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

The density of titanium alloy is small, so the inertia of liquid titanium is small when flowing. The fluidity of molten titanium is poor, resulting in low casting flow rate. The difference between the casting temperature and the mold temperature (300°C) is large and the cooling is fast. The casting is carried out in a protective atmosphere. Defects such as pores will inevitably appear on the surface and inside of titanium castings, which have a great impact on the quality of titanium castings. The following describes titanium alloy forgings. surface treatment methods.

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, 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 sandblasting followed by pickling.

1. Sand blasting:

For sand blasting of titanium castings, it is generally better to use white corundum coarse blasting. The sand blasting 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 produce violent sparks. 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 adhering sand, surface sintered layer and part of the oxide layer on the surface of the casting are removed. The rest of the surface reaction layer structure should be quickly removed by chemical pickling.

2. Pickling:

Pickling can quickly and completely remove the surface reaction layer without contamination from other elements on the surface. Both HF-HCl and HF-HNO3 pickling solutions can be used for pickling titanium, but the HF-HCl pickling solution has a larger hydrogen absorption capacity, while the HF-HNO3 pickling solution has a smaller hydrogen absorption capacity and can control HNO3 The concentration of HF reduces hydrogen absorption and can brighten the surface. 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 cavities 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, then grind the surface to expose the pores, and use laser repair welding. Surface pore defects can be directly repaired by local laser welding.

Grinding and polishing

1. Mechanical grinding:

Titanium 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 abrasives with good thermal conductivity. For super-hard abrasives, such as diamond, cubic boron nitride, etc., the polishing linear speed is generally 900~1800m/min. Otherwise, grinding burns and micro-cracks will easily occur on the titanium surface.

2. Chemical polishing

Chemical polishing achieves the purpose of leveling and polishing through the oxidation-reduction reaction of metal in chemical media. The advantage is that chemical polishing has nothing to do with the hardness of the metal, the polishing area, and the structural shape. All parts in contact with the polishing fluid are polished. No special complicated equipment is required, and the operation is simple. It is more suitable for polishing titanium denture brackets with complex structures. 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.

Coloring

In order to increase the beauty of titanium dentures and prevent the discoloration of titanium dentures from continued oxidation under natural conditions, surface nitriding, atmospheric oxidation and anodizing can be used to color the surface to form a light yellow or golden yellow color, thereby improving the durability of titanium dentures. Beauty. The anodizing method utilizes the interference of light by the titanium oxide film to produce natural color. Colorful colors can be formed on the titanium surface by changing the cell voltage.