Forging of titanium and titanium alloy forging
The forging of pure titanium and titanium alloy ingots must be carried out first. The process of blanking (also called rough forging, B-forging) is very important. It can be said that it is the first step in the forging of titanium ingots. The main step of this step is The purpose is to break up the coarse Weigangite grain boundaries, homogenize the metallurgical structure, reduce the proportion of high-temperature B-phase, and fuse the as-cast pores inside the ingot. At the same time, the volume of the ingot blank is reduced to provide a suitable size blank for the next processing.
Titanium and titanium alloy ingots have coarse grains. Like the steel ingots, their microstructure is as-cast widmanite structure. This grain and microstructure process is less plastic. Therefore, the first step in the processing of titanium ingots must be performed first. Blanking, (also called rough forging, initial forging, B-forging) temperatures are selected to be 150-250°C above the a+B/B transition temperature (that is, the phase transition temperature), at which temperature the deformation resistance of the titanium ingot is low and the plasticity is high. Larger deformations can be used to increase the forgeability, which can be sufficient to break up the coarse as-cast structure, and at the same time it can also increase productivity and reduce consumption. Special attention should be paid to the fact that the main deformation must be completed before the temperature is still above the phase transition point. But the first few hammers must be played lightly and quickly, and the backs must be repeated. Prevent internal cracks due to too much starting deformation.
In the process of blanking, if the total deformation reaches 70 to 80%, or the total forging ratio reaches 3.5 to 5, as long as the forging is performed at the appropriate deformation temperature within the specified deformation temperature range, the coarse as-cast state of the blank should be said. The tissue can basically be broken down to obtain a more uniform, fine fibrous tissue. Both its tensile strength and plasticity are improved. However, if the amount of deformation during the blanking process is too low, that is, when the number of drawing times is not enough, the as-cast microstructure cannot be effectively broken, and the residual as-cast microstructure is later refined at a lower temperature than the phase transformation point. Forging can not be eliminated and broken, forming a net or semi-reticulate a, which is called tissue genetics. Even if other tissues become equiaxed a-structure, the entire metallographic field of view is uniform, and this discontinuous a is distributed along the original grain boundary, which is the main reason for the unqualified organization. Therefore, the purpose of the coarse Wei body structure to be destroyed in the blanking stage cannot be expected to be solved in the subsequent intermediate blank forging.
In the specific operation of forging, two methods can be selected. First, the smelted titanium ingot is directly cut into segments according to the length of about 2.5 times of the diameter, and the drawing is directly performed multiple times. The second is to first forge the titanium ingot into a billet or a billet, and then cut it according to the appropriate length, and then perform multiple times of drawing at a temperature above the phase change point. Which process is chosen mainly depends on the conditions of the equipment. Especially the setting of the robot. However, compared with the first method, it is better to directly open the ingot after the ingot is cut, because the first method is hammered in all directions after the ingot is cut, which is easier to improve the heart. The state of the organization, the sampling of the finished product is the same in all directions. Second, the forging ratio is higher, and the grain boundary crushing effect is obviously better. Even if the forged square blank is cut and then B forged, it seems that there is no effect of directly cutting the ingot with the cut section. One reason here is that when metal forging reaches a certain forging ratio, the deformation is more difficult than the ingot deformation. If the goal of breaking the coarse annular as-cast grain boundary is not completed in the blanking stage, it is difficult to forge the residual trace of the grain boundary after the subsequent phase change point. It will form our common organization as an equiaxed a or needle a, and the entire field of view is also relatively uniform, that is, there is an undefined a grain boundary, and can not be delivered. Because the grain boundary must be broken above the a+B/B transition temperature, the subsequent forging is carried out at 30 to 50 °C below the phase transition point. The energy shortage cannot completely break the grain boundary, and the grain boundary is still retained. It is. The ASM4928 standard in the United States is a strip that does not remain along the grain boundary. In fact, many standards in our country are stricter than this. As long as the titanium alloy standard required by the metallographic organization does not allow the existence of grain boundaries. Since the grain boundary is not eliminated, it will remain in the metallographic structure of the final product, and this genetically refined grain boundary morphology will become the source of fatigue cracks. Whether it is military or civilian use of such organizations, we should have a consensus that the determination of whether or not the metallographic organization is qualified or the arbitration as long as there are traces of grain boundaries, is a non-conforming product, there is no room for negotiation.