An Investigation on Microstructure and Mechanical Properties of Nd:YAG Laser Beam Weld of Copper Beryllium Alloy
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PRECIPITATED-HARDENING copper beryllium alloy is widely used in various kinds of springs due to its low elastic module and high strength. This alloy is solution annealed to raise its formability and precipitation hardened (age hardened) to increase its strength. Figure 1 shows the binary diagram of Cu-Be and the behavior of C17200 alloy. The microstructure consists of a phase in the 650 C to 900 C temperature range. By quenching from this temperature range, the supersaturated a is produced. If the aging treatment for the copper beryllium alloy is carried out below the 550 C, the CuBe intermetallic phase would precipitate at the grain boundaries in the forms of GP zone, c¢¢, c¢, and c.[1] Few investigations have been carried out on the precipitation behavior of copper and copper alloys.[2–5] Their finding suggested the following precipitation sequences for the Cu-Be alloy: supersaturated a fi GP zone fi coherent c¢¢ fi semicoherent c¢ fi incoherent c. Era and Kishitake[4] studied the microstructure of agetreated alloy by transmission electron microscopy (TEM) bright field. Their TEM images revealed the S.A.A. AKBARI MOUSAVI, Associate Professor, and S.T. NIKNEJAD, Research Assistant, are with the School of Metallurgy and Materials Engineering, University College of Engineering, University of Tehran, Tehran 14395-515, Iran. Contact e-mail: [email protected] Manuscript submitted October 18, 2008. Article published online April 21, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
striations emerged within the a grains due to contraction of lattice parameter. The C17200 alloy exhibits two different behaviors during solidification. In the equilibrium conditions, the a phase nucleates and the alloy keeps its single-phase condition below the solidus line. If the solidification is carried out in the nonequilibrium condition, which is usual in any low energy fusion welding processes, it is more likely that b phase is formed by peritectic reaction. The b phase does not reserve its stability at room temperature and decomposes to a and c. Few investigations have been carried out on the microstructure and properties of copper and copper alloys weldment. Zalkind et al.[6] studied the effect of postweld age treatment on the electron beam weld of the C17200 alloy. They found no sharp increase on the hardness of weld metal after the age treatment due to precipitation of incoherent c phase. On the contrary, they showed that hardness of the heat-affected zone was raised significantly after aging heat treatments. The authors attributed the reasons to the severe thermal cycles formed during the welding and subsequent solution annealing occurring in the HAZ. Nd:YAg pulsed laser welding is known as an appropriate process for joining materials with the high thermal conductivity coefficient. Nd:YAG pulsed laser welding also enables welding of plates with little nominal thickness due to low heat input introduced in joint sections. In this research, butt welding of copper beryllium plate was carried out with the nominal thickness of 0.2 mm in order
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