The density of titanium alloy is small, so the inertia […]
The density of titanium alloy is small, so the inertia of titanium liquid is small when it flows, and the fluidity of molten titanium is poor, resulting in low casting flow rate. The difference between the casting temperature and the mold temperature (300℃) is large, the cooling is fast, and the casting is carried out in a protective atmosphere. It is inevitable that there will be defects such as pores on the surface and inside of the titanium castings, which have a great impact on the quality of the titanium castings. The following describes the titanium alloy forgings The surface treatment method.
Removal of surface reaction layer
The surface reaction layer is the main factor affecting the physical and chemical properties of titanium castings. Before the titanium castings are ground and polished, the surface contamination layer must be completely removed to achieve a satisfactory polishing effect. The surface reaction layer of titanium can be completely removed by pickling after sandblasting.
1. Sandblasting: Generally, white corundum rough blasting is better for the blasting of titanium castings. The pressure of sandblasting is smaller than that of non-precious metals, and it is generally controlled below 0.45Mpa. Because, when the injection pressure is too high, the sand particles impact the titanium surface to produce intense sparks, and the temperature rise can react with the titanium surface to form secondary pollution and affect the surface quality. The time is 15~30 seconds, and only the sticky sand, surface sintered layer and part and oxide layer on the surface of the casting can be removed. The remaining surface reaction layer structure should be quickly removed by chemical pickling.
2. Pickling: Pickling can quickly and completely remove the surface reaction layer without the pollution of other elements on the surface. Both HF-HCl series and HF-HNO3 series pickling liquids can be used for pickling of titanium, but HF-HCl series pickling liquids have a large hydrogen absorption capacity, while HF-HNO3 series pickling liquids have small hydrogen absorption and can control HNO3 The concentration of HF can reduce hydrogen absorption, and the surface can be brightened. Generally, the concentration of HF is about 3% to 5%, and the concentration of HNO3 is about 15% to 30%.
Treatment of casting defects
Internal pores and shrinkage cavity internal defects: can be removed by hot isostatic pressing, but it will affect the accuracy of the denture. It is best to use X-ray inspection to remove the exposed pores on the surface and use laser repair welding. Surface pore defects can be directly repaired by laser local welding.
Grinding and polishing
1. Mechanical grinding: Titanium has high chemical reactivity, low thermal conductivity, high viscosity, low mechanical grinding and grinding ratio, and easy to react with abrasives. Ordinary abrasives are not suitable for grinding and polishing of titanium. It is better to use good thermal conductivity. For ultra-hard abrasives such as diamond, cubic boron nitride, etc., the polishing linear speed is generally 900~1800m/min. Otherwise, grinding burns and micro-cracks may occur on the titanium surface.
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 the hardness of the metal, the polishing area and the shape of the structure. All parts in contact with the polishing liquid are polished, no special complicated equipment is required, and the operation is simple. It is more suitable for polishing complex structure titanium denture stents. However, the process parameters of chemical polishing are difficult to control, and it is required to have a good polishing effect on the denture without affecting the accuracy of the denture.
In order to increase the beauty of titanium dentures and prevent the discoloration of titanium dentures from continuous oxidation under natural conditions, surface nitriding treatment, atmospheric oxidation and anodic oxidation surface coloring treatments can be used to form light yellow or golden yellow on the surface to improve the titanium denture’s Beauty. The anodic oxidation method uses the interference effect of the titanium oxide film on the light to produce natural color, which can form colorful colors on the titanium surface by changing the cell voltage.