Bolts, screws, or welds are typical methods for joining dissimilar metals. Although each can produce strong joints, all have shortcomings. For example, it's impossible to weld together materials with vastly different melting points such as stainless steel and aluminum. Also, the high temperatures inherent to the process can degrade materials. Holes required for fasteners weaken joints, and the fasteners themselves may have different strength and thermal behavior than the clamped parts, further complicating design.
Eliminating these concerns is a less-conventional but highly effective method for joining different metals: explosion bonding. It uses a controlled detonation of ammonium nitrate and fuel oil to drive together two or more metals at speeds to 1,600 ft/sec creating contact pressures to 600,000 psi. The intense pressure fuses the metals and turns a few atomic layers of each to plasma. The plasma spurts out ahead of the angled collision zone effectively cleaning the surfaces prior to bonding. Bonds appear wavy because metals behave as viscous liquids under these conditions. Heating is highly localized and of extremely short duration so metals retain most of their original mechanical properties, even at the joint or bond line. The approach lets engineers focus on the best materials for a design rather than the limitations of a fastening or joining process.
For example, a part requiring a highstrength, corrosion-resistant mount able to readily conduct electrical current over one surface is made by bonding stainless steel and copper plate. Another example is a steelaluminum composite that is both strong and lightweight. This particular combination is highly regarded in the aerospace industry where launch costs can reach thousands of dollars per pound. Explosive bonding can also attach thin, diffusion-inhibiting interlayers such as tantalum and titanium, which then allows conventional weld-up installations. Not only is the process versatile, the resulting bonds are extremely strong and able to withstand significant thermal expansion. Moreover, bonds seal against ultrahigh vacuum, an important metric for many medical and semiconductor applications.
Although the high-collision force generally doesn't affect a material's bulk mechanical properties, it does change its shape. Softer and thicker materials tend to thin more from the pressure than harder and thinner sections. Consider an explosionbonded clad of copper and stainless for example: A copper plate that's 0.250 X 12.00 X 24.00 in. initially, might end up at 0.220 X 12.25 X 24.25 in., while the relatively harder stainless steel goes from 0.250 X 12.00 X 24.00 in. to 0.240 X 12.12 X 24.20 in.Flatness is another issue. Explosionbonded materials typically require postmechanical flattening. When clad systems contain one or more metals that are crack sensitive, such as titanium, molybdenum, or tungsten, flattening may have to be done at elevated temperatures or after a thermal stress-relief treatment. Flatness is controllable to about 0.020 in. over a 6 12-in. section 0.005 to 3.000 in. thick, and scales with section size. In other words, smaller sections can be made flatter.
Critically important are the bond lines themselves. Careful control of plasma flow and the resulting wave pattern at the bond line are key to quality bonds. This is accomplished by adjusting detonation velocity, explosive load, and interface spacing. However, the wave's inherent shape and peak-to-peak amplitude (0.020 to 0.040 in. typically) make it difficult to hold precise bond-line tolerances. Bond-line linearity varies with the particular material combination and thickness but, in general, can be held to ±.020 in. on flat composites and ±0.025 in. on cylindrical composites, although tighter tolerances are possible.
A bond line is identified by visual inspection and by taking ultrasonic thickness measurements of the area. "Knowing bond-line location is critical to subsequent machining operations because the lines act as control points," explains David Brasher, chief metal-lurgist for High Energy Metals,Port Townsend, Wash., speaking on behalf of partner firm, J&S Fabrication, also of Port Townsend. J&S NC machines the explosion-bonded composites to customer specs.
Bond lines and material properties dictate cutter speeds, as well as cut depths. For example, to machine through a copper-alloytantalum-stainless-steel bond, heat transfer, cutter speed, and depth, must all be controlled precisely to prevent gouging the cladding. Although exact settings are proprietary, cutters traverse areas other than bond lines at one speed and go slower across the bond lines themselves. Despite the complications of simultaneously machining different materials, it's possible to hold tolerances to ±0.002 in., even ±0.0002 in. in some cases.