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Material properties and machining properties of titanium and titanium alloy

One.Properties of titanium and titanium alloys

Titanium and titanium alloy have many excellent properties, mainly reflected in the following aspects:

  1. High intensity.Titanium alloy has a very high strength, its tensile strength is 686 -- 1176MPa, and the density is only about 60% of steel, so the specific strength is very high.

  2. High hardness.The hardness HRC of titanium alloy (annealed) is 32 -- 38.

  3. Low elastic modulus.The elastic modulus of titanium alloy (annealed) is 1.078×10-1.176×10MPa, about half that of steel and stainless steel.

  4. High and low temperature performance.At high temperature, titanium alloy can still maintain good mechanical properties, its heat resistance is much higher than aluminum alloy, and the operating temperature range is wide, the current new heat resistant titanium alloy operating temperature can reach 550-600℃;At low temperature, the strength of titanium alloy is increased than that at room temperature, and has good toughness, low temperature titanium alloy can maintain good toughness at -253℃.

  5. Titanium has strong corrosion resistance.Titanium in the air below 550℃, the surface will quickly form thin and dense titanium oxide film, so in the atmosphere, seawater, nitric acid and sulfuric acid and other oxidizing media and strong alkali, its corrosion resistance is better than most stainless steel.

Two.Processing properties of titanium and titanium alloy

  1. Machining performance

    Titanium alloy high strength, hardness, so the requirements of processing equipment power, mold, tool should be higher strength and hardness.When cutting, the chip and the front cutter surface contact area is small, tip stress is large.Compared with 45 steel, although the cutting force of titanium alloy is only 2/3 -- 3/4, but the chip and the cutting face contact area is smaller (only 1/2 -- 2/3 of 45 steel), so the cutting edge of the tool to bear the stress is greater, the tip or cutting edge easy to wear;The friction factor of titanium alloy is large, but the thermal conductivity is low (only 1/4 and 1/16 of iron and aluminum respectively).The contact length between the tool and the chip is short, the cutting heat accumulates in the small area near the cutting edge and is not easy to send out, these factors make the cutting temperature of titanium alloy is very high, causing the tool wear to speed up and affect the processing quality.Because of the low modulus of elasticity of titanium alloy, the workpiece springback is large during machining, which is easy to cause the aggravation of wear on the back of the tool and the deformation of the workpiece.Titanium alloy at high temperature chemical activity is very high, easy and air hydrogen, oxygen and other gas impurities chemical reaction, the formation of hardening layer, and further aggravate the wear of the tool;In titanium alloy cutting, the workpiece is easy to bond with the surface of the tool, plus a high cutting temperature, so the tool is easy to produce diffusion wear and bonding wear.

  2. Grinding performance

    The titanium alloy has active chemical properties and is easy to bond and adhere to the abrasive at high temperature, which blocks the grinding wheel, resulting in increased grinding wear, decreased grinding performance and difficult to guarantee grinding accuracy.Grinding wheel wear also increases the contact area between the grinding wheel and the workpiece, leading to the deterioration of heat dissipation conditions, a sharp rise in the temperature of the grinding area, and the formation of large thermal stress in the grinding surface layer, resulting in local burns of the workpiece and grinding cracks.The high strength and toughness of titanium alloy make it difficult to separate the grinding chips, increase the grinding force and increase the grinding power.Titanium alloy has low thermal conductivity, small specific heat and slow heat conduction during grinding, which leads to heat accumulation in the grinding arc area, causing a sharp rise in the temperature of the grinding area.

  3. Extrusion processing performance

    During the extrusion processing of titanium and titanium alloy, the extrusion temperature should be high and the extrusion speed should be fast to prevent the temperature drop too fast, and the contact time between the high temperature billet and the mold should be shortened as far as possible.Therefore, new heat-resistant mould materials should be selected for extrusion mould, and the conveying speed of billet from heating furnace to extrusion cylinder should also be fast.Due to the fact that metals are easily polluted by gas during heating and extrusion, appropriate protective measures should be taken.Suitable lubricant should be selected when extruding to prevent the bonding mold, such as the use of envelope extrusion and glass lubrication extrusion.Due to the large deformation thermal effect and poor thermal conductivity of titanium and titanium alloy, special attention should be paid to prevent 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 heat reverse extrusion, the die temperature is low, the surface temperature of the blank in contact with the die drops rapidly, and the temperature inside the blank rises due to deformation heat.Due to the low thermal conductivity of titanium alloy, when the surface temperature drops, heat from inner billet cannot be transferred to the surface in time to supplement, resulting in surface hardening layer, which makes it difficult to continue deformation.At the same time, the surface and the inner layer will produce a large temperature gradient, even if the formation, also easy to cause deformation and uneven structure.

  4. Forging and pressing performance

    Titanium alloy is very sensitive to forging process parameters. The change of forging temperature, deformation, deformation and cooling speed will cause the change of microstructure and properties of titanium alloy.In order to better control the microstructure 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 range of 1000-1200 ℃, the plasticity reaches the maximum value and the allowable deformation degree reaches 70%-80%.Titanium alloy forging temperature range is narrow, should be strictly in accordance with the (alpha + beta)/ beta transition temperature (except billet casting), otherwise the beta grain will grow dramatically, reduce the room temperature plasticity;Alpha titanium alloy are usually (alpha + beta) two-phase area forging, because (alpha + beta)/beta subroutine above forging temperature is too high, will lead to the beta phase, a beta titanium alloy ending forging and finish forging must be higher than (alpha + beta)/beta transition temperature.The deformation resistance of titanium alloy increases rapidly with the increase of deformation speed, and the forging temperature has a greater influence on the deformation resistance of titanium alloy.The content of interstitial elements (such as O, N, C) also has a significant effect on the forging property of titanium alloy.

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