Large and complex titanium alloy castings melt mold precision casting technology
Titanium alloy has the advantages of low density, high specific strength, good corrosion resistance, etc. It is widely used in aviation, aerospace, shipping, electronics and other fields. With the development of aviation and aerospace industry, the load, strength, rigidity and other requirements of the equipment is getting higher and higher, large and complex titanium alloy castings as a whole gradually replace the traditional “multi-component” structure. Especially in the field of aviation manufacturing, the engine in the need for high thrust-to-weight ratio, but also have a longer life, higher reliability and economy and meet the airworthiness certification requirements, accelerating the titanium alloy castings to the functionality, integration, lightweight, large-scale development, casting has been upgraded from the traditional meaning of the blank for the overall function of the near-net shape parts. The casting structure is becoming more and more complex, the outer contour size is getting bigger and bigger, the minimum wall thickness is getting thinner and thinner; the dimensional accuracy is getting higher and higher, and the metallurgical quality requirements are almost harsh; the requirements for the casting’s reliability, safety, and long-time stability are getting more and more clear. All these put forward more and more high requirements for large titanium alloy structural parts.
1 Domestic large-scale titanium alloy castings development status quo
In the early 1990s, China’s aviation engine titanium alloy magazine using split casting and then welded into a whole part production, in the process of use due to the large amount of welding of the magazine, parts of the rigidity of the poor, easy to produce fatigue cracks, parts of the reliability and service life decline, difficult to realize the full-life use. With the aviation engine performance requirements continue to improve, split casting and then welded into a whole part of the magazine class components have failed to meet the requirements of the use of aircraft engines, the need for better structural rigidity, more reliable overall castings.
Domestic from the 1990s to carry out the development of large-scale titanium alloy castings, the whole casting of the intermediary magazine was the first to be used in the XX10 engine. In recent years, China’s development of titanium alloy casting size continues to increase, from the profile size of Φ890 mm aero-engine intermediary magazine, the development of the profile size of 1,372 mm × 782 mm × 621 mm of a titanium alloy shaped structural parts.
For a long time, large and complex titanium alloy casting is mainly model task-oriented technology development, the development of general-purpose technology is relatively slow, resulting in titanium alloy casting technology progress can not catch up with the current casting quality and size of the development needs, so that the current development and production of large and complex titanium alloy casting preparation has always existed in the form of low dimensional accuracy, metallurgical defects, batch stability is not enough to other aspects of the technical bottleneck.
2 Large and complex titanium alloy castings manufacturing technology
In the 1980s to the early 1990s, China has basically formed a titanium alloy precision casting technology system, in recent years began to focus on near-net shape investment casting technology engineering application technology research. Casting technology is to realize the casting product quality conformity of the foundation, quality conformity is the product can realize the necessary protection of the equipment required function. From casting technology to product functionality is a complex process, the casting technology through the quality of consistency to achieve product functionality. Therefore, the casting technology foundation is solid, especially whether it can break through the bottleneck technology, key technology, is a large complex titanium alloy castings can be safe and reliable to realize its due function of the top priority. The following is an analysis of the dimensional accuracy control technology and metallurgical quality control technology in the titanium alloy casting process.
2.1 Large and complex titanium alloy castings dimensional accuracy control technology
Precision casting process is complex, from the wax mold to the shell, and then to the casting, the shape of the successive replication. The shape and dimensional stability of the wax mold and shell and the precision transfer process have a significant impact on the final acquisition of high-precision castings. Subsequent welding, heat treatment and machining of the casting will bring about deformation of the casting.
2.1.1 Key technical difficulties
Large and complex titanium alloy castings by the structural limitations and manufacturing process of multi-factors, resulting in casting deformation and dimensional contraction of the diversity of factors affecting the deformation, to achieve deformation and dimensional control must be clarified separately from the deformation of each factor to produce deformation of the mechanism, in order to formulate an effective control method. The main factors that cause deformation of castings are as follows.
(1) Wax mold deformation. Due to the complex structure, after the wax mold is opened, the live blocks can only be taken out one by one. At this time, one part of the wax mold is in contact with the mold live block, and the other part is exposed, which results in the constraints on each part of the wax mold can not be lifted at the same time and the outside temperature of the contact is also different, resulting in different shrinkage; and due to the large area of thin wall and unequal wall thickness difference, the wax mold itself shrinkage is also inconsistent. In the process of wax mold storage, the storage method, environmental temperature changes, self-weight and so on will also bring about the deformation of the wax mold.
(2) Shell deformation. Large and complex module will be deformed in the process of shell making under the influence of self-weight in the process of drying and storing the module; the shell is to copy the size of the wax mold in the room temperature environment, and the temperature of the shell will be increased to the casting temperature after pouring, and the temperature change of the shell will cause its size to change; the shell will be deformed by the effect of the liquid metal and the centrifugal force in the process of pouring.
(3) Deformation of casting solidification. In the wax mold set installation risers and sprue and casting pouring process, the casting cooling environment and solidification contraction are affected by pouring risers, due to the casting of its own structure of non-uniformity, large thin-walled easy to deform, large size contraction difference and other characteristics, by the pouring riser system of the temperature field and the contraction of the impact of the stress is even greater.
(4) Welding deformation. Within the scope of technical standards, some defects in the casting can be repaired by filler welding, but the filler welding process due to local temperature and organization changes in the stress generated by the casting will lead to deformation.
(5) structural deformation. Compared with general castings, the size and wall thickness of large titanium alloy castings vary greatly, and the size fluctuations are more significantly affected by the process environment in the process of mold making, shell making, pouring, hot isostatic pressing and heat treatment.
2.2 Large and complex titanium alloy castings quality control technology
2.2.1 Key technical difficulties
Due to the process control is more difficult, large and complex titanium alloy castings defects than small and medium-sized castings to increase the likelihood. From the point of view of the existing production and processing of large titanium alloy castings, castings after pouring almost all have a certain number of metallurgical defects, save by patch welding, after the blank is qualified, in the fluorescence inspection of the processing process, there will be part of the casting there is a fluorescence display. The main factors affecting the quality of castings are as follows.
(1) The increasing size of the outer contour increases the flow distance of the melt, prolongs the contact time between the melt and the casting, and increases the chance of interaction between the melt and the casting.
(2) The wall thickness difference is increasing, which increases the chance of shrinkage, porosity and stress concentration.
(3) The minimum wall thickness is getting smaller and smaller, increasing the possibility of undercasting and underpouring.
(4) The casting structure is becoming more and more complex, so that the liquid metal filling process is in the process of multi-pipe flow, it is very easy to involve foreign objects into the casting as inclusions, slag and other defects.
2.2.2 Technical solutions
(1) Use numerical simulation to analyze the temperature field under the heat condition of the shell and the liquid metal flow field, temperature field and stress field change rule when pouring. Construct the temperature measurement system under actual working conditions, measure the temperature state and metal liquid flow in the actual production process, so as to check and correct the numerical simulation results.
(2) Improve the shell temperature control method. By means of local heat preservation or radical cooling of the mold shell to achieve differentiated and precise control of the mold shell heat dissipation conditions, with the pouring process design, optimize the liquid metal solidification sequence to reduce the casting defects caused by incomplete filling and insufficient make-up shrinkage.
(3) Aiming at the structural characteristics of the components, combined with the computer simulation of the optimized pouring scheme, study the impact of the casting preheating temperature on the performance of the liquid metal filling and shrinkage, to obtain the optimized casting preheating temperature, to reduce the defects of flow marks, underpouring, shrinkage and shrinkage holes, and to achieve the complete forming of the castings.
(4) The computer finite element simulation technology is used to analyze the influence of temperature field on the formation of shrinkage holes and other defects in solidification, and predict the location of defects. Based on the results, optimize the three-dimensional casting structure, pouring riser and other pouring system design. Using X-ray flaw detection, penetration testing and other non-destructive means of inspection, combined with the casting anatomy, analysis of metallurgical defects and their distribution pattern of the casting, and compared with the finite element simulation results to verify, iteratively process parameter settings, optimization of the pouring process.
(5) Adopt hot isostatic pressing technology to eliminate internal shrinkage and shrinkage hole defects in castings.
3 large-scale complex titanium alloy investment casting process cases
A titanium alloy shaped structure casting is a typical large and complex titanium alloy castings. Titanium alloy castings manufacturing process flow is long (from feeding to casting into the warehouse needs to go through more than 70 main processes), any process operation quality of the casting final quality will have an impact. The main processes are: wax mold pressing, wax mold combination, paint shell making, melting and pouring, sand blowing and polishing, exhausting and welding, pickling and fluorescence, hot isostatic pressing, multiple X-ray flaw detection, orthopedics, and final inspection by machining. Due to the titanium alloy melt is very active, pouring melt is mostly obtained by vacuum self-consumption electrode condensing shell furnace, but because of this melting method metal liquid superheat is not high, resulting in the melt itself viscosity, poor fluidity, usually centrifugal pouring method for casting production.
3.1 Molding process
3.1.1 Mold making process
Wax mold size control is the first part of casting size control, in order to ensure the accuracy of the wax mold size, and at the same time verify the feasibility of the process parameters, the wax molds with different preheating temperatures, wax injection pressure and holding time are pressed, and the wax molds are dimensionally scribed and inspected, three-dimensional scanning and destructive comprehensive dimensional measurements are carried out, to determine the suitable parameters of the mold making process.
3.1.2 Wax mold size control technology
Wax mold tire molds and measuring tools are designed to correct the conformity of wax molds. Measurement results of the deformation of the wax mold surface corresponding to the casting surface show that the designed mold and gauge can effectively control the deformation of the wax mold, and the dimensions of the wax mold can be controlled to about 0.5 mm after curing, checking and controlling by the gauge, and local correction of the mold.
3.2 Pouring System Design
3.2.1 Computerized Process Simulation
Using the casting process simulation software to show the different process parameters of the process plan for pouring and solidification simulation analysis, to provide a basis for the optimization of casting process parameters.
3.2.2 Analysis of simulation results
From the results of the analysis in Figure 5, it can be seen that Scheme 1 has a better filling and shrinkage effect, which is mainly due to the high centrifugal speed, which is conducive to the filling and shrinkage of the alloy. If the centrifugal speed is reduced, the preheating temperature of the shell must be greatly increased and the shrinkage channel must be enlarged. However, both solutions have insufficient capacity at the bearing holes, so the shrinkage effect is not good, and it is necessary to increase the shrinkage of the thick parts.
3.2.3 Determination of pouring system
Due to the casting has the characteristics of large external dimensions and thin wall, the following two problems are mainly considered in the wax mold combination process plan:
(1) Designing a suitable ratio between inner sprue and cross sprue as well as center sprue, in order to ensure that sufficient pressure is obtained in the cavity;
(2) Since the wax molds are asymmetric thin-walled profiles, the combination scheme needs to consider that the modules are prone to torque due to weight imbalance during the rotation of the shell making process, resulting in the breakage of the modules. Special tooling is made for coating.
3.2.4 Anti-deformation design
Considering the large span of casting fascia is easy to deform, in order to ensure the strength of the wax mold and avoid the deformation of the wax mold and the casting after the process, we design, manufacture, and install a combination of technological tendons in the corresponding position of the wax mold to connect the casting fascia with a large span, forming an anti-deformation framework.
3.2.5 Specialized coating tooling design
The casting wax mold structure is an asymmetric thin-walled shaped structure, which is often cracked or broken at the connection parts such as sprue and riser in the process of assembling and coating, which increases the risk of casting deformation, flameout, porosity and high-density slagging defects. In order to improve the force situation of the die set, play the role of anti-deformation and protection of the die set by the tooling, minimize the risk of cracks or breakage of the die set, and improve the physical quality of the casting, special combination tooling is designed and manufactured to prevent the deformation of the wax die in the process of combination and coating.
3.3 Shell making technology
Wax molds of castings with large size, thin wall and poor overall strength are prone to collapse or micro-cracks during the coating process. Flying fins are formed on the inner surface of the cavity after dewaxing, and they are involved in the metal liquid to form slag when pouring. Therefore, it is necessary to enhance the strength of the wax mold set with a reinforcing frame, and to be careful during operation to prevent the mold set from collapsing or the wax mold from cracking.
3.3.1 Shelling operation
Since the module profile size exceeded the specification limitations of the existing paint production robot, it could only be painted by hand, which increased the difficulty of module dipping and sanding uniformity, and the stability of the painting process was poor. For this reason, a special coating crane shaft was designed and manufactured to rotate the coating by crane and hand.
3.3.2 Shell-making process material research
High-density inclusions and fluorescence linear display have been the main defects of titanium alloy castings, and poor shell quality stability is one of the main reasons. In order to further improve the quality of titanium alloy castings and shorten the production cycle, alkaline shell-making materials (silica sol-based) instead of acidic shell-making materials (zirconium diacetate-based), the shell by the paint roasting, the surface quality is good, no surface cracks and the phenomenon of the surface layer falling off. X-ray shows that the casting of high-density slag defects have been greatly reduced.
3.4 Melting and pouring technology research
Correct selection and control of melting process parameters, is to ensure that the key link to obtain high-quality castings. Because titanium alloy is active metal, molten state is easy to react with N2, O2, H2 and other gases, so the titanium alloy melting and pouring process should be carried out in a vacuum state, not only to prevent oxidation of titanium liquid, but also to prevent the alloy content of N2, O2, H2 exceeds the standard requirements.
Process parameters are determined.
(1) vacuum. To prevent oxidation of molten titanium liquid, select a higher vacuum, vacuum pressure needs to be less than 4 Pa.
(2) Electrical parameters. Due to the casting profile size, thin wall thickness, to get a complete casting, a higher melting temperature is required, for vacuum arc melting, in the case of ensuring that the voltage can not be too high, the key to improve the temperature is to try to increase the melting current. At the same time, in order to make the equipment melting process is in a safe state, in improving the melting current at the same time to prevent arc breakage and partial arc. Comprehensive analysis of the above, the melting electric parameters used are: melting voltage of 34 to 50 V; melting current of 28 000 to 32 000 A; melting volume calculated according to the weight of the module.
(3) Centrifugal speed. Improve the centrifugal rotational speed is the key to the filling of large, thin-walled castings, according to the theoretical formula for:
Where: n for the centrifugal disc speed (r/min); G for the gravity coefficient; R for the centrifugal disc rotation center to the casting of the shortest distance (cm). Considering the characteristics of the structure of the casting, the centrifugal speed of 200 r/min was selected for the calculation.
In addition, because of the casting outer contour size is large, the production of a special furnace box to ensure that the shell has enough strength to withstand the centrifugal force under the design speed.
3.5 Castings after treatment size control
3.5.1 Castings heat treatment anti-deformation tooling design
By comparing the size of castings before and after hot isostatic pressing, it is found that there is a certain amount of deformation of castings after hot isostatic pressing. For this reason, according to the casting in the hot isostatic pressing process furnace loading mode, from the casting to avoid deformation considerations, the design of hot isostatic pressing anti-deformation tooling. At the same time in order to meet the requirements of field development, welding and manufacturing of a simple hot isostatic pressing cardboard, after the application of the castings to prevent deformation played a certain effect, hot isostatic pressing of the castings by the scribing check surface deviation can be controlled in 1.5 mm or so.
3.5.2 Optimization study of casting vacuum creep heat straightening process
In order to ensure the size and shape of the casting after deformation and positional accuracy, the design and manufacture of hot straightening molds, and hot straightening process experiments. Two overall hot straightening tooling design ideas were optimized in the development of castings.
After using the optimized straightening mold to straighten the castings, the dimensional deviation of the curved surface can be controlled at about 1.5 mm after the castings are checked by scribing and three-coordinate fitting.
3.6 Development results of a titanium alloy shaped structural parts
(1) The technical measures taken in the casting process, such as mold making, assembling, coating, melting and pouring, and hot-straightening, are effective and controllable.
(2) The quality of castings meets the acceptance requirements of GJB2896A Class I B, and the size of the state in accordance with HB6103-2004/CT7. After the installation test, it meets the requirements for use.
4 Conclusion
Large-scale complex titanium alloy castings have become the development trend of titanium alloy investment casting, China’s related technology is still a big gap compared with foreign countries. In order to reduce quality fluctuations, improve casting quality, the following casting key process control is particularly important:
(1) Determine the reasonable mold parameters and wax mold anti-deformation measures is the key to control the dimensional accuracy of large and complex titanium alloy castings precision casting; the use of thermal straightening to control the size of the casting is an important method to deal with the deformation of large and complex titanium alloy castings precision casting dimensions;
(2) For large and complex titanium alloy castings, the centrifugal rotational speed should be increased appropriately, the preheating temperature should be increased, and the risers should be increased to make up for the shrinkage in the parts with smaller centrifugal radius and thicker parts, which can effectively improve the quality;
(3) The use of computer simulation to optimize the design of the pouring system can shorten the development cycle of large and complex titanium alloy castings and quickly improve product quality.
