Titanium rod and titanium alloy are important metal structural materials rising in the 1950s. Among them, TC4 alloy is the most widely used + Lu type titanium alloy at home and abroad. Because of its high specific strength, good corrosion resistance, and excellent comprehensive performance, it is widely used in aerospace, chemical machinery, pharmaceutical engineering, and other industries. The common defects of TC4 alloy include segregation and inclusion of alloying elements, residual casting structure, embrittlement layer, hydrogen embrittlement, embrittlement, and forging crack.
A unit found a crack of about 4.3mmx0.68mm on the surface of a TC4 titanium alloy bar processed product during machining. In order to analyze the cause of the defect and avoid the occurrence of similar defects, the author used various physical and chemical inspection methods to analyze the TC4 titanium alloy product.
The process of changing the forging ring of TC4 rod is to heat twice at 950 ° C, forge the cake material, and expand the cake material to form the ring material. The defects may be caused by cracking due to improper changing forging process, or the metallurgical defects such as inclusions, segregation, porosity, etc. in the rod itself, and the cracking may be caused by metallurgical defects in the subsequent changing forging process.
Through theoretical calculation, it is concluded that the design of this set of special fixtures for the overpass of a computerized flat knitting machine is completely in line with the requirements of theoretical design. Therefore, the author manufactured the real object of this set of fixtures and installed it on the workbench of the vcentei-70 high-precision CNC machining center for actual machining verification. Then put the finished parts of the horizontal machine overpass into the three coordinate measuring machine for accurate accuracy measurement, and the angle error of the two included angles is within the range of 0.2 ° ~ 0.6 °, which meets the technical requirements. Since this set of fixtures can easily and quickly realize the processing and positioning of overpass parts without any other means of alignment, it is suitable for mass production of overpass parts.
According to the left and right characteristics of the defect, it can be judged that the defect is formed by the oxidized sponge titanium in the melting process. After the weak oxidation and melting of the sponge titanium, a low-density segregation defect area is formed. After the severe oxidation and melting of the sponge titanium, oxygen-rich inclusions are formed. This defect is a metallurgical defect. The oxygen-enriched carbon and a small amount of nitrogen in the defect area will increase the hardness and embrittlement of this area, and sometimes form brittle phase T ^ AL. During the hot forging process, the deformation ability and deformation coordination ability of this area are very poor, and the coarse flakes a and bright blocks formed by melting are difficult to break. The structure is easy to be coarse and cracks are easy to occur, which are relatively harmful to the material.
According to the melting process, such defects are easy to appear at the bottom of the ingot. When melting and arc striking, the unstable arc striking current is easy to cause the sponge titanium scattered at the bottom to oxidize and melt insufficiently, forming inclusions and segregation areas, which are not easy to find during flaw detection. In order to prevent the damage caused by this defect to the product, the bottom of the ingot shall be sawed off with a length of 6% ~ 8% of the nominal diameter.
