CHARACTERISTICS OF TITANIUM AND TITANIUM ALLOYS

Titanium and its alloys have many excellent properties, which are mainly embodied in the following aspects:

1. High strength. Titanium alloy has high strength, its tensile strength is 686 – 1176 MPa, and its density is only about 60% of steel, so its specific strength is very high.
2. High hardness. The hardness HRC of titanium alloy (as annealed) is 32-38.
3. Low modulus of elasticity. The elastic modulus of titanium alloy (as annealed) is 1.078x 10-1.176x 10MPa, which is about half of that of steel and stainless steel.
4. Excellent performance at high and low temperatures. At high temperature, titanium alloy can still maintain good mechanical properties, its heat resistance is much higher than that of aluminium alloy, and its working temperature range is wider. At present, the working temperature of new type of heat-resistant titanium alloy can reach 550-600 °C. At low temperature, the strength of titanium alloy is higher than that at normal temperature, and it has good toughness. Low temperature titanium alloy has -25 °C. It also maintains good toughness at 3 °C.
5. Titanium has strong corrosion resistance. Titanium can form thin and compact titanium oxide film on the surface of Titanium in air below 550 °C. Therefore, its corrosion resistance is better than that of most stainless steels in oxidizing media such as atmosphere, sea water, nitric acid and sulfuric acid and strong alkali.

Processing Properties of Titanium and Titanium Alloys

1. Cutting performance

Titanium alloy has high strength and hardness, so it requires high power of processing equipment and high strength and hardness of dies and tools. In cutting process, the contact area between chip and rake face is small, and the stress of tool tip is large. Compared with 45 steel, the cutting force of titanium alloy is only 2/3-3/4, but the contact area between chips and rake face is smaller (only 1/2-2/3 of 45 steel), so the cutting edge of the tool bears more stress and the cutting edge is easy to wear; the friction coefficient of titanium alloy is large, while the thermal conductivity is low (only 1/4 and 1/4 of iron and aluminum respectively). / 16) The short contact length between cutting tool and chip and the accumulation of cutting heat in a small area near the cutting edge, which makes the cutting temperature of titanium alloy very high, resulting in faster tool wear and affecting the processing quality. Because of the low elastic modulus of titanium alloy and the large rebound of workpiece in cutting process, it is easy to cause worn tool flank and workpiece deformation. Titanium alloy has high chemical activity at high temperature, which is easy to react with hydrogen and oxygen impurities in air to form hardening layer, and further aggravate tool wear. In metal cutting, the workpiece material is easy to bond with the tool surface, and the cutting temperature is very high, so the tool is easy to produce diffusion wear and bond wear.

2. Grinding performance

Titanium alloys are active in chemical properties, easy to affinity and adhere to abrasives at high temperatures, blocking grinding wheels, resulting in worsening of grinding wheels, reducing grinding performance and ensuring grinding accuracy. The wear of grinding wheel also increases the contact area between grinding wheel and workpiece, resulting in deterioration of heat dissipation conditions, sharp increase of temperature in grinding area, and formation of greater thermal stress in grinding surface layer, resulting in local burns of workpiece and grinding cracks. Titanium alloy has high strength and toughness, which makes it difficult to separate grinding debris, increase grinding force and increase grinding power consumption. Titanium alloy has low thermal conductivity, low specific heat and slow heat conduction during grinding, which results in heat accumulation in the grinding arc zone and sharp increase of temperature in the grinding zone.

3. Extrusion Processing Performance

When extruding titanium and titanium alloys, it is required that the extrusion temperature be high and the extrusion speed be fast, so as to prevent the temperature from falling too fast. At the same time, the contact time between the high warm billet and the die should be shortened as far as possible. Therefore, a new type of heat-resistant die material should be selected for extrusion die, and the transportation speed of billet from heating furnace to extrusion cylinder should also be fast. In view of the fact that metals are easily polluted by gases during heating and extrusion, appropriate protective measures should be taken. Suitable lubricants should be selected during extrusion to prevent bonding die, such as sleeve extrusion and glass lubrication extrusion. Because the deformation thermal effect of titanium and titanium alloys is large and their thermal conductivity is poor, special attention should be paid to preventing overheating during extrusion deformation. The extrusion process of titanium alloy is more complicated than that of aluminum alloy, copper alloy and even steel, which is determined by the special physical and chemical properties of titanium alloy. When titanium alloy is formed by conventional hot back extrusion, the die temperature is low, and the surface temperature of the billet contacted with the die drops rapidly, while the temperature of the billet increases due to the heat of deformation. Because of the low thermal conductivity of titanium alloy, the heat of inner billet can not be transferred to the surface in time after the surface temperature drops, and the surface hardening layer will appear, which makes the deformation difficult to continue. At the same time, the surface layer and the inner layer will produce a large temperature gradient, even if it can be formed, it is easy to cause deformation and uneven structure.

4. Forging Processing Performance

Titanium alloys are very sensitive to forging process parameters. The changes of forging temperature, deformation amount, deformation and cooling rate will cause the changes of structure and properties of titanium alloys. In order to better control the structure and properties of forgings, advanced forging technologies such as hot die forging and isothermal forging have been widely used in the forging production of titanium alloys in recent years.

The plasticity of titanium alloy increases with the increase of temperature. In the temperature range of 1000 – 1200 °C, the plasticity reaches the maximum, and the allowable deformation degree reaches 70% – 80%. Titanium alloy forging temperature range is narrow, should be strictly controlled according to (α+β)/β transition temperature (except ingot billet), otherwise the β grain will grow sharply and reduce room temperature plasticity; α titanium alloy is usually forged in (α+β) two-phase zone, because the forging temperature above (α+β)/β transition line is too high, which will lead to β brittle phase, β titanium alloy’s initial forging and final forging. Forging must be higher than (α+β)/β transition temperature. Deformation resistance of titanium alloy increases rapidly with the increase of deformation speed, and forging temperature has a greater impact on deformation resistance of titanium alloy. Therefore, conventional forging must be completed with the least cooling in the forging die. The content of interstitial elements (such as O, N, C) also has a significant effect on the Forgability of titanium alloys.