Titanium alloys, with their significant advantages including high specific strength, excellent medium-temperature performance, corrosion resistance, and superior weldability, have been widely used in key load-bearing components of aircraft and their engines, making them an indispensable metal structural material. Statistics show that titanium alloys account for approximately 30% of the weight of foreign aircraft, fully demonstrating the vast potential for titanium alloys in the aviation industry. However, titanium alloys are not perfect. Their inherent properties present numerous challenges in production and processing, which can easily lead to various defects in forgings. In-depth analysis of these defects and the implementation of effective quality control measures are crucial to ensuring the quality of titanium alloy forgings.
Common Defect Types and Analysis of Titanium Alloy Forgings
(I) Segregation Defects
There are numerous types of segregation defects in titanium alloy forgings. In addition to the common β segregation, β spotting, titanium-rich segregation, and striped α segregation, interstitial α-stable segregation (Type I α segregation) and aluminum-rich α-stable segregation (Type II α segregation) are particularly dangerous. Type I α segregation is often surrounded by tiny pores and cracks and contains gases such as oxygen and nitrogen, making it extremely brittle. Type II α segregation is also accompanied by cracks and exhibits high brittleness, which not only reduces the alloy's mechanical properties but also adversely affects properties such as thermal stability. The presence of these segregation defects seriously threatens the quality and performance of titanium alloy forgings.
(II) Inclusions
Inclusions on the billet surface are another major risk in titanium alloy forgings. During the forging process, these inclusions often become crack initiation sites, causing cracks to form along the forgings. Alternatively, after corrosion treatment, visible foreign matter may appear in the forgings. These inclusions are often composed of metals with high melting points and high densities, and their formation is primarily due to two factors. On the one hand, high-melting-point, high-density elements in titanium alloys fail to fully melt, leaving residues in the matrix, such as molybdenum inclusions. On the other hand, smelting raw materials (especially recycled materials) may contain carbide tool chips, or improper electrode welding processes (titanium alloys are generally smelted using vacuum consumable electrode remelting), such as tungsten arc welding, can leave high-density inclusions such as tungsten inclusions. Titanium alloy forgings with such inclusions will severely impact their performance and are not permitted for use.
(III) Holes
Holes are not uncommon in titanium alloy forgings. They may occur singly or in densely distributed clusters. The presence of holes significantly accelerates the propagation of low-cycle fatigue cracks, leading to premature fatigue failure in the forging, seriously impacting the service life and safety of the forging.
(IV) Cracks
Forging cracks are a common defect in titanium alloy forgings. Due to the titanium alloy's high viscosity, poor fluidity, and poor thermal conductivity, the forging process results in high surface friction, significant internal deformation nonuniformity, and a large temperature difference between the inside and outside. These factors, combined, can easily create shear bands (strain lines) within the forging. In severe cases, cracks can develop along the direction of maximum deformation stress, forming forging cracks.
(V) Overheating
Titanium alloy has poor thermal conductivity, making it prone to overheating during hot working. In addition to improper heating causing overheating of the forging or raw materials, the thermal effects of deformation during the forging process can also easily cause overheating. Overheating can cause changes in the titanium alloy's microstructure, resulting in an overheated Widmanstätten structure, which in turn affects the performance and quality of the forging.
Quality Control Measures for Titanium Alloy Forgings
(I) Strict Control of Raw Material Quality
Raw material quality is the foundation of the quality of titanium alloy forgings. When purchasing raw materials, select reputable suppliers with consistent quality and conduct rigorous raw material inspection and acceptance. Ensure that the chemical composition, physical properties, and other indicators of raw materials meet relevant standards to eliminate forging defects caused by raw material quality issues at the source.
(II) Emphasize Ultrasonic Testing of Blanks and Semi-finished Products
Ultrasonic testing is an effective nondestructive testing method that can detect internal defects such as cracks and holes in titanium alloy forging blanks and semi-finished products. Regular ultrasonic testing of blanks and semi-finished products can promptly identify potential defects and take appropriate measures to prevent them from further developing during subsequent heating processes, which could lead to a decline in forging quality. During ultrasonic testing, operations must strictly adhere to relevant standards and specifications to ensure the accuracy and reliability of test results.
III. Conclusion
Titanium alloys have broad application prospects in the aviation industry, but defects such as segregation defects, inclusions, holes, cracks, and overheating in titanium alloy forgings seriously affect their quality and performance. To ensure the quality of titanium alloy forgings, effective quality control measures must be implemented, starting with raw material quality control and ultrasonic testing of blanks and semi-finished products. Only in this way can we produce high-quality titanium alloy forgings, meet the aviation industry's demand for high-performance metal structural materials, and promote the sustainable development of the aviation industry. At the same time, with the continuous advancement of technology, it is necessary to further study the properties and defect formation mechanism of titanium alloys, and continuously optimize production processes and quality control methods to improve the quality and reliability of titanium alloy forgings.
