Successful joining of TMC components by diffusion bonding can be accomplished at pressures and temperatures lower than normal HIP runs. To process preconsolidated C-channels into spars, as shown in Fig. 9.23, they can be joined together in a back-to-back fashion. For secondary diffusion bonding in a HIP chamber, the parts are assembled and encapsulated in a leak free steel envelope or bag. Since the bag is subjected to the same high pressure and temperature as the parts, the ability of the steel bag to withstand the HIP forces is critical for success. If the bag develops a leak, the isostatic pressure is lost, and so is the bonding pressure. To minimize the risk associated with the extreme pressures and temperatures required for initial consolidation, lower temperatures and pressures can be used for secondary diffusion bonding. Lower temperatures and pressures reduce the risk of bag failures during the secondary HIP bonding cycle. However, since TMC is inherently stiffer than conventional sheet metal
Hard Tools TMC Spar Using
Fig. 9.23. Secondary Diffusion Bonding of TMC Spars
Hard Tools TMC Spar Using
Fig. 9.23. Secondary Diffusion Bonding of TMC Spars components, it generally requires higher pressures to achieve secondary diffusion bonding. A machined filler is placed in the corner to fill the void created by the C-channels. By not pinning the TMC components together, and allowing them to slide a little in relation to one another, complete bonding is achieved. Incomplete bonding or damage can occur if movement is not permitted. Successful bonding can be achieved with HIP pressures as low as 5 ksi at 1600° F for two hours.
Superplastic forming and diffusion bonding (SPF/DB) can be used to take advantage of elevated temperature characteristics inherent in certain titanium alloys. Structural shapes, which combine superplastic forming and bonding of the parent metal, can be fabricated from thin titanium alloy sheets to achieve high levels of structural efficiency. Components fabricated from TMC do not lend themselves to superplastic forming. However, since they retain the diffusion bonding capability of the matrix alloy, TMC components can be readily bonded with superplastically formed sheet metal substructures. A TMC reinforced SPF/DB structural panel retains the structural efficiency of TMC, while possessing the fabrication simplicity of a superplastically formed part. Two potential methods of fabricating TMC stiffened SPF/DB panels are shown in Fig. 9.24. The core pack, which forms the substructure, can be fabricated from a higher temperature titanium alloy, such as Ti-6Al-2Sn-4Zr-2Mo, rather than Ti-6Al-4V which is normally used in SPF/DB parts. Ti-6Al-2Sn-4Zr-2Mo has good SPF characteristics at a moderate processing temperature (1650° F). The final shape and size of the substructure is determined by the resistance seam welding pattern of the core pack prior to the SPF/DB cycle, which is inflated during the SPF/DB cycle with argon gas to form the substructure. More detailed information on SPF/DB of titanium can be found in Chapter 4 on Titanium.
Conventional non-destructive testing (NDT) techniques, including both through-transmission and pulse echo ultrasonics, can be used for the detection of manufacturing defects. Conventional ultrasonic and X-ray have the ability to find many TMC processing defects, including lack of consolidation,
TMC Facesheet ^
TMC Reinforced Structure
Fig. 9.24. Methods for Making SPF/DB TMC Reinforced Parts f TMC
TMC Reinforced Structure
Fig. 9.24. Methods for Making SPF/DB TMC Reinforced Parts delaminations, fiber swimming, and fiber breakage. Ultrasonics can find defects as small as 3/64 in. in diameter, with through-transmission giving the best resolution. Normal radiographic X-ray inspection can also be used to examine TMC components.
Titanium matrix composite is extremely difficult to machine. The material is highly abrasive, and tool costs can be high. Improper cutting not only damages tools but can also damage the part as well. Abrasive waterjet cutting, illustrated in Fig. 9.25, has been found to work well on TMC. Typical machining parameters are 45 000 psi water pressure, dynamically mixed with #80 garnet grit, at a feed rate of 0.5-1.2 ipm. The waterjet cutter has multi-axis capability and can make uninterrupted straight and curved cuts. A diamond cut-off wheel also produces excellent cuts. However, this method is limited to straight cuts and is rather slow. The diamond cut-off wheel is mounted on a horizontal mill, and cutting is done with controlled speeds and feeds. Because this method is basically a grinding operation, cutting rates are typically slow; however, the quality of the cut edge is excellent.
Wire EDM is also a flexible method of cutting TMC. EDM is a non-contact cutting method which removes material through melting or vaporization by high frequency electric sparks. Brass wire (0.010 in. in diameter) can be used at a feed rate of 0.020-0.050 ipm. The EDM unit is self-contained with its own coolant and power system. EDM has the advantage of being able to make small diameter cuts, such as small scallops and tight radii. Like waterjet cutting, the EDM method is programmable and can make uninterrupted straight or curved cuts.
Several methods can be used to generate holes in TMC. For thin components (e.g., 3 plies), with only a few holes, the cobalt grades of high speed steel (e.g., M42) twist drills can be used with power feed equipment; however, tool wear is so rapid that several drills may be required to drill a single hole. Punching has also been used successfully on thin TMC laminates. Using conventional dies, punching is fast and clean, with no coolant required. Punching results in appreciable fiber damage and metal smearing. However, most of the disturbed metal can be removed and the hole cleaned up by reaming. However, some holes may require several passes, and several reamers, to sufficiently clean up the hole. In some instances, the diameter of the reamer is actually reduced due to wear of the reamer cutting edges, rather than increasing the diameter of the hole. Punching is not used extensively because of the large number of fastener holes required in the internal portion of structures. In addition, load-bearing sections are normally too thick for punching.
Neither conventional twist drills nor punching will consistently produce high quality holes in TMC, especially in thicker material. The use of diamond core drills has greatly improved hole drilling quality in thick TMC components. High quality holes can consistently be produced with diamond core drills. The core drills are tubular with a diamond matrix built-up on one end. This construction is similar to some grinding wheels. A typical core drill and coolant chuck is shown in Fig. 9.26. The important parameters for successful diamond
Metal Matrix Composites Impregnated Diamond
core drilling are drill design, coolant delivery system, drill plate design, type of power feed drilling equipment used, and the speeds and feeds used during drilling. During drilling, the core drill abrasively grinds a cylindrical core plug from the material. Some fabricators even mix an abrasive grit with the water coolant to improve the material removal rate. Multiple drill set-ups on a TMC structure are shown in Fig. 9.27. The drill plate can hold several drill motors, which allows the operator to operate more than one at a time. The drill plates must be stiff enough to produce a rigid setup. A properly drilled hole can hold quite good tolerances, depending on the thickness of the TMC: ±0.0021 in. in 4 plies, ±0.0030 in. in 15 plies, and ±0.005 5 in. in 32 plies.
Another method for joining thin TMC components into structures is resistance spot welding. Conventional 50 kW resistance welding equipment (Fig. 9.28), with water cooled copper electrodes, has been successfully used to spot weld thin TMC. Fabricators often use conventional titanium (e.g., Ti-6Al-4V) to set the initial welding parameters. As with any spot welding operation, it is important to thoroughly clean the surfaces before welding. Initial welding parameters should be verified by metallography and lap shear testing.
Was this article helpful?