During cutting and machining operations, one of the most typical defects is work hardening.
Because Ti‑6Al‑4V undergoes severe plastic deformation during cutting, the surface layer becomes extremely hard, which accelerates tool wear and causes tearing, galling, and scratching on the machined surface. Another common issue is high cutting temperature caused by low thermal conductivity. Most heat generated during machining concentrates in the cutting zone instead of dissipating into the workpiece or chip. This high temperature leads to tool adhesion, diffusion wear, and built‑up edge (BUE), resulting in poor surface roughness, dimensional deviation, and even microcracks. In addition, due to the low elastic modulus of titanium alloy, springback and vibration easily occur during turning or milling, causing dimensional instability, taper, and chatter marks on the surface.
In hot working processes such as forging and rolling, defects are closely related to temperature control and deformation uniformity.
Cracking is a critical defect, especially at the surface or edges. High‑temperature alpha case embrittlement, uneven heating, or excessive deformation rate can lead to edge cracks, surface cracks, or internal intergranular cracks. Another typical defect is alpha case formation, a hard and brittle oxygen‑enriched layer produced when the alloy is heated in air without protective atmosphere. This alpha case reduces ductility, fatigue performance, and impact resistance. In addition, improper forging may lead to coarse grain structure, which reduces strength and toughness. Uneven deformation can also cause flow lines disturbance, which significantly reduces fatigue life in key structural components.
During heat treatment, improper parameters can induce overheating, overaging, or residual stress concentration.
Overheating leads to abnormal grain growth and brittle microstructure. Overaging in the STA process reduces strength and hardness significantly. Insufficient cooling or uneven heating causes residual stress, which may result in deformation, warping, or even delayed cracking after machining. For welded components, improper post‑weld heat treatment easily causes weld residual stress and distortion, affecting dimensional stability.
In welding processing, common defects include porosity, incomplete fusion, and weld cracking.
Titanium has high chemical affinity for oxygen, nitrogen, and hydrogen at high temperatures. If the protective atmosphere is insufficient, gas absorption occurs, leading to porosity and increased brittleness. Hydrogen absorption is particularly harmful and may cause delayed hydrogen cracking. In addition, rapid heating and cooling during welding produce large residual stress, resulting in weld distortion and hot cracking. The weld and heat‑affected zone often show hardening and loss of ductility, which reduces the overall structural integrity.
During casting, typical defects include shrinkage cavity, shrinkage porosity, inclusions, and segregation.
Titanium has a high melting point and high chemical reactivity, so it must be melted under vacuum or protective atmosphere. Improper pouring temperature and mold filling easily lead to shrinkage and porosity. Nonmetallic inclusions reduce fatigue strength significantly. Chemical segregation may cause uneven microstructure and performance.
In summary, due to its inherent physical and chemical properties, Grade 5 titanium alloy is prone to work hardening, surface galling, high temperature tool wear, alpha case, cracks, residual stress, porosity, and distortion during processing.
These defects can be effectively reduced or avoided by strictly controlling processing parameters, using special tools, applying protective atmospheres, optimizing heating and cooling procedures, and implementing reasonable post‑processing heat treatments. Understanding these common defects and their formation mechanisms is essential for improving product qualification rate, ensuring mechanical performance, and extending service life in industrial applications.
