In modern manufacturing, titanium alloy is widely used in aerospace, automotive manufacturing, and medical devices due to its excellent properties, such as high strength, low density, and good corrosion resistance. However, titanium alloy faces a thorny issue during hot forging and extrusion processes: its strong tendency to adhere. This adhesion not only affects production efficiency but also leads to surface defects on forgings and damage to molds. This article will delve into the causes of titanium alloy adhesion, solutions, and key elements of the forging process to provide readers with a comprehensive understanding of this field.
Titanium alloy is more prone to adhesion with the mold surface during processing than other metals. The main reason is the high chemical reactivity of titanium. When titanium alloy comes into contact with tools, the oxide film undergoes a reduction reaction. As the sliding time increases, large areas of pure surfaces come into contact, leading to surface welding. Welding usually occurs in areas where metal flows intensely along the contact surface, posing significant challenges to hot forging and extrusion.
To prevent adhesion and reduce friction during deformation, selecting the right lubricant is crucial. The lubricant for titanium alloy forging must meet the following basic requirements:
Form a Strong and Continuous Protective Film: The lubricant should form a strong and continuous protective film on the surface of the blank throughout the deformation process to prevent direct contact between the blank and the mold.
Prevent Oxidation and Gas Contamination: During heating and deformation, the lubricant should protect the blank from oxidation and gas contamination to ensure surface quality.
Good Thermal Insulation: The lubricant should have good thermal insulation properties to reduce heat loss during the transfer of the blank from the furnace to the mold and during the deformation process, maintaining the temperature stability of the blank.
Chemical Stability: The lubricant should not chemically react with the surfaces of the blank and the mold to avoid damage.
Easy Application and Removal: The lubricant should be easy to apply to the blank surface and suitable for mechanized operations. Moreover, it should be easily removed from the forging surface after forming without affecting subsequent processing.
Long-Term Lubrication Performance: The lubricant should maintain good lubrication performance over an extended period to ensure the smooth progress of the entire forging process.
However, lubricants used for forging steel and other non-ferrous metals often do not meet all the above requirements. For example, a paste lubricant made from a mixture of graphite, water glass, and oil can cause cracks on the surface of titanium alloy. This is because a brittle α layer forms on the surface, which is easily damaged during deformation. These cracks act as stress concentrators and can propagate into the metal interior during continued deformation, severely affecting the quality of the forging.
For titanium alloy, glass lubricant is currently the most effective lubricant available. Under forging and partial extrusion deformation conditions, if the composition of the glass coating is properly selected, it can maintain liquid friction and effectively reduce adhesion.
The glass lubricant mainly consists of glass powder, stabilizer, binder, and water. The stabilizer is usually clay or bentonite, and the binder can be water glass, casein glue, or sulfite alcohol waste. Water serves as the diluent. For example, 1000g of glass powder can be mixed with 400g of water. The selection of glass powder is crucial during preparation. Different types of glass powder are required for titanium alloy forging at different temperature ranges. For example, glass powder No. 6 or No. 2 can be used for forging at temperatures ranging from 950 to 850°C; for forging at temperatures ranging from 1080 to 800°C, a mixture of glass powder No. 4 (80%) + No. 5 (20%) or No. 2 (60%) + No. 3 (40%) can be used.
To ensure the lubricant adheres firmly to the blank surface, the blank should be sandblasted before application. The main application methods are as follows:
Dip Coating: The blank is immersed in the suspension. Before dipping, the suspension must be thoroughly stirred to ensure uniformity. The suspension can be used at room temperature or heated to below 80°C. Generally, the blank is just dipped into the suspension. After dipping, the blank is air-dried for 20 to 30 minutes or dried in an oven at 60 to 80°C. The coating thickness on the blank surface after drying is approximately 0.2 to 0.3mm. If the coating thickness is uneven, it should be removed and reapplied.
Brush Coating: The lubricant is applied to the blank surface with a brush. This method is simple but results in less uniform coating thickness.
Spray Coating: The lubricant is sprayed onto the blank surface using a sprayer. Before spraying, it is best to preheat the blank in an oven to around 150°C. Spray coating provides a uniform thickness and is easy to control, making it an ideal application method.
After the blank coated with glass lubricant is dried, it should be placed on a corrosion-resistant steel or heat-resistant alloy base plate when heated in a box furnace. If an automated or semi-automated furnace is used, the blank should be placed in a box fixture. Sufficient spacing should be maintained between blanks to prevent contact and adhesion during heating. The blanks should not be stacked on the base plate. Special tongs should be used to handle the blanks in the furnace, and the contact area between the tongs and the blanks should be minimized.
The use of glass lubricant has significantly improved several indicators in the titanium alloy forging process:
Reduced Temperature Drop: Since glass lubricant is a good thermal insulator, the temperature drop during the transfer of the heated blank from the furnace to the mold is reduced. The forging temperature of titanium alloy blanks coated with protective-lubricating paint is about 60 to 80°C higher than that of uncoated blanks.
Increased Mold Life: The continuous film formed by the glass lubricant on the blank surface has good thermal insulation, reducing heat conduction to the tool and lowering the die cavity surface temperature by 100 to 150°C. This increases mold life by 20% to 30%.
Improved Metal Flow: The lubricant reduces the temperature difference between the blank surface and the core, which helps improve the uniformity and plasticity of metal flow and also reduces the unit forging pressure.
Prevention of Metal Oxidation: The melting point of the glass in the lubricant is lower than the temperature at which metal oxidizes rapidly. Oxidation of the metal is prevented from the moment the glass begins to melt and forms a viscous film on the heated blank. Research shows that when using protective-lubricating paint, the thickness of the α layer on the surface of the blank heated to the deformation temperature is less than 0.1mm, while without the paint, the α layer thickness is 0.3 to 0.5mm.
Reduced Friction Coefficient: The film formed by the glass lubricant on the blank surface acts as a lubricant during deformation, reducing the friction coefficient and facilitating metal filling in deep cavities. Calculations and measurements show that when using glass lubricant, the friction coefficient u is 0.04 to 0.06.
The titanium alloy forging process is a complex one involving several key elements. These elements are detailed as follows.
The surface of the blank must be rough machined or ground to ensure surface quality. After turning or centerless grinding of the bar stock, the blank is usually cut using a band saw, and gas cutting should be avoided to prevent damage to the surface quality and subsequent processing performance of the blank.
Before heating, the furnace bottom should be cleaned of slag and scale to ensure that the atmosphere inside the furnace is oxidative (with a slow saturation process for hydrogen). To minimize oxidation of titanium alloy, protect against hydrogen, prevent gas contamination, and avoid grain growth, the time the alloy spends at the heating temperature should be the shortest necessary to heat through the entire cross-section.
The mold must be preheated in advance, generally kept at 250 to 350°C for more than 12 hours. The mold should be lubricated before forging. Lubrication can improve the low flowability of titanium alloy and prevent forgings from sticking to the mold. The lubricant can be a mixture of colloidal graphite and water or a mixture of graphite and MoS₂ (oil-based or water-based).
The forging reduction is generally 40% to 80%. After the final heating, the entire metal should have uniform deformation, and the deformation temperature should be uniform throughout to prevent cracking at too low a temperature. For α-phase titanium alloy, it is even more important to provide sufficient deformation, as grain refinement of α-phase titanium alloy cannot be achieved through heat treatment but only through deformation.
The brittle oxide layer formed on the surface of titanium alloy forgings can cause the underlying metal to crack during the next forging pass. Therefore, the oxide layer should be removed after each forging pass, usually by sandblasting.
When designing molds for titanium alloy forgings, the shrinkage rate is smaller than that for steel forgings, with a ratio of 1:1.87. When using molds of the same depth and complexity, the mold for forging titanium alloy should be 50% thicker than that for forging steel, and larger fillet radii should be used. The surface finish of the mold cavity is also required to be higher.
Forging in the full β phase can improve the forging properties of titanium alloy at high temperatures or increase the notch toughness of forgings. To obtain forgings with high comprehensive properties, the microstructure of the alloy after β forging should be one where the equiaxed α phase in the transformed β phase is controlled within the range of 15% to 30%. An excess of equiaxed α phase will reduce notch toughness, while insufficient equiaxed α phase will reduce elongation. For forgings with an excess of α phase, notch toughness can be restored by heat treatment at a temperature below the β phase transformation (15 to 30°C) according to normal heat treatment practices.
Titanium alloy holds an irreplaceable position in modern industry due to its excellent properties. However, adhesion during the forging process has long been a key factor limiting production efficiency and product quality. By correctly selecting and using glass lubricant, the adhesion problem of titanium alloy can be effectively solved, improving forging efficiency and forging quality. Strictly controlling each aspect of the forging process, such as billet preparation, heating, lubrication, forging, cleaning, and mold design, is also an important guarantee for the successful forging of titanium alloy. With continuous technological progress, the titanium alloy forging process will be continuously improved, providing stronger support for the development of high-end manufacturing.
