Due to the high temperature, titanium alloys are prone to chemical reactions with gases such as oxygen, nitrogen, and hydrogen, forming oxides, nitrides, hydrides, etc., resulting in a decrease in the mechanical and corrosion resistance of welds. Moreover, the thermal conductivity of titanium alloy is only 1/10 of that of copper and 1/5 of that of iron, which means that the heat generated during titanium alloy welding is difficult to dissipate and can easily cause overheating of the weld, resulting in problems such as coarse grains, uneven microstructure, and stress concentration. The thermal expansion coefficient of titanium alloy is 1.5 times that of iron and 2 times that of aluminum. This means that the temperature changes generated during titanium alloy welding can cause significant deformation and stress, which can easily lead to problems such as weld cracking, deformation, and bending. According to the characteristics of titanium alloys, there may be problems such as porosity, cracks, deviation, undercutting, and accumulation during laser welding of titanium alloys. So how to solve the problems that occur during laser welding of titanium alloys?
There are usually several commonly used solutions:
1. Gas protection: This is the most important method to avoid the formation of pores and cracks, mainly by using inert gases (such as argon) to protect high-temperature areas during laser welding, preventing chemical reactions between gases such as oxygen, nitrogen, and hydrogen in the air and titanium alloys. Gas protection can be carried out through methods such as front and back blowing, side blowing, and back blowing, or by using a glove box laser welding machine to weld inside the box.
2. Parameter control: This is an important method to avoid deviation and undercutting, mainly based on the material characteristics of titanium alloy and the shape of the workpiece, selecting appropriate laser power, speed, defocus amount and other parameters, so that the laser beam can uniformly melt titanium alloy and form an ideal melt pool shape and aspect ratio. Parameter control can be achieved by adjusting devices such as lasers, workbenches, and lenses.
3. Preheating or post heat treatment: This is an effective method to reduce or eliminate the generation of cracks, mainly by increasing the initial temperature of titanium alloy or reducing the final temperature of titanium alloy to reduce the shrinkage force and thermal stress of the weld. Preheating or post heat treatment can be carried out using methods such as resistance heating, induction heating, flame heating, etc.
4. Forced cooling: This is an effective method to avoid excessive oxidation or infiltration, mainly by accelerating the cooling rate in high-temperature areas to reduce the time of contact between titanium alloy and air. Forced cooling can be carried out by means of water cooling, air cooling, spray cooling, etc.
5. Control laser power and speed: Laser power and speed are important parameters that affect the formation of the molten pool and heat input. If the laser power is too high or the speed is too slow, it can cause the molten pool to be too large, fill with too much metal, and cause accumulation. Therefore, it is necessary to choose the appropriate laser power and speed based on the material characteristics and workpiece shape of titanium alloy, so as to make the size and aspect ratio of the molten pool moderate and avoid accumulation.
6. Control the gap and groove of the workpiece: The gap and groove of the workpiece are important factors affecting the formation and filling amount of the weld. If the gap of the workpiece is too large or the groove is too deep, it will cause the weld to require more filler metal, which is easy to cause accumulation. Therefore, it is necessary to choose appropriate workpiece gaps and groove forms based on the thickness and joint type of titanium alloy, so that the weld can effectively connect the workpiece and maintain a good appearance and size.
7. Adjust welding position: The welding position refers to the relative position of the laser beam and the surface of the workpiece. If the laser beam deviates from the center of the weld or is not perpendicular to the surface of the workpiece, it will cause asymmetry or unevenness of the molten pool, resulting in deviation. Therefore, it is necessary to adjust the distance, angle, direction and other parameters between the laser beam and the workpiece surface based on the shape of the workpiece and the type of joint, so that the laser beam can accurately illuminate the center of the weld seam.
