Joint zone evolution in infrared bonded steels with copper filler
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JOINING of steels for low temperature applications has traditionally been achieved with vacuum brazing at temperatures between 1120 ⬚C and 1150 ⬚C for hours using copper as the filler material.[1–6] Steels joined under such processing conditions are recommended for room-temperature applications, such as brake line hose connectors and steering wheels on automobiles. There are millions of steel parts prepared with vacuum brazing annually. Due to vacuum requirements, traditionally these parts are prepared by remote vacuum brazing plants and stored in a large quantity for assembly lines. Huge inventory spaces and cost are required. Previous studies have shown that with conventional vacuum brazing, due to long processing time, active penetration of molten copper along the grain boundaries of steel in brazing lead to the formation of cracks and failure of components.[1] The degree of intergranular attack of steels by molten copper increased with increasing time of processing.[2] Growth of columnar phase containing Fe, Cu, and C was observed in the joint. The joint strength increased with the increasing amount of the columnar phase.[5,6] These studies pointed out disadvantages of vacuum furnace brazing including vacuum requirements, flux requirements, long processing time, weakened steel parts due to the liquid copper attack, and high inventory cost. In the past ten years, a new joining technique using infrared was developed.[7–12] Infrared joining provides bonds that are superior or similar to those prepared with either the vacuum brazing or diffusion bonding processes as reported in the literature. The greatest advantages of infrared bonding over existing commercially available joining processes are its low cost and potential of being easily adapted to become a work station in the production line. This will significantly J.H. LI, Graduate Student, and R.Y. LIN, Professor, are with the Department of Materials Science and Engineering, University of Cincinnati, Cincinnati, OH 45221-0012. Manuscript submitted March 16, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS B
reduce the inventory cost for the parts that otherwise would have to be prepared separately and stored for assembly line needs. Other advantages include no vacuum requirement even for joining superalloys or titanium alloys, controlled joint interfaces with limited reactions, little to no base material property deterioration due to rapid processing, and potentials of localized heating for joining large machine parts. So far, infrared joining techniques of steels, nickel-based superalloys, titanium alloys, aluminides, silicon carbide, and titanium-matrix composites have been successfully developed. In this article, infrared joining of steels with copper as the filler material was investigated. II. EXPERIMENTAL A. Materials 1008 low carbon steel coupons of 20 ⫻ 5 ⫻ 2 mm in dimension were cut and polished to 1200 mesh for joining. Commercially available copper sheets (Alfa, Copper Foils, 0.25 mm thickness, 99.9985 pct pure) were rolled to 50 m in thickness
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