Emission spectroscopy of plasma during laser welding of AISI 201 stainless steel
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I. I N T R O D U C T I O N
THE
loss of volatile alloying elements from the weld pool is a serious problem in the laser welding of many important engineering a l l o y s If an alloy contains one or more volatile components, its fabrication using a high power density laser beam leads to significant vaporization of alloying elements and inadequate control of composition and properties of the weld. Pronounced changes in the alloy composition due to laser welding of important aluminum alloys m and high manganese stainless steels 121 are familiar examples of this problem. In a recent publication, f3[ we discussed the mechanism of alloying element vaporization during laser welding of high manganese stainless steels. The vaporization involves transport of alloying elements from the interior of the weld pool to the surface, aided by the recirculating fluid motion in the weld pool. It was demonstrated that for a given temperature distribution at the weld pool surface, the rate of vaporization was significantly influenced by the presence of plasma. Indeed, plasma is known to influence the chemical nature of the molten metal surfaces as evidenced by the recent measurements I4~ of interracial tensions between low pressure argon plasma and molten iron and copper. The importance of the role of plasma in welding is well documented in the literature. Weld properties are known to be influenced by the characteristics of the plasma formed during welding.iS] Most of the previous work on the characterization of plasma was undertaken to identify the species present in the welding arcs and to determine the arc temperature using emission spectroscopy. During arc welding of various iron-based alloys, spectral lines of iron, manganese, chromium, and nickel were identiffed. E6-~21Mills I~3,~41 demonstrated that the temperature of the welding arc was in the range of 5000 to 6000 K M.M. COLLUR, formerly Graduate Student, Pennsylvania State University, is with Allegheny Ludlum Steel Corporation, Brackenridge, PA. T. DEBROY, Associate Professor, is with the Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802. This paper is based on a presentation made in the T.B. King Memorial Symposium on "Physical Chemistry in Metals Processing" presented at the Annual Meeting of The Metallurgical Society, Denver, CO, February, 1987, under the auspices of the Physical Chemistry Committee and the PTD/ISS. METALLURGICAL TRANSACTIONS B
during G T A welding of 21Cr-6Ni-9Mn (weld current: 100 amperes; electrode gap: 0.06 inch) and 304L stainless steels. Glickstein 171estimated the arc temperature to be 11,000 K near the cathode and 8000 K toward the anode for a 100 amperes G T A welding arc with a 2 m m arc gap during welding of Ni-Cr-Fe alloy (alloy 600). As a result of the previous w o r k [6-14] o n arC welding of iron-based alloys, the species present in the welding arcs and the arc temperature range are now fairly well understood. However, such information has been rather scarce for laser processing.
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