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Two characteristics of heating titanium alloy

The first characteristic of titanium alloy heating is:

Compared with copper, aluminum, iron and nickel, titanium has a low thermal conductivity.When large billet is heated, the cross section temperature difference is large.While the thermal conductivity of copper, iron and nickel-based alloy decreases with the increase of temperature, the thermal conductivity of titanium alloy increases with the increase of temperature.

The second characteristic of titanium alloy heating is:

They react strongly with the air when the temperature is raised.When heated above 650℃, titanium reacts strongly with oxygen, while when heated above 700℃, it also reacts with nitrogen, forming a deeper surface layer saturated with these two gases.For example, when the titanium billet with a diameter of 350mm is heated to 1100-1150 by means of surface heating, it needs to be held for more than 3 ~ 4h in the temperature range of strong reaction between titanium and gas, and an aspirating layer with a thickness of more than 1mm May be formed.This getter will deteriorate the deformation properties of the alloy.

When heated in the oil furnace with reducing atmosphere, hydrogen absorption is particularly strong, hydrogen energy diffuses into the alloy during the heating process, reducing the plasticity of the alloy.When heated in an oil furnace with oxidizing atmosphere, the hydrogen absorption process of titanium alloy slows down significantly.Hydrogen absorption is slower when heated in an ordinary box furnace.

Therefore, titanium alloy blank should be heated in electric furnace.When flame heating is necessary, the atmosphere in the furnace should be slightly oxidized to avoid hydrogen embrittlement.No matter which type of furnace is heated, titanium alloy should not interact with refractory materials. Stainless steel plate should be placed on the bottom of the furnace.Do not use heat-resistant alloy plates containing more than 50% nickel to prevent billets from being welded to the plates.

In order to obtain uniform fine grain structure and high mechanical properties of forgings and die forgings, the residence time of the blank at high temperature must be guaranteed to be the shortest when heated.Therefore, in order to solve the problem of low thermal conductivity of titanium alloy and serious inspiratory at high temperature during heating process, segmented heating is usually adopted.In the first stage, the billet is slowly heated to 650 ~ 700 ° c, then rapidly heated to the desired temperature.Because titanium is less inspiratory below 700℃, the overall permeation effect of segmented heating oxygen in the metal is much smaller than that of ordinary heating.

The residence time of billet at high temperature can be shortened by using segmental heating.Although the thermal conductivity of titanium is low at low temperature, it is similar to that of steel at high temperature. Therefore, when titanium is heated to 700℃, it can be heated to high temperature faster than steel.

For surface quality of high precision forgings, or allowance for smaller important forgings, such as compressor blade, disc, etc.), billet is best in a protective atmosphere heating (argon or helium), but such large investment, high cost, and after still have been the danger of air pollution, so the production using coated glass lubricant protective coating, and then in the ordinary box type resistance furnace heating.Not only does the glass lubricant prevent oxide from forming on the billet surface, it also reduces the thickness of alpha layers and lubricates during deformation.

If the work is interrupted for a short time, the temperature of the furnace with billet shall be lowered to 850℃. When the work is continued, the furnace temperature shall be raised to the starting forging temperature again at the speed possible by the furnace power.When work is interrupted for a long time, the billet should be cast out and cooled on asbestos board or dry sand.

Free forging is mainly used for the primary processing of ingots, that is, the production of circular face, square section or rectangular section of the bar semi-finished.When free forging is more economically reasonable than die forging in single or small batch production, it is also commonly used to produce large blanks.

From ingot to finished bar, the forging process is usually divided into three stages.

  1. 1. The breakdown

    Its initial forging (billet) temperature is 150 ~ 250℃ above the beta transition point, when the plasticity of the cast structure is at its best.The ingot should be deformed at the beginning by a light or quick strike until the primary coarse grain structure is broken.The degree of deformation must be kept within 20% ~ 30%.The ingot is wrought into the desired section and then cut into a blank of a given size.

    The plasticity increases after the casting structure is broken.Aggregation recrystallization is intensified with the increase of temperature, longer holding time and grain refinement. In order to prevent the formation of aggregation recrystallization, the forging temperature must be gradually reduced with grain refinement, and the heating and holding time should also be strictly controlled.

  2. Upsetting in multiple directions

    It begins forging at 80 ~ 120 ° c above the temperature of the point of transition, alternating 2 ~ 3 upsetting and pulling lengths, and alternately changing the axis and edges.In this way, the recrystallization fine grain structure with the feature of deformation in the beta region can be obtained uniformly throughout the blank section.Such upset may not be necessary if the blank is rolled on a rolling mill.

  3. The second upset

    It is the same as the first multi-directional upset, but the initial forging temperature depends on whether the finished product after forging is the blank of the next process or the delivered product.If the next process of blank, the initial forging temperature is 30 ~ 50℃ higher than the temperature of the beta transition;If the product is delivered, the initial forging temperature is 20 ~ 40℃ below the temperature of the initial transformation. Due to the low thermal conductivity of titanium, when upsetting or drawing the blank on the free forging equipment, if the preheating temperature of the tool is too low, the impact speed of the equipment is low, and the deformation degree is large, x-shaped shear band is often formed on the longitudinal section or cross section.This is especially true for non - isothermal upsetting on hydraulic presses.This is because the tool temperature is low, and the blank contact with the tool causes the surface layer of the metal blank to be chilled. In the process of deformation, the deformation heat generated by the metal has no time to conduct heat around, and a large temperature gradient is formed from the surface layer to the center, resulting in a strong flow of the metal strain zone.The greater the degree of deformation, the more obvious the shear band, and finally the crack formed under the action of the sign opposite tensile stress.Therefore, in free forging titanium alloy, the strike speed should be faster, shorten the contact time between the blank and the tool as far as possible and preheat the tool to a higher temperature as far as possible, but also properly control the deformation degree in a stroke.

When forging, edges and corners cool fastest.Therefore, it is necessary to turn the blank several times and adjust the hammer force to avoid acute Angle.Hammer forging, the initial stage should be light hit, deformation degree is not more than 5% ~ 8%, then can gradually increase the amount of deformation.

Die forging is usually used to produce the final blank with a shape and size close to the finished product, followed by only heat treatment and cutting.The forging temperature and deformation degree are the basic factors that determine the microstructure and properties of the alloy.The heat treatment of titanium alloy is different from that of steel.Therefore, the technical specification of the final working step of die forging of titanium alloy is of special importance.

In order to make the die forging of titanium alloy have higher strength and plasticity at the same time, the overall deformation of the blank must be no less than 30%, the deformation temperature should not exceed the transformation temperature, and the temperature and deformation degree should be distributed evenly in the whole deformed blank as far as possible.

The microstructure and properties of die forgings of titanium alloy are less uniform than those of steel forgings.After the recrystallization heat treatment in the hot flow zone of metal, the low power is fuzzy crystal and the high power is equiaxed fine crystal.In the area of difficult deformation, due to small or no deformation, the microstructure always remains the state before deformation.So in die forging some important titanium alloy parts, such as compressor disk, blade, etc.), in addition to control deformation of the deformation temperature under the TB and the appropriate degree, the control of the original blank group is very important, otherwise, the coarse grain structure or certain defects inherited forgings, and subsequent heat treatment and cannot eliminate, will lead to the forging scrap.

When forging complex titanium alloy forgings, the temperature of the metal may exceed TB of the alloy even if the heating temperature is strictly controlled in the sharp deformation area where the thermal effect is locally concentrated.For example, when forging titanium alloy blank with i-shaped cross section, the hammer is too heavy, and the temperature of the middle part (web area) is about 100℃ higher than that of the edge part due to deformation thermal effect.In addition, in the area of difficult deformation and the area with critical deformation degree, it is easy to form coarse crystal structure with low plasticity and durable strength during the heating process after die forging.Therefore, the mechanical properties of the forging with complicated shape are often unstable.

Although reducing the die forging heating temperature can eliminate the risk of local overheating of the blank, it will lead to a sharp increase in deformation resistance, increasing tool wear and power consumption. When using more powerful equipment to hammer the die forging, multiple taps can also reduce the local overheating of the blank.However, it is necessary to increase the heating time to compensate the heat loss caused by the contact between the blank and the cold mould.However, it is not suitable to use hammer forging for the alloy, because the repeated heating in the die forging process will have an adverse effect on the mechanical properties.Compared with the forging hammer, the working speed of the press (hydraulic press, etc.) is greatly reduced, which can reduce the deformation resistance and deformation thermal effect of the alloy.When die forging titanium alloy on hydraulic press, the unit die forging force of blank is about 30% lower than that of hammer, which can improve the life of die.Reduced thermal effects also reduce the risk of metal overheating and temperature rise over TB.

When forging with press, under the same condition of unit pressure and forging with forging hammer, the heating temperature of blank can be reduced by 50 ~ 100℃.In this way, the interaction between the heated metal and the periodic gas and the temperature difference between the blank and the die are correspondingly reduced, thus improving the uniformity of deformation, the microstructure uniformity of the die forging is also greatly improved, and the consistency of mechanical properties is also improved.

The most obvious increase in the deformation rate is the surface shrinkage, which is the most sensitive to the microstructure defects caused by overheating.

Titanium alloy deformation is characterized by a more difficult flow into deep and narrow die grooves than steel.This is because the titanium deformation resistance is high, with the tool

High friction and the contact surface of the blank cooling too fast.In order to improve the fluidity and die life of titanium alloy.The usual practice is to increase the Angle and radius of the forging die and use lubricant: forging die burr bridge height is larger than steel, generally about 2mm.

In order to make the groove fill up easily, it is sometimes possible to restrict or accelerate the flow of metal to a part of the groove by using uneven burr groove at the bridge.For example, a long square box-shaped forging (as shown in figure 12) with thin front and rear side walls;The left and right side walls are thicker.When the burr groove shown in b-b is used around the box, the metal flows into the left and right side walls with little resistance, which makes the metal flow to the thin front and rear side walls difficult and the filling is not satisfactory.Later, b-b burr groove was still used on the front and rear side walls, while a-a burr groove was used on the left and right side walls. Due to the wide size of the bridge and the obstruction of the damping groove, the thin side walls on the front and rear were completely filled.

And the metal is more economical than the use of the aforementioned burr groove.

One of the most effective ways to improve the fluidity and reduce the deformation resistance of titanium alloy is to increase the preheating temperature of the die.Isothermal die forging and hot die forging developed in recent 20 or 30 years at home and abroad have provided a feasible method for solving the forming of large and complex titanium alloy precision forgings.This method has been widely used in the production of titanium alloy forgings.

When die forging titanium alloy with closed die forging method, the die life is reduced due to high pressure.Therefore, closed die forging must strictly limit the volume of the original blank, which complicates the preparation process.Whether to adopt closed die forging should be considered from both cost and process feasibility.During open die forging, the burr loss accounts for 15% ~ 20% of the weight of the blank, the clamping part of the technical waste (if according to the die forging conditions must leave this part) accounts for 10% of the weight of the blank.The relative loss of rough edge metal usually increases with the reduction of the weight of the blank. For some forgings with asymmetric structure, large difference in section area and difficult parts to fill, the cost of rough edge can be as high as 50%.Although closed die forging has no burring loss, the process is complex and more transition groove is needed, which will undoubtedly increase the auxiliary cost

Titanium alloy at high temperature has the deadweight bending tendency, thus increasing the product in die forging, cooling and heat treatment distortion tendency, distortion is particularly obvious is the section changes dramatically, or section is very thin products, so often need to titanium alloy forging alignment to meet the size requirements.

Different from aluminum alloy, titanium alloy is not easy to carry out cold correction, because of its high yield strength and elasticity coefficient, resulting in a great resilience, so titanium alloy forging alignment mainly rely on creep correction and thermal correction, the former is more common.Creep correction of most titanium alloys can be completed in annealing and aging process, whose temperature is annealing and aging temperature.However, if the annealing and aging temperature is lower than about 540 ~ 650℃, the time required for creep correction may be extended for different alloys.

Creep correction requires simple or complex fixtures and molds.Heat alignment in molds is usually used for medium size forgings. The forgings are heated to annealing or aging temperature for heat alignment, and then stress release is performed below the temperature for heat alignment.



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