Four-step surface treatment of titanium alloy forgings: sandblasting, casting, grinding, coloring

The density of titanium alloy is small, so the inertia of titanium liquid flow is small, and the poor fluidity of molten titanium leads to low casting flow rate. Casting temperature and mold temperature difference (300℃) is large, cooling fast, casting in a protective atmosphere, titanium castings surface and internal inevitably have pores and other defects appear, the quality of titanium castings is greatly affected, the following describes the surface treatment method of titanium alloy forgings.

1. Removal of the surface reaction layer
The surface reactive layer is the main factor affecting the physical and chemical properties of titanium castings. Before grinding and polishing titanium castings, the surface contaminated layer must be completely removed to achieve satisfactory polishing effect. The surface reaction layer of titanium can be completely removed by the method of pickling after sandblasting.

(1) Sandblasting: The sandblasting treatment of titanium castings is generally better with white corundum rough blasting, and the sandblasting pressure is smaller than that of non-precious metals, which is generally controlled below 0.45Mpa. Because when the injection pressure is too large, the sand impact on the titanium surface produces fierce 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 sticky sand, surface sintered layer and part and oxidation layer on the surface of the casting can be removed. The remaining surface reactive 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 produce pollution of other elements. Both HF-HCL series and HF-HNO3 series pickling solution can be used for pickling titanium, but HF-HCL series pickling solution has a larger hydrogen absorption capacity, while HF-HNO3 series pickling solution has a small hydrogen absorption capacity, which can control the concentration of HNO3 to reduce hydrogen absorption, and can carry out bright treatment on the surface. Generally, the concentration of HF is about 3%~ 5%. The concentration of HNO3 is about 15% to 30%.

2. Treatment of casting defects
Internal pores and shrinkage holes internal defects: can be removed by hot isostatic pressing technology, but it will affect the accuracy of denture, it is best to use X-ray detection, surface grinding to remove the exposed pores, with laser welding. Surface porosity defects can be repaired directly by laser local welding.

3. Grinding and polishing
(1) Mechanical grinding: titanium chemical reactivity is high, low thermal conductivity, large viscosity, mechanical grinding grinding ratio is low, and easy to react with abrasives, ordinary abrasives should not be used for titanium grinding and polishing, it is best to use good thermal conductivity of superhard abrasives, such as diamond, cubic boron nitride, polishing line speed is generally 900~1800m/min. Otherwise, the surface of titanium is prone to grinding burns and micro-cracks.

(2) Chemical polishing: chemical polishing is through the REDOX reaction of metal in the chemical medium to achieve the purpose of leveling and polishing. Its advantage is that chemical polishing has nothing to do with the hardness of the metal, the polishing area and the structure shape, and the parts in contact with the polishing liquid are polished, no special complex equipment is required, and the operation is simple, which is more suitable for the polishing of complex structure titanium denture support. However, the process parameters of chemical polishing are difficult to control, which requires a good polishing effect without affecting the accuracy of the denture.

4. Coloring
In order to increase the beauty of titanium dentures and prevent the discoloration of continuous oxidation of titanium dentures under natural conditions, surface nitriding treatment, atmospheric oxidation and anodic oxidation surface coloring treatment can be used to form light yellow or golden yellow on the surface and improve the beauty of titanium dentures. Anodizing method uses the interference of titanium oxide film on light, natural color, can be changed by changing the tank voltage on the surface of titanium to form colorful colors.