When exposed to high temperatures, titanium can react with elements in the air, such as oxygen (O), hydrogen (H), nitrogen (N), and elements in embedded materials like silicon (Si), aluminum (Al), and magnesium (Mg). This reaction forms a surface contamination layer on the casting, which can degrade its properties, leading to increased hardness, reduced elasticity, and increased brittleness.
Due to its low density, titanium has low fluidity and a low casting rate when in its liquid state. Additionally, the significant temperature difference between the casting and mold temperatures causes rapid cooling during casting. These factors contribute to the formation of defects like surface and interior pores in titanium castings, which significantly impact their quality.
Therefore, surface treatment for titanium castings is crucial and presents unique challenges. Titanium's properties, including low thermal conductivity, surface hardness, low elasticity, high viscosity, low electrical conductivity, and susceptibility to oxidation, require special processing methods and operational approaches beyond conventional surface treatment methods.
Cleaning Methods
Sandblasting
Coarse sandblasting is commonly used for titanium castings, with the blasting pressure controlled below 0.45 MPa. Excessive pressure can cause sparks and potential reactions with the titanium surface, leading to secondary pollution and surface quality issues.
Acid Washing
Acid washing rapidly and completely removes the surface reaction layer without introducing contamination from other elements.
Grinding and Polishing
Mechanical Grinding
Titanium's high reactivity, low thermal conductivity, and high viscosity make conventional abrasives unsuitable. Using highly thermally conductive superabrasives like diamond is recommended. The polishing line speed is typically between 900 and 1800 m/min to prevent surface grinding burns and microcracks.
Ultrasonic Grinding
Ultrasonic vibration facilitates the grinding and polishing process by causing relative movement between the abrasive grains and the surface being polished or ground.
Electrochemical Mechanical Compound Grinding
This method combines mechanical grinding with electrochemical action by using conductive abrasives, an electrolyte, and voltage application. It reduces surface roughness and enhances glossiness.
Barrel Grinding
Centrifugal force generated by the rotation and revolution of a grinding barrel reduces surface roughness through friction between the barrel's contents and abrasives. This method is automated and efficient but does not enhance surface glossiness.
Chemical Polishing
Chemical polishing achieves leveling and polishing through oxidation-reduction reactions of metals in a chemical medium. It is not dependent on metal hardness, polished area, or structural shape, and it does not require complex equipment.
