Weldability of titanium and its alloys Ceramic laser cutting machine

Date:2022/5/12 9:48:35 / Read: / Source:本站

Weldability of titanium and its alloys Ceramic laser cutting machine

1) Effect of gas impurities on weldability Titanium has high chemical activity. It is generally believed that it starts as long as the temperature exceeds 400 °C, and it reacts violently with oxygen in the air above 600 °C to generate very stable oxides. At high temperatures, it also has a high solubility for oxygen, nitrogen, and hydrogen, which are solid-dissolved in metals. The dissolved hydrogen reduces the toughness of titanium and titanium alloys and increases its notch sensitivity. Dissolving a small amount of oxygen and nitrogen significantly increases the hardness of titanium and titanium alloys and reduces the plasticity. It can be seen that titanium and titanium alloys must be welded under high-purity inert gas or high vacuum conditions to avoid embrittlement. Titanium and titanium alloys also react with carbon to form carbides at high temperatures. Proper carbon content increases strength best, but in metals, excessive carbide content can cause embrittlement. Hydrogen is th e main cause of cold cracks and pores. Because the solubility of hydrogen in the α phase below 300 ℃ is very small, only 0.002% at room temperature, the general atmosphere content of the weld exceeds this limit. When the weld or heat affected zone is cooled to below 300°C after welding, supersaturated hydrogen is precipitated in the form of titanium hydride (y-phase). Since the a-y transition is accompanied by an increase in volume, intergranular stress may be generated and microcracks may be caused. Under the action of external stress, it can expand into macroscopic cracks. If the amount of hydrogen and atmosphere in the weld is also high, the plasticity will not be high, which will increase the sensitivity of the weld metal to cold cracks. www.dwinauto .com

2) Effect of welding thermal cycle and alloying elements on weldability Pure titanium is a close-packed hexagonal crystal structure below the transformation temperature (892 ° C), called a phase, and above the transformation temperature, it will be transformed into a body- centered cubic lattice, It's called phase B. The transition temperature is affected by the content of impurity elements in titanium. Phase B grains tend to overheat and grow when heated. Due to the high melting temperature of titanium (1668°C), the small thermal conductivity and volumetric heat capacity, the high temperature area during welding is larger than that of steel, aluminum and copper, the high temperature residence time is long, and the cooling speed is slow, which makes the high temperature B phase. The grains are easily overheated and grown to reduce plasticity. In order to prevent the occurrence of this problem, it is required to use process parameters with a fast cooling rate for welding. However, the qu The enching tendency of titanium is also large. Under the action of the welding thermal cycle, the structure obtained by heating to the B phase region and then cooling down is acicular α phase, which is similar to the martensite structure in steel and has poor plasticity. Therefore, the overheating and hardening tendencies must be taken into account at the same time, and the appropriate welding process parameters must be selected. TIG welding is currently the most widely used method for welding titanium and titanium alloys. Others such as plasma arc welding, vacuum electron beam welding, brazing and laser welding are also used in different occasions. www.dwinauto.com

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