturing titanium alloys.
Subsequently, I participated in several titanium alloy welding projects, continuously learning and gaining a deeper understanding of titanium alloy welding. I also summarized some experiences that can be used as reference for future welding of titanium alloys.
2. Welding Process Measurements
(1) Proper selection of welding method:
The commonly used welding methods for titanium and titanium alloys include argon arc welding, submerged arc welding and vacuum electron beam welding, among which tungsten inert gas (TIG) welding is the most widely used. Therefore, we chose TIG welding.
(2) Correct selection of welding wire:
When selecting welding wire, it is necessary to ensure that the content of harmful impurities is as low as possible while meeting the plasticity and strength requirements of the weld.
Generally, the principle is to use welding wire with the same composition as the base material. In our actual welding, we used TA4 titanium alloy welding wire which had a similar composition to the base material.
Later, to save materials, we cut the remains of the base material into narrow strips and used them as welding wire, which also yielded good results.
(3) Strict control of material purity:
Welding titanium alloys is prone to the formation of hydrogen pores and sometimes carbon monoxide pores. To avoid porosity, it is necessary to strictly control the content of impurities such as hydrogen, oxygen and carbon in raw materials.
We carried out physical and chemical tests on both the base material and the welding wire, and the welding wire we used also underwent vacuum annealing treatment to ensure that the hydrogen and oxygen content did not exceed the specified limits.
(4) Shielding gas:
When the thickness of titanium alloy sheet is less than 3.2mm, pure argon gas is used for shielding. As the batch of orders we received mainly consisted of thin sheets with a thickness of 2mm, we used pure argon gas for shielding, with a purity requirement of ≥99.99%.
The oxygen content should be less than 0.002%, the nitrogen content less than 0.005%, the hydrogen content less than 0.002%, the moisture content less than 0.001mg/L, and the relative humidity ≤5%.
Furthermore, for sheet thicknesses greater than 3.2 mm, argon-helium shielding gas can be used to increase penetration depth, improve arc thermal power and increase welding productivity. The argon-helium ratio is 25% argon and 75% helium.
(5) Surface preparation:
Firstly, mechanical cleaning is carried out to remove dirt and oxide scale from the welding area by cutting, sandblasting, blasting or wire brushing.
Secondly, chemical cleaning is carried out using solvents such as alcohol, acetone and carbon tetrachloride to remove oil, dust and other contaminants from the welding surface and welding wire.
Then, a pickling process using HNO3 mixed with HF (10:1) is conducted to remove oxides until a silvery-white metallic shine is obtained.
After pickling, a thorough rinse is carried out with running water, followed by cleaning the surface of the welding wire and the welding area with acetone or alcohol before welding.
Finally, scraping is carried out in areas where acid pickling is not possible, using a hard alloy scraper to remove approximately 0.025mm of the metal surface within a range of 15-20mm from the groove and joint edge.
(6) Reinforcement of protection measures:
Although titanium and titanium alloys are low in sulfur and phosphorus impurities, they can still develop stress cracks and cold cracks when not properly protected.
Furthermore, titanium is easily oxidized at high temperatures during welding. If the weld metal and the zone close to the high-temperature weld, whether at the front or back, are not effectively protected, they will be subject to contamination by impurities such as air, resulting in further embrittlement.
Experimental evidence has shown that strengthening gas shielding in the welding area can improve the weld structure and prevent crack formation. We use the following protection methods:
First, a rotating local protection device was made. We created a local protection device with a “curtain” size of 15mm×20mm, which rotates in relation to the part during welding. The gap between the workpiece and the rotating surface of the “curtain” should be as small as possible, generally ≤0.5 mm.
Secondly, the use of a protective cover. We made a shielding layer with a thickness of about 1mm using pure copper plates. The hood is 20-40 mm wide and 80-120 mm long. The principle is that the gas stream enters through the inlet tube and is evenly distributed through a row of small holes or slits in the distribution tube before being blown upwards.
The outer jacket of the hood reflects the gas flow downwards, and after passing through several layers of fine copper wire mesh, it is evenly distributed in the welding. The main requirement for the hood is that the shielding gas flowing out is distributed evenly in a laminar flow rather than a turbulent flow. The internal transition of the hood must be smooth, with minimal dead corners. During welding, the hood is positioned against the rear of the welding area and moves along with the welding torch.
Lastly, back protection. During welding, the temperature at the back of the weld bead also exceeds 400°C, therefore it also requires protection. Therefore, we made a copper backplate that can provide shielding gas. The backing plate is placed on the back of the weld bead to isolate it from air and accelerate cooling in the welding area.
When the structure of the part is complex and it is difficult to implement shielding covers or back protection, the box welding method with vacuum evacuation and filling with argon gas can be used. This ensures that the workpiece is welded in an inert atmosphere with general gas protection.
