Titanium alloys offer outstanding advantages such as low density, high strength, excellent corrosion resistance, and superior overall mechanical properties, making them indispensable core structural materials in the fields of aerospace equipment, medical devices, and high-end chemical processing equipment. Die forging, as the core process for forming titanium alloy components, directly determines the microstructure, mechanical properties, dimensional accuracy, and in-service reliability of the forgings.
Titanium alloys have a narrow plasticity range, high resistance to deformation, are prone to oxidation at high temperatures, and exhibit poor fluidity, making them extremely sensitive to process parameters throughout the entire die forging process. To consistently improve the quality of finished titanium alloy forgings, this paper systematically reviews the seven core process stages of die forging. It conducts an in-depth analysis covering billet preparation, heating regimens, lubrication processes, forging parameters, post-processing and cleaning, die design, and forging control in the two-phase region, thereby providing a comprehensive technical reference for the industrial production of precision titanium alloy die forgings.
I. Blank Preparation Process Prior to Die Forging
High-quality blanks are the foundation for ensuring the quality of die-forged parts. The surfaces of titanium alloy raw materials are prone to defects such as scale, scratches, inclusions, and cracks. If these materials are fed directly into the furnace for forging, these defects are highly likely to propagate into internal damage and surface defects during the forming process. Therefore, all die-forging billets must undergo standardized precision machining. Bar stock is uniformly shaped using turning and centerless grinding to ensure the billet surface is flat and smooth, free of pits, residual oxide layers, and machining impurities. Cold cutting with a band saw is the preferred method for blank cutting; oxy-fuel cutting is prohibited. This effectively avoids issues such as heat-affected zones, surface oxidation, and coarse grain structure caused by oxy-fuel cutting, ensuring the uniformity and integrity of the blank's matrix structure and laying the foundation for stable subsequent forging.
II. Control of the High-Temperature Heating Process
During the high-temperature heating process, titanium alloys are highly susceptible to hydrogen and oxygen absorption, which can cause material embrittlement and microstructural degradation. Therefore, the heating process is a critical step in controlling the quality of die forging. Before heating, scale, slag, and all types of debris must be thoroughly removed from the furnace bottom to ensure the furnace chamber is clean and free of contamination sources. Heating in an oxidizing atmosphere during production effectively suppresses hydrogen saturation in titanium alloys, reduces contamination by gaseous elements, and minimizes high-temperature oxidation defects. Heating temperatures and holding times must be strictly controlled during production to avoid abnormal grain growth caused by excessively high temperatures or prolonged holding times, thereby preventing a decline in the forging's toughness and fatigue properties. At the same time, a die preheating protocol must be followed, with the preheating temperature controlled within the range of 250 to 350 degrees Celsius and held for at least 12 hours. Adequate preheating significantly reduces the temperature difference between the billet and the die, thereby lowering surface thermal stress and the risk of cracking in the forged part, and improving forming stability.
III. Forging Lubrication Technology
Due to the poor high-temperature plasticity and flow properties of titanium alloys and their strong tendency to adhere to dies, defects such as die sticking, scoring, and flaking are highly likely to occur during the die forging process; therefore, specialized lubrication techniques are essential. Proper lubrication can significantly improve metal flow properties, reduce forming resistance, prevent direct adhesion between the forging and the die, and effectively enhance the surface quality and dimensional accuracy of the forged parts. The industry-standard, well-established lubrication system primarily uses graphite as the medium, employing colloidal graphite aqueous solutions, oil-based graphite mixtures, or water-based graphite coatings. These are evenly sprayed to cover the die cavity surface, forming a stable, high-temperature-resistant lubricating layer that meets the requirements of high-temperature die forging operations for titanium alloys.
IV. Control of Core Forging Parameters
Deformation, deformation temperature, and deformation uniformity are the three core parameters in the die forging of titanium alloys. For conventional die forging of titanium alloys, the effective deformation must be consistently maintained within the range of 40 to 80 percent. During the final forging stage, it is essential to ensure sufficient deformation of the entire billet, uniform metal flow, and consistent temperature distribution. Low-temperature, under-deformed forming is strictly prohibited to prevent shear cracks and folding defects from forming on the surface and within the forging. For two-phase titanium alloy components, heat treatment cannot achieve grain refinement; microstructural refinement and performance enhancement rely entirely on plastic deformation. Therefore, it is essential to ensure sufficient deformation both per forging operation and overall. Through significant plastic deformation, coarse microstructures are broken down to obtain uniform, fine matrix grains, thereby ensuring a balanced match between the strength and toughness of the forged part.
V. Post-Forging Surface Cleaning Process
After high-temperature forging of titanium alloys, a dense, brittle oxide layer rapidly forms on the surface. This oxide layer is highly hard and brittle. If not cleaned promptly, the hard oxide layer will exert pressure on the base metal during the next forging pass, causing surface cracking, peeling, and the propagation of inclusions, which severely affects the quality of the finished product. Therefore, after each forging pass, a dedicated surface cleaning process must be performed. The sandblasting process should be prioritized to thoroughly remove surface scale, contamination layers, and defect layers, ensuring that the forming interface for the next pass is clean and the microstructure is continuous, thereby achieving stable and controllable quality throughout multiple forging passes.
VI. Key Considerations for the Design of Specialized Forging Dies
The design logic for titanium alloy die-forging dies differs significantly from that of traditional steel forging dies and requires specialized optimization tailored to the deformation characteristics of titanium alloys.
First, there is a difference in shrinkage rates: the shrinkage rate of titanium alloy forgings is much lower than that of steel. The shrinkage ratio between titanium and steel is consistently 1:1.87, so die dimensional compensation must be calculated separately; parameters used for steel forgings cannot be applied here.
Second is die strength design. Under conditions of equivalent die cavity depth and structural complexity, titanium alloys exhibit greater forming resistance. The overall thickness of the die must be increased by 50 percent compared to steel forging dies to enhance the die's overall rigidity and resistance to deformation.
Third, structural details must be optimized: the radius of the die cavity fillet should be moderately increased to reduce metal flow resistance and stress concentration; simultaneously, the overall surface finish of the die cavity should be improved to minimize forging scratches and gouges, thereby enhancing the surface finish of the forged part and ensuring forming stability.
Conclusion
Titanium alloy die forging is a high-precision, multi-coupled, and highly interdependent system of processes, in which billet preparation, heating regimens, lubrication systems, forging parameters, surface cleaning, die design, and microstructural control of the two-phase region directly determine the final component quality. Through standardized and refined process control across the entire production process, common defects such as oxidation, cracking, die sticking, and microstructural inhomogeneity can be effectively avoided, ensuring the stable production of high-performance, highly consistent titanium alloy precision forgings. This provides a solid process foundation for the large-scale production of high-end aerospace equipment and various precision titanium components.
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