The density of the titanium alloy is small, so the inertia of the titanium liquid is small, and the flowability of the molten titanium is poor, resulting in a low casting flow rate. The difference between the casting temperature and the mold temperature (300 ℃) is large, and the cooling is fast. The casting is carried out in a protective atmosphere. There are inevitable defects such as pores on the surface and inside of the titanium casting, which greatly affects the quality of the titanium casting. Surface treatment method.
First, Removal of the surface reaction layer
The surface reaction layer is the main factor affecting the physical and chemical properties of titanium castings. Before grinding and polishing the titanium castings, 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.
- Sand blasting: Generally, the white sand blasting treatment of titanium castings is better to use white corundum rough blasting. The pressure of sand blasting is smaller than that of non-precious metals. It is generally controlled below 0.45Mpa. Because, when the injection pressure is too large, the sand particles hit the titanium surface to generate a fierce spark, and the temperature increase can react with the titanium surface, forming secondary pollution, which affects the surface quality. The time is 15 ~ 30 seconds. Only the sticky sand, sintered layer and part of the surface of the casting and the oxide layer can be removed. The remaining surface reaction layer structure should be quickly removed by chemical pickling.
- Pickling: Pickling can quickly and completely remove the surface reaction layer, and the surface will not be polluted by other elements. Both HF-HCl series and HF-HNO3 series pickling solutions can be used for pickling of titanium, but the HF-HCl series pickling solution has a large amount of hydrogen absorption, while the HF-HNO3 series pickling solution has a small amount of hydrogen absorption, which can control HNO3. The concentration of hydrogen is reduced, and the surface can be treated brightly. Generally, the concentration of HF is about 3% ~ 5%, and the concentration of HNO3 is about 15% ~ 30%.
Second, the treatment of casting defects
Internal pores and shrinkage cavity internal defects: can be removed by hot isostatic pressing (hot isostatic pressing), but it will affect the accuracy of the denture. It is best to use X-ray flaw detection, the surface of the exposed pores are removed, and laser repair welding is used. Surface pore defects can be repaired directly by laser local welding.
Third,Grinding and polishing
- Mechanical grinding: Titanium has high chemical reactivity, low thermal conductivity, high viscosity, low mechanical grinding grinding ratio, and is easy to react with abrasives. Ordinary abrasives are not suitable for titanium grinding and polishing. It is best to use good thermal conductivity The super-hard abrasive, such as diamond, cubic boron nitride, etc., the polishing line speed is generally 900 ~ 1800m / min. It is appropriate, otherwise, the titanium surface is prone to grinding burns and micro cracks.
- Chemical polishing: Chemical polishing is to achieve 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 structural shape. All parts in contact with the polishing liquid are polished, no special complex equipment is needed, and the operation is simple. It is more suitable for polishing titanium complex denture support with complex structure. However, the technical 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 aesthetics of titanium dentures and prevent the continued discoloration of titanium dentures under natural conditions, surface nitriding, atmospheric oxidation and anodizing surface coloring treatments can be used to make the surface light yellow or golden yellow to improve the titanium denture Beauty. The anodizing method utilizes the interference effect of the titanium oxide film on the light to naturally develop color, and can form a colorful color on the surface of the titanium by changing the bath voltage.