The density of titanium alloy is small, the inertia of titanium liquid is small, and the fluidity of titanium liquid is poor, resulting in low casting flow. The temperature difference between the casting temperature and the mold is large, and the cooling rate is fast. The casting is carried out under a protective atmosphere. There are inevitably defects such as pores on the surface and inside of the casting, which have a great impact on the quality of the titanium casting. The following describes the surface treatment methods of titanium alloy parts processing.
1. Removal of the surface reaction layer
The surface reaction layer is the main factor affecting the physical and chemical properties of titanium castings. Before the titanium casting is ground and polished, the surface contamination layer must be thoroughly removed to achieve a satisfactory polishing effect. Pickling after sandblasting can completely remove the surface reaction layer of titanium.
Pickling: Pickling can quickly and completely remove the surface reaction layer, and the surface is not contaminated by other elements. The Hf-hcl system and Hf-hno3 system can be used for pickling titanic acid, but the Hf-hcl system has a large hydrogen absorption capacity, while the Hf-hno3 system has a small hydrogen absorption capacity. The hno3 concentration can be controlled to reduce the hydrogen absorption capacity and the surface can be brightened. . In general, the concentration of HF is about 3% to 5%, and the concentration of HNO3 is about 15% to 30%.
2. Treatment of casting defects
Internal pores and internal defects of the shrinkage cavity: Hot isostatic pressure can be eliminated, but it will affect the accuracy of the denture. X-ray flaw detection is used to remove exposed pores on the surface, and laser repair welding has a better effect. Surface pores can be repaired by laser welding.
Three, polishing
1. Mechanical grinding: Titanium has the characteristics of high chemical reactivity, low thermal conductivity, high viscosity, low mechanical grinding ratio, and easy reaction of abrasive tools. Ordinary abrasives are not suitable for grinding and polishing titanium. You can use super-hard abrasives with good thermal conductivity, such as diamond, cubic boron nitride, etc., and the polishing linear speed is generally 900-1800m/min, otherwise, the titanium surface is prone to grinding burns and micro-cracks.
2. Chemical polishing: Chemical polishing is the purpose of leveling and polishing through the oxidation-reduction reaction of metal in a chemical medium. The advantage is that chemical polishing has nothing to do with metal hardness, polishing area and structure shape. All parts in contact with the polishing liquid are polished without special complicated equipment.
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