be arranged relative to each other in configuration s which allowed the incident intensity to range from 6 to

A technique which employs high-intensity, pulsed lasers to bond metallic foils to different metal sub­ strates is reported. Laser-generated, large amplitude thermal and pressure waves heat and compress a coating /substrate combination and mix the two materials both by plastic or hydrodynamic motions, and by diffusion at rates determined by the elevated temperatures. The degree of mixing, the rate of cooling, and thus the depth, homogeneity and metallurgical structure of the bond depend on the laser intensity, fluence, and material prop­ erties. Optical microscopy and Robinson backscatter electron imaging of cross-sections of several such coat­ ings indicate that they are unusually well-bonded, suggesting good resistance to abrasion and corrosion, and that they exhibit unique distributions of elements. Here we report on an initial exploration of the wide range of material processing parameters offered by the use of pulsed lasers. In a series of surface coating experiments, metal foils, typically 25 ym thick, were bonded to dissimilar metal substrates under a variety of laser conditions by a technique which we have called "laser stamping." The laser stamping process makes use of both the heat and pressure7 supplied to the foil surface by a high intensity pulsed laser. The procedure is simple; under vacuum conditions, a foil sheet of the coating material is placed in contact with the substrate and irradiated with a laser pulse. Thermal and compressive waves are generated which travel through the materials at different velocities. If the thermal wave reaches the coating/substrate interface during irradiation, both materials will melt and, under the in­ fluence of the laser-induced pressure gradients, mix. If the laser pulse is of such short duration that it terminates before the thermal wave reaches the interface, the pressure wave may still cause the materials to mix by plastic flow, similar to the process of explosive bonding. The subsequent thermal wave, if sufficient­ ly strong, can then remelt the mixture and allow alloy formation. Thus, the bonding mechanism and resulting bond characteristics may be pre-determined by selecting the appropriate laser parameters. Experimental Technique Early experiments conducted in our laboratory demonstrated that aluminum, titanium, tantalum, and tungsten could be bonded to copper and steel substrates by laser stamping with a low energy (~ 15 J) laser focussed to an intensity of 8 x 109 W/cm2. Since the coatings produced in these tests were generally limited to areas of less than 1 mm2 by the constraints of laser energy, pulse duration, and desired intensities, two higher power­ ed lasers, located at Avco Everett Research Laboratory and Battelle Memorial Institute, were used to create larger coated areas and to study changes in the bonding mechanism with variations in laser and material prop­ erties. The characteristics of all three laser systems are summarized in Table 1. In each case, the laser was focused by a lens or copper mirror. The foils and substrates were mounted together and placed in the position relative to focus which yielded the desired laser flux. The experiments were conducted in vacuo (< 20 jam Hg) to maximize the peak laser-induced pressure and to inhibit oxidation of the metals.