The density of titanium alloy is small, so the inertia of titanium liquid flow is small, and the fluidity of molten titanium is poor, resulting in low casting flow rate. The temperature difference between the casting temperature and the mold (300°C) is large, the cooling is fast, and the casting is carried out in a protective atmosphere. Defects such as pores on the surface and inside of the titanium casting will inevitably appear, which has a great impact on the quality of the titanium casting. Next, the titanium forging The surface treatment tips are organized as follows:
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 grinding and polishing titanium castings, the surface contamination layer must be completely removed to achieve a satisfactory polishing effect. The surface reaction layer of titanium alloy flange can be completely removed by pickling after sandblasting.
1. Sandblasting: For the sandblasting treatment of titanium castings, it is generally better to use white corundum for rough spraying. 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, forming secondary pollution and affecting the surface quality. The time is 15 to 30 seconds, and only the sand sticking, surface sintering layer and part of the casting surface and the oxide layer are 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, and the surface will not be polluted by other elements. Both HF-HCl and HF-HNO3 pickling solutions can be used for pickling of titanium, but the HF-HCl series pickling solution has a large hydrogen absorption capacity, while the HF-HNO3 series pickling solution has a small hydrogen absorption capacity, which can control HNO3 The concentration of HF reduces hydrogen absorption, and can brighten the surface. Generally, the concentration of HF is about 3%~5%, and the concentration of HNO3 is about 15%~30%.
2. 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 flaw detection to remove the exposed pores on the surface and use laser repair welding. Surface porosity defects can be directly repaired by local laser welding.
3. Grinding and polishing
1. Mechanical grinding: titanium alloy forged ring has high chemical reactivity, low thermal conductivity, high viscosity, low mechanical grinding ratio, and is easy to react with abrasives. Ordinary abrasives are not suitable for grinding and polishing titanium. It is best to use good thermal conductivity. For superhard abrasives, such as diamond, cubic boron nitride, etc., the polishing line speed is generally 900~1800m/min. Otherwise, grinding burns and microcracks are prone to occur on the titanium surface.
2. Chemical polishing: Chemical polishing is to achieve the purpose of leveling and polishing through the oxidation-reduction reaction of metals in chemical media. Its advantage is that chemical polishing has nothing to do with the hardness of the metal, the polishing area has nothing to do with the structural shape, all parts in contact with the polishing liquid are polished, no special complicated equipment is required, the operation is simple, and it is more suitable for polishing complex titanium denture brackets. 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 precision of the denture.
4. Coloring
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 can be used for surface coloring treatment to make the surface light yellow or golden yellow and improve the titanium dentures. beauty. The anodic oxidation method utilizes the interference effect of the titanium oxide film on the light to produce natural color, and can form colorful colors on the titanium surface by changing the cell voltage.
