Combining the characteristics of titanium alloy and the structural characteristics of frame-type titanium alloy, this article briefly introduces from the aspects of process plan planning, processing equipment selection, tooling design ideas, common tool types and processing considerations.
Framework titanium alloy parts processing technology plan
Frame type titanium alloy parts, the common is titanium alloy casing (see Figure 1). According to the different requirements of product use and its processing characteristics, there are currently four main processes: ① Segmental finishing, re-welding, and supplementary processing of welded joints. ②Sectional rough machining, re-welding, and overall CNC finishing. ③ Forging as a whole, and then roughing and finishing. ④Integrated casting, then rough and precision machining.
Selection of machining equipment for frame titanium alloy parts
If the scheme of segmented finishing, re-welding and re-processing of welded joints is adopted, the processing equipment can select the appropriate five-sided gantry machining center, large-scale five-axis linkage machining center or CNC lathe according to the condition of the welded joint and the contour size of the part Wait.
If the process of section rough machining, re-welding, and overall CNC precision machining is used, the frame-type titanium alloy parts adopting this kind of process scheme are generally large in size, complicated in structure, many welds, and easy to deform, so before welding Generally, more margin is left on the end face and surface. When planning the finishing process, it is necessary to switch as few procedures as possible, because once there are too many procedures, it is difficult to control the cumulative error and the deformation factors of the product. Generally speaking, it is preferred to choose a large gantry turning and milling compound center (see Figure 2 and Figure 3); secondly, a CNC lathe and a five-axis linkage machining center (see Figure 4) are recommended for joint processing.
Such as the use of overall forging (casting), and then coarse and fine machining. The machining blank prepared by this process scheme has a relatively uniform machining allowance and relatively little removal allowance, which can shorten the machining time. In the machining process, the inner and outer walls of the frame parts and the surrounding windows need to be rough-finished. For this reason, when selecting the processing equipment, select the appropriate CNC lathe (turning rough finishing), large-scale gantry five-axis machining center (internal Finish the rough machining of the outer wall and the surrounding window), large-scale gantry turning and milling compound center, etc.
Framework titanium alloy parts machining tooling design ideas
Considering that the frame-type titanium alloy parts are generally thin-walled, easily deformed, and complex in structure, according to their structural analysis, the following basic principles must be followed when assisting the design of tooling.
- (1) Satisfies the stability and reliability of workpiece positioning during the use of tooling, has sufficient bearing or clamping force, and has a rigid structure suitable for parts.
During roughing, the clamping clamping force can be large to prevent the parts from loosening during the large cutting process with a large cutting width and depth; when finishing, the parts are easily deformed, and the clamping force of the tooling is slightly smaller, but it must be Prevent loose parts. For thin-walled structures that are particularly easily deformed, auxiliary supporting tooling structures must be added appropriately.
- (2) As far as possible, fully consider the unification or association between the design criteria of parts (use criteria), the positioning criteria of tooling and the inspection criteria of parts.
- (3) The method in the clamping process is simple and the operation is fast.
- (4) Satisfies that the tooling has reliable repeatability during the adjustment, simultaneous transfer or replacement process.
- (5) Avoid complex structures and expensive costs as much as possible. Strictly follow the principles of manual, pneumatic, hydraulic and servo design priority.
- (6) It is necessary to design a limit anti-rotation device in the circumferential direction to prevent the secondary clamping from missing the circumferential reference. If the batch size is large, pneumatic and hydraulic automatic fixtures can be designed to realize the automatic giving-up of the pressing block and automatic recovery of the pressing during processing. If it is a side slot (hole) and other situations that may be cut into the tooling when processing the part, generally speaking, appropriate concessions should be considered in the design. The better solution is to recommend the matching part of the tooling to be the same Material to avoid quality accidents caused by tool damage.
For parts with high precision requirements for thin-walled titanium alloy parts, the clamping of the tooling may cause new deformation of the parts. Because during the cutting process, the fluctuation effect between the cutting force and the clamping force will produce a coupling effect, resulting in the additional stress of improper clamping, the residual stress after cutting, and the residual stress inside the workpiece are redistributed. New deformation after processing. Therefore, for special parts, it is recommended to use the “transition shape adjustable support” or “transition shape flexible tooling” method (see Figure 5), that is, according to the shape of the transition shape of the part in the free state, the design is fully adjustable (Flexible) Support and clamp.
When processing the inner wall of large thin-walled titanium alloy parts, from the perspective of cost saving, a tooling structure that combines a bowl-shaped bag holder and a core support (see Figures 5 and 6) is generally used. This method can be more Effectively achieve reliable clamping of tooling.
Frame type titanium alloy parts machining commonly used tool type selection
Starting from the three comprehensive factors of improving efficiency, improving quality, and reducing costs, tools for processing frame-type titanium alloys are expected to have both high thermal hardness and good wear resistance; both hope to have good impact resistance , And hope to have better toughness; both hope to have a higher thermal conductivity, but also hope to have a lower chemical activity. From these hopes, the tool materials commonly used for processing titanium alloys are mainly cemented carbide, polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN). After actual production verification, cemented carbide and PCD tools are considered to be ideal tool materials for processing titanium alloys.
At present, the choice of tool materials for processing titanium alloys is the highest priority and the most widely respected is hard alloy tools. Because cemented carbide has the characteristics of relatively low cost, good thermal conductivity, high hardness, toughness and red hardness. According to its chemical composition, it can be divided into tungsten-cobalt (YG), tungsten-cobalt-titanium (YT) and added rare carbides (YW). At present, it is still widely used in industrial production is tungsten-cobalt carbide YG8 , YG6, YG3, etc. For example, YG8 blades are usually used for rough turning and intermittent turning, YG3 blades are used for fine turning and continuous turning, and YG6 blades are used for general machining. If the rare-earth fine-grained cemented carbide YA6, YD15, YG10H, YS2, etc. are used, the tool life and machining efficiency can be improved. The reason why the tungsten-cobalt-titanium (YT) carbide tool is not selected in practice is that the YT blade contains titanium, and a strong affinity will occur between it and the titanium alloy being processed, and the tool tip will soon stick out.
The speed of processing titanium alloy with cemented carbide can reach more than 45 m / min, but when the cutting speed continues to increase, the temperature of the contact surface between the tool and the workpiece rises rapidly. Under the action, it causes the diffusion and loss of W and Co elements in the tool material, reduces the hardness and toughness of the tool, and causes severe plastic deformation, bonding wear and diffusion wear of the cemented carbide tool, resulting in tool failure. Therefore, cemented carbide tools are only suitable for titanium alloys with a cutting speed of less than 75 m / min.
Polycrystalline diamond cutters have extremely high hardness and ultra-high wear resistance. They also have the advantages of high elastic modulus, high thermal conductivity, sharp cutting edge, low friction coefficient, and low affinity with non-ferrous metals. This type of tool is suitable for finishing and super finishing of titanium alloys.
Polycrystalline cubic boron nitride (PCBN) tool, although its hardness is slightly lower than diamond, but it has better characteristics than diamond is the thermal stability is much higher, can reach more than 1 200 ℃ (heat resistance temperature of diamond is only 700 ~ 800 ℃), chemically inert, and no chemical reaction with titanium alloy at 1 200 ℃. Compared with cemented carbide tools, PCBN tools have the characteristics of high cutting speed, good surface roughness quality and long tool life. For this reason, PCBN tools are more suitable for the finishing of titanium alloys. However, due to the high brittleness of PCBN tools, intermittent cutting or sudden allowance cutting must be avoided during cutting.
Other precautions for processing titanium alloy parts
Do not seek a high cutting speed, but only to increase the cutting pass reliably. Excessive cutting speed will lead to overheating of the cutting edge, serious blade adhesion and diffusion wear. The change of the cutting amount is not obvious to the temperature change, so reducing the cutting speed and increasing the cutting amount is a reasonable and suitable cutting method.
In order to improve the heat dissipation conditions and enhance the cutting edge when turning, the rake angle is generally 5 ° -9 °; in order to overcome the friction caused by springback, the flank of the cutter body is generally 10 ° -15 °; when drilling , Shorten the length of the drill bit, increase the thickness of the drill core and the amount of guide cone, the durability of the drill bit can be improved several times.
Indispensable cutting fluid, and water-soluble cutting fluid is more suitable. However, cutting fluids containing chlorine or other halogen elements and sulfur must not be used. Such cutting fluids will adversely affect the mechanical properties of titanium alloys.
For the easily deformable titanium alloy frame parts, in principle, it is recommended to carry out detailed measurements before CNC finishing, and carefully analyze and redistribute the processing allowance and re-check the processing coordinates according to the measurement results.
The milling process is followed by three-axis machining and then five-axis machining, and then face machining and hole machining. Maintain the uniformity of the CNC programming and processing coordinate system, and merge the procedures as much as possible to reduce flipping. All tools, fixtures and other devices in contact with the titanium alloy must be cleaned in advance. The cleaned titanium alloy parts should be protected from grease pollution and avoid direct contact with hands. Operators or inspectors should wear clean gloves for operation or inspection.