Four steps of surface treatment of titanium alloy forging: sandblasting, casting, grinding and coloring

The density of titanium alloy is small, so the inertia of liquid titanium is small, and the poor fluidity of molten titanium results in low casting flow rate. The temperature difference between the casting temperature and the mold (300 ℃) is large, the cooling is fast, the casting is carried out in a protective atmosphere, and the defects such as porosity are inevitable on the surface and inside of titanium castings, which have a great impact on the quality of titanium castings. The following describes the surface treatment methods of titanium alloy forgings.

1、 Removal of surface reaction layer

The surface reaction layer is the main factor that affects the physical and chemical properties of titanium castings. Before the grinding and polishing of titanium castings, the surface pollution 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.

1. Sandblasting: generally, white corundum is better for the sandblasting of titanium castings, and the sandblasting pressure is smaller than that of non precious metals, generally controlled below 0.45MPa. Because, when the jet pressure is too large, the sand particles impact the titanium surface to produce intense sparks, and the temperature rise can react with the titanium surface, forming secondary pollution, affecting the surface quality. The time is 15-30 seconds, only removing the sand sticking, surface sintering layer and part of oxide layer on the casting surface. The rest of the surface reaction layer structure should be removed quickly by chemical pickling.

2. Pickling: pickling can quickly and completely remove the surface reaction layer, and the surface will not produce other elements pollution. Hf-hcl system and hf-hno3 system can be used for titanium acid pickling, but hf-hcl system has a large amount of hydrogen absorption, while hf-hno3 system has a small amount of hydrogen absorption, which can control the concentration of HNO3 to reduce the hydrogen absorption, and the surface can be bright treated. 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 internal defects of shrinkage cavity: it can be removed by hot isostatic pressing, but it will affect the accuracy of denture. It is better to use X-ray flaw detection to remove the exposed pores on the surface and repair welding with laser. The surface porosity can be repaired by laser welding.

3、 Grinding and polishing

1. Mechanical grinding: titanium has high chemical reactivity, low thermal conductivity, high viscosity, low mechanical grinding grinding ratio, and it is easy for abrasive tools to react. Ordinary abrasive is not suitable for titanium grinding and polishing, so it is the best choice It is better to use super hard abrasive with good thermal conductivity, such as diamond, cubic boron nitride and so on. The polishing linear speed is generally 900 ~ 1800m / min. otherwise, the surface of titanium is prone to grinding burns and microcracks.

2. Chemical polishing: chemical polishing is to achieve the purpose of leveling and polishing through the oxidation-reduction reaction of metal in the chemical medium. Its advantages are that chemical polishing has nothing to do with the hardness of metal, polishing area and structure shape. All parts contacting with polishing fluid are polished without special and complicated equipment. It is easy to operate and is more suitable for polishing complex titanium denture bracket. However, the process parameters of chemical polishing are difficult to control. It is required to have good polishing effect on denture without affecting the precision of denture.

4、 Coloring

In order to increase the aesthetic feeling of titanium denture and prevent the discoloration of titanium denture under natural conditions, surface nitriding, atmospheric oxidation and anodizing can be used to form a light yellow or golden yellow surface, so as to improve the aesthetic feeling of titanium denture. The anodizing method uses the interference of titanium oxide film on the light to produce natural color, which can form colorful color on the titanium surface by changing the cell voltage.