Common heat treatment methods include annealing, solution treatment, and aging. Annealing is used to relieve internal stresses, enhance ductility and structural stability, thereby achieving better comprehensive properties. Typically, the annealing temperature for α alloys and (α+β) alloys is selected below the α+β to β phase transformation point by 120 to 200°C. Solution treatment and aging involve rapid cooling from the high-temperature region to obtain the martensite α' phase and the metastable β phase. Subsequently, holding at an intermediate temperature allows these metastable phases to decompose, resulting in fine, dispersed phases such as α phase or compounds, which serve to strengthen the alloy. The quenching of (α+β) alloys is usually performed below the α+β to β phase transformation point by 40 to 100°C, while the quenching of metastable β alloys is carried out above the α+β to β phase transformation point by 40 to 80°C. The aging treatment temperature is generally between 450 and 550°C.
In summary, the heat treatment process for titanium alloys can be summarized as:
(1) Stress Relieving: The purpose is to eliminate or reduce residual stresses generated during the processing. Prevents chemical erosion in some corrosive environments and reduces deformation.
(2) Full Annealing: The purpose is to achieve good toughness, improve processing properties, facilitate further processing, and enhance dimensional and structural stability.
(3) Solid Solution Treatment and Aging: The purpose is to enhance its strength. Alpha titanium alloys and stable beta titanium alloys cannot undergo strengthening heat treatment, and only annealing is performed during production. Alpha-beta titanium alloys and metastable beta titanium alloys containing a small amount of alpha phase can be further strengthened through solid solution treatment and aging.
In addition, to meet the special requirements of workpieces, industrial applications also employ metal heat treatment processes such as double annealing, isothermal annealing, β heat treatment, and deformation heat treatment.
Titanium alloys are particularly difficult to machine when their hardness exceeds HB350, and they tend to exhibit adhesion to the blade and are hard to cut when the hardness is below HB300. However, the hardness of titanium alloys is just one aspect of their difficulty in machining. The key lies in the comprehensive influence of the chemical, physical, and mechanical properties of titanium alloys on their machinability. Titanium alloys have the following machining characteristics:
(1) Low deformation coefficient: This is a significant characteristic of titanium alloy machining, with a deformation coefficient less than or nearly equal to 1. The sliding friction distance of the chips on the leading face of the tool is greatly increased, accelerating tool wear.
(2) High cutting temperature: Due to the low thermal conductivity of titanium alloys (only 1/5 to 1/7 of No. 45 steel), the contact length between the chip and the front cutting face is extremely short. The heat generated during cutting is not easily dissipated, concentrating in a smaller area around the cutting zone and the cutting edge, resulting in a very high cutting temperature. Under the same cutting conditions, the cutting temperature can be more than double that of cutting No. 45 steel.
(3) High cutting force per unit area: The main cutting force is about 20% less than when cutting steel. Due to the extremely short contact length between the chip and the leading edge, the cutting force per unit contact area is greatly increased, which is prone to chipping. At the same time, because of the low elastic modulus of titanium alloys, bending deformation is easily caused under radial force during processing, leading to vibration, increasing tool wear, and affecting the precision of the parts. Therefore, the process system should have good rigidity.
