Investment Casting

Titanium alloys are difficult to cast due to their high reactivity; they will react with both the atmosphere and the casting mold. However, investment casting

Fig. 4.25. Titanium Investment Casting Source: The Boeing Company

procedures are now available, that in combination with HIP after casting, can produce aerospace quality near net shaped parts that can offer cost savings over forgings and built-up structure.21 An example of the complexity that can be achieved with investment casting is shown in Fig. 4.25. This casting replaced 22 parts resulting in a significant cost savings.

Although a large number of titanium alloys have been successfully cast, by far the majority of titanium casting is done with Ti-6-4 (about 90% of all castings), with CP titanium representing the majority of the other 10%. It should be noted that there are no specific titanium casting alloys, and the same compositions that are used for wrought products are also used for castings. This is due to the lack of problems, such as fluidity or lower mechanical properties, which have been encountered with the wrought compositions for other metals. The advantages of investment castings over wrought titanium are lower costs for near net shaped complex parts, shorter lead times, and the ability to prototype new parts at reasonable costs. In general, the more complex the part, the better are the economics of using a casting. Rapid prototyping, using processes such as stereolithography, can be used to generate patterns for casting from CAD files.

Cast titanium parts approach the mechanical properties of wrought product forms. Static strength is usually the same while ductility is somewhat lower. Fracture toughness and fatigue crack growth rate are often as good, or better, than for wrought material. While fatigue strength is lower, HIP processing is used to close internal porosity and improve fatigue performance as shown in Fig. 4.26.

103 104 105 106 107 Cycles to Failure

Fig. 4.26. Comparison of Cast and Wrought Fatigue Strength

103 104 105 106 107 Cycles to Failure

Fig. 4.26. Comparison of Cast and Wrought Fatigue Strength

In the investment casting process,22 shown in Fig. 4.27, a pattern of the part is produced from wax. The waxes are formulated to give smooth, defect-free surfaces, be stable, maintain tolerances, and have a relatively long shelf life. The wax patterns are then robotically dipped in a fine ceramic slurry that contains refractories such as silica or alumina. The coated patterns are then stuccoed with dry coarser particles of the same material to make the slurry dry faster and insure adhesion between the layers. The dipping and stuccoing process is repeated until the desired thickness is obtained, usually 6-8 times. Once the mold is completely dry, it is placed in an oven and the wax is melted out. The ceramic mold is then fired at about 1800° F. The titanium melt is produced by vacuum arc remelting titanium in a water cooled copper crucible before pouring it into the mold. Sufficient preheat of the melt and preheating of the molds is used to maximize flow to achieve complete mold filling. As-cast titanium has a microstructure typical of titanium alloys worked in the beta field, which has lower ductility and fatigue strength than equiaxed structures. Due to the slow cooling rate from the HIP temperature, titanium castings are often heat treated to refine the microstructure and eliminate grain boundary alpha, large alpha plate colonies, and individual alpha plates.

One of the problems with investment castings has been shell inclusions (Fig. 4.28), which are small pieces of the ceramic shell that flake off during casting and can cause contamination that adversely affects fatigue strength. Very extensive and expensive non-destructive inspection procedures have been

Wax Injection Wax Assembly Slurry Coating

Stuccoing

Wax Injection Wax Assembly Slurry Coating

Stuccoing

Dewaxing

Firing

Casting

Shell Removal

Dewaxing

Firing

Casting

Shell Removal

Finishing Inspection

Fig. 4.27. Investment Casting Process23

developed to detect this defect and others in castings. To eliminate surface contamination, castings are usually chemical milled after casting. Unfortunately, titanium investment castings have a tendency not to completely fill the mold during casting, particularly for large and/or complex castings. Weld repair of surface defects must be carefully done to avoid oxygen and hydrogen pickup. Repairs are usually done using GTAW with filler wire. ELI filler wire is often used to help minimize the potential for oxygen contamination. Weld repaired castings must be stress relieved after welding. Fortunately, test programs have shown that the fatigue properties of castings with properly conducted weld repairs are not degraded.

To improve the fatigue properties, all aerospace grade titanium castings are processed by HIP to close off any internal porosity. Typical HIP processing is done under argon pressure of 15 ksi at 1750° F for 2h. It should be noted

Fig. 4.28. Shell Inclusion Cross-Section

that HIP will collapse internal porosity but will not close off surface connected porosity, hence the need to repair those areas by welding prior to HIP.

While most Ti-6-4 titanium castings are supplied in the mill annealed condition, some are supplied in the beta-SOTA condition. In this heat treatment, a beta solution heat treatment at 1875° F in a vacuum for 1h is followed by rapid cooling to room temperature. The castings are then aged at 1550° F for 2 h followed by air cooling. While some aerospace specifications require this heat treatment, a round robin test program conducted by the major aerospace companies in the U.S. showed that it offered no major advantages over a standard mill anneal, i.e. heating to around 1550° F for 2 h followed by air cooling,24 except in high cycle fatigue (108 cycles) as experienced in some helicopter applications.

Was this article helpful?

0 0

Post a comment