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Common surface treatment methods for titanium alloy forgings

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 rate. The temperature difference between the casting temperature and the mold (300 ℃) is large, and the cooling is fast. Casting is carried out in a protective atmosphere, and defects such as pores are inevitable on the surface and inside of titanium castings, which have a significant impact on the quality of titanium castings. The following are the surface treatment methods for titanium alloy forgings.

1、 Removal of 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, it is necessary to completely remove the surface contamination layer in order to achieve satisfactory polishing results. The surface reaction layer of titanium can be completely removed by sandblasting and acid washing.

1. Sandblasting: For titanium castings, it is generally better to use coarse white corundum for sandblasting treatment. The sandblasting pressure is smaller than that of non precious metals, and is generally controlled below 0.45Mpa. Because when the injection pressure is too high, the sand particles impact the titanium surface and generate intense sparks. The temperature increase can react with the titanium surface, forming secondary pollution and affecting surface quality. The time is 15-30 seconds, and only the sticky sand, surface sintered layer, and some oxidation layer on the surface of the casting can be removed. The remaining surface reaction layer structures should be quickly removed by chemical acid washing.

2. Pickling: Pickling can quickly and completely remove the surface reaction layer without producing contamination from other elements on the surface. Both HF-HCl and HF-HNO3 acid washing solutions can be used for titanium acid washing, but the HF-HCl acid washing solution has a larger hydrogen absorption capacity, while the HF-HNO3 acid washing solution has a smaller hydrogen absorption capacity. The concentration of HNO3 can be controlled to reduce hydrogen absorption, and the surface can be polished. Generally, the concentration of HF is around 3%~5%, and the concentration of HNO3 is around 15%~30%, which is suitable.

2、 Treatment of casting defects

Internal porosity and shrinkage internal defects: can be removed using hot isostatic pressing technology, but it will affect the accuracy of dentures. It is best to use X-ray inspection, surface grinding to remove exposed porosity, and laser repair welding. Surface porosity defects can be directly repaired by laser localized welding.

3、 Grinding and polishing

1. Mechanical grinding: Titanium has high chemical reactivity, low thermal conductivity, high viscosity, low grinding ratio, and is prone to reaction with abrasive tools. Ordinary abrasives are not suitable for grinding and polishing titanium, and it is best to use superhard abrasives with good thermal conductivity, such as diamond and cubic boron nitride. The polishing line speed is generally 900-1800m/min. It is advisable, otherwise, the titanium surface is prone to grinding burns and microcracks.

2. Chemical polishing: Chemical polishing is achieved by the oxidation-reduction reaction of metals in a chemical medium to achieve the purpose of leveling and polishing. Its advantage is that chemical polishing is independent of the hardness, polishing area, and structural shape of the metal. All parts in contact with the polishing solution are polished without the need for special and complex equipment. It is easy to operate and 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 dentures without affecting their accuracy.

4、 Coloring

In order to enhance the aesthetic appeal of titanium dentures and prevent further oxidation and discoloration under natural conditions, surface nitriding treatment, atmospheric oxidation, and anodizing methods can be used to color the surface, forming a light yellow or golden color, and improving the aesthetic appeal of titanium dentures. Anodizing method utilizes the interference effect of titanium oxide film on light, resulting in natural coloring. It can form colorful colors on the surface of titanium by changing the slot voltage

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