The influence of structure and temperature on titanium castings
Publish Time:
2020-10-28
The main factors influencing casting performance are the thermal conductivity, strength, and yield strength of the mold, as well as its stability with molten titanium. Using molds with high thermal conductivity (i.e., high heat capacity), high strength, and poor yield strength, such as machined graphite molds for casting titanium and titanium alloy castings, often results in surface cracks. Surface cracks are the most common defects in titanium and titanium alloy castings and are generally cold cracks. To eliminate surface cracks, it is best to use molds with low thermal conductivity, moderate strength, and good yield strength, such as rammed molds and shell molds. Secondly, the stability of the mold with molten titanium also greatly affects the surface cracks of titanium and titanium alloy castings. When casting Ti-5Al alloy castings in ordinary lead dioxide, electrofused alumina, and magnesia shell molds, due to the contamination of the casting surface by interstitial elements such as H, C, O, and N, the C, N, and H content on the casting surface is not much different from the inside, but the O content of the surface layer is about an order of magnitude greater than the center, causing an α-embrittlement layer to form on the casting surface, which easily produces surface cracks under thermal stress. Therefore, the surface state of the casting is one of the main factors influencing the formation of cold cracks.
Proper preheating of the mold can remove some of the adsorbed gases from the mold surface, reduce the harm of interstitial elements, and reduce the cooling rate of the casting in the mold. Under the condition of ensuring that the casting is full, reducing the temperature of the poured titanium liquid can reduce the mutual reaction between the titanium liquid and the mold and avoid the formation of a serious α-surface embrittlement layer. These are helpful in preventing surface cracks in the casting.
Castings with complex structures and large wall thickness differences have relatively poor crack resistance, while castings with relatively uniform wall thickness are better. Therefore, when designing and producing castings with complex structures and large wall thickness differences, more attention should be paid to the structure and process design of the castings, and a reasonable gating system should be adopted to strictly control the parameters of the process.
keyword： Crack,Titanium alloy
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