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

Due to its low density, titanium alloy has small inertia when liquid titanium flows, resulting in poor fluidity of molten titanium and consequently a low casting flow rate. The significant temperature difference between the casting temperature and mold temperature (300°C) leads to rapid cooling. Since casting is performed under protective atmosphere, pores and other defects are inevitably present on the surface and within the titanium castings, significantly affecting their quality. The following discusses surface treatment methods for titanium alloy forgings.

1. Removal of Surface Reaction Layer
The surface reaction layer is the dominant factor deteriorating the physical and chemical properties of titanium castings. To obtain high-quality polishing surfaces, complete removal of the surface contaminated layer is a prerequisite for grinding and polishing procedures. A combined process of sandblasting and acid pickling can thoroughly eliminate the surface reaction layer of titanium castings.

1.1 Sandblasting
White corundum is preferred for coarse sandblasting of titanium castings. The sandblasting pressure should be lower than that for ordinary non-ferrous metals, generally controlled below 0.45 MPa. Excessively high blasting pressure enables sand particles to impact the titanium surface at high speed, generating intense local sparks and high temperature. This triggers surface chemical reactions and secondary contamination, which damages the surface integrity of titanium castings. The blasting time is controlled within 15–30 seconds, which is sufficient to remove surface adhesive sand, sintered layer and partial oxide layer. The residual surface reaction layer will be further removed by subsequent acid pickling.

1.2 Acid Pickling
Acid pickling can completely remove the residual surface reaction layer efficiently without introducing additional elemental contamination. Two pickling systems, HF-HCl and HF-HNO3 mixed solutions, are commonly used for titanium treatment. Compared with the HF-HCl system that causes severe hydrogen absorption, the HF-HNO3 system features lower hydrogen absorption. By adjusting the HNO3 concentration, hydrogen absorption can be further suppressed, and a bright surface finish can be achieved. The optimal formula is determined as 3%–5% HF and 15%–30% HNO3.

2. Repair of Casting Defects
Internal defects such as pores and shrinkage cavities are common in titanium castings. Although hot isostatic pressing can eliminate internal defects, it may compromise the dimensional accuracy of titanium denture castings. Therefore, a targeted repair process is adopted: X-ray inspection is firstly performed to detect the location and size of internal defects. The surface layer above the defects is then ground off to expose internal pores, followed by laser welding repair. For superficial pore defects, direct local laser welding repair is applicable.

3. Grinding and Polishing Treatment
Titanium is characterized by high chemical activity, low thermal conductivity and strong surface adhesion, resulting in a low grinding efficiency in mechanical processing. It is prone to chemical reactions with ordinary abrasives and grinding tools, which makes conventional polishing materials inapplicable for titanium processing. Reasonable grinding and polishing processes are essential to obtain smooth and defect-free titanium surfaces.

3.1 Mechanical Grinding
Superabrasives with excellent thermal conductivity, such as diamond and cubic boron nitride, are ideal grinding materials for titanium alloy. The optimal linear polishing speed is controlled at 900–1800 m/min. Polishing speeds beyond this range will easily cause surface burn marks and microcracks on titanium castings, reducing surface quality and service performance.

3.2 Chemical Polishing
Chemical polishing relies on redox reactions between the metal surface and chemical medium to realize surface leveling and finishing. It is not limited by metal hardness, polishing area or structural complexity, enabling uniform polishing of all solution-contacted surfaces. This process requires no sophisticated equipment and features simple operation, making it especially suitable for polishing titanium denture frameworks with complex structures. However, its process parameters are sensitive and difficult to control. Precise parameter adjustment is required to achieve a uniform and bright polishing surface without damaging the dimensional accuracy of denture parts.

4. Surface Coloring Treatment
To improve the aesthetic performance of titanium dentures and prevent natural oxidation and discoloration, surface coloring treatments including nitriding, atmospheric oxidation and anodizing can be adopted to form pale yellow or golden protective films, optimizing the visual effect of workpieces. Among them, anodizing utilizes the light interference effect of in-situ generated titanium oxide films to realize natural color development. Diversified colorful surface effects can be obtained by adjusting the anodizing bath voltage.