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Vacuum heat treatment control of machining deformation of engine cast titanium casing

ZT3 and ZT4 alloys have good comprehensive properties, and the working temperature can reach 500 ℃ and 350 ℃ respectively. They can be used for engine compressor parts and aircraft structural parts. They are cast titanium alloys widely used in the aviation industry. It is used to manufacture the 2, 3, 4-5 and 6-7 rectifier housings of the turbojet 13A engine, replacing the heat-resistant stainless steel lCr11Ni2w2MoV forgings originally designed and used, which can reduce the structural weight of the aero-engine by nearly 20kg and improve the engine thrust-to-weight ratio. 15%. At the same time, it can greatly save raw materials and shorten the production cycle, and the economic benefits are considerable.

In the as-cast casing structure, except for the 3-stage casing, which is an integral structure, the other stages are two separate half-rings, and bolts are used to form a whole longitudinally on both sides of the connection. Among them, the diameter of the largest titanium casing is φ22mm, the maximum axial height is 148.3mm, and the minimum wall thickness of the cylinder is only 2.5±0.025mm. Titanium casing, a large thin-walled special-shaped structural part, has the phenomenon of end face warping and serious ovality increase after machining, which makes the part unable to meet the technical requirements. Therefore, it is necessary to study the deformation control of cast titanium casing in order to facilitate the popularization and use of new technologies.

The machining procedure of the casing is:

Rough turning of the blank → wire cutting into two half rings → two half rings are connected into a whole with bolts and then finishing turning → inner hole turning with spiral groove → nickel and graphite spraying → trimming and forming → finished product.

According to the test statistics, the two halves of the ring are warped and the ovality becomes larger immediately after the wire cutting, which makes the next process difficult to carry out. After spraying nickel graphite in the casing, the machining amount of the inner hole is increased, the turning depth is increased by 0.6mm, the pitch turning amount is increased by 0.8mm, and the surface roughness is Ra12.5. After finishing machining, the overall deformation of the casing is serious, the end face is warped up to 2mm, and the ovality exceeds 0.9mm. The largest deformation is at the junction of the two half rings, and the maximum residual tensile stress at this location can reach 53.9×104Pa. Therefore, the deformation of the casing is mainly caused by the thermal stress, structural stress and machining residual stress during casting.

In order to eliminate the internal stress of the cast titanium casing and ensure the stable size of the casing structure, it is necessary to arrange vacuum annealing at different processing stages of the casing to eliminate the internal stress of the parts. For this reason, it is determined to carry out vacuum annealing, vacuum stress relief annealing and vacuum correction annealing of castings respectively after rough machining and finishing to obtain the solution of the problem.

A vacuum furnace is used to anneal the cast titanium casing. The effective working size of the equipment is φ800mm×920mm, the ultimate vacuum degree is 6.7×10-3Pa, and the maximum operating temperature is 1300℃. The actual temperature of the sample in the vacuum furnace is detected by the fixed-point thermocouple and DRO30 multi-point calibrator.

The microstructure of the alloy after vacuum annealing was observed by optical metallography. According to the change of mechanical properties and microstructure characteristics of the samples after different annealing specifications, the vacuum heat treatment process conditions are determined. Using inert gas pulse chromatography, the oxygen content and hydrogen content of the sample before and after annealing were measured for reference in determining the vacuum annealing conditions.

Selection of Annealing Temperature

In order to select the appropriate annealing temperature, the room temperature properties and high temperature durability properties of ZT4 alloy after annealing at different temperatures were tested, and the annealing temperature was 600-800 °C. The results show that there is no significant change in room temperature properties and high temperature endurance properties between the annealed state (600-800 ℃) and the as-cast state.

The results of metallographic analysis and residual stress measurement show that after the alloy is annealed at 600-800 ℃, its as-cast microstructure is still retained, and the residual stress (casting stress and machining stress) of the casing blank is also eliminated.

The purpose of annealing of cast titanium casing is to eliminate internal stress, improve the structure stability and stabilize the structure size of the parts, and the selected test temperature is lower than the β→α transformation temperature (990℃) of the alloy, and the microstructure after annealing is not stable. Changes should be normal. Based on this, it is determined that the annealing temperature of ZT3 alloy casting blank is 650 ℃, and that of ZT4 alloy is 700 ℃, and the time is 3 to 4 hours; the annealing temperature for eliminating machining residual stress is selected as 600 ℃.

Vacuum heat treatment process

Titanium is a chemically active element, which is easily adsorbed and chemically reacted with oxygen, hydrogen, nitrogen and volatile substances in the air that have an adverse effect on the properties of the alloy. Once the titanium alloy is contaminated by the above substances, its plasticity and toughness will be drastically reduced, and its strength will also be adversely affected. In order to prevent the occurrence of the above phenomenon, it can only be achieved by using a vacuum heat treatment process.

Production practice shows that if the vacuum is too low, it is easy to cause the surface of the casing to be contaminated by oxygen and generate oxides. Relevant military standards and technical conditions clearly stipulate: “Parts with oxides darker than light yellow should be scrapped”. Especially for the annealing of the casing without machining allowance, this phenomenon is more serious. If the vacuum is too high, it is not only difficult to reach the equipment, but also can evaporate the elements on the surface of the alloy, which affects the structure and properties of the alloy. Therefore, we control the vacuum degree during vacuum annealing within the range of 0.13-0.013Pa.

In ordinary heat treatment furnaces, the heat exchange between the heat source and the processed parts is carried out by convection and radiation; for vacuum heat treatment furnaces, heat exchange is only realized by heat radiation. On the other hand, because the heat capacity and heat dissipation of the vacuum furnace body are smaller than those of ordinary heat treatment furnaces, the furnace body heats up quickly, and the temperature lag of the processed parts is serious. In order to grasp the relationship between the actual temperature rise of the sample and the indicated temperature of the furnace body under the existing equipment conditions, φ10mm, φ20mm, and φ60mm test bars were processed, and a φ8mm blind hole was drilled in the center of the test bar for installing thermocouples. Point the thermometer to check the temperature of each part. It can be seen that the size of the sample has a great influence on the temperature hysteresis of the sample. At the same time, under the condition that the heat conduction is mainly conducted by convection, the phenomenon that the temperature rise of the specimen lags behind the temperature rise of the furnace body and its temperature difference are most obvious in the middle temperature region.

The cast titanium casing is a special-shaped structural part with a large change in wall thickness, especially when the correction annealing heating is carried out with a clamp, the temperature difference between different parts of the casing will be large. For this reason, two preheating and heat preservation stages are added in the heating operation, one is 300°C/h and the other is 450°C/h, so that the temperature of each part of the part is as close to the furnace temperature as possible and reduces the thermal stress during the heating process. , to reduce deformation.

During the cooling process, in order to prevent stress deformation caused by too fast cooling rate, it is determined to adopt the method of slow cooling with the furnace to below 200 °C and then air cooling. The method of quick cooling without nitrogen or argon can still prevent the surface of the casing from being polluted by impure gas.

The cast titanium casings of all levels produced by this process method have been tested on the ground for a long time and installed and tested to show that the properties of the cast titanium casings meet the design requirements. In addition, the appearance and fluorescence inspection of the decomposed cast titanium casing after a long-term test drive did not find any abnormality, and the structure and dimensions were stable without obvious deformation. The measured ellipticity changed only 0.00-0.06mm.

Selection of vacuum heat treatment equipment: The RVA series vacuum heat treatment furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment process of titanium alloys. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum heat treatment process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.