Composition control in laser surface alloying
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I.
INTRODUCTION
LASER surface alloying is an exciting new process spawned by the advent of high power continuous lasers. It is the process o f modifying the surfaces of relatively inexpensive substrates by adding small quantities o f alloying elements to a pool o f molten metal produced by local melting using an intense, collimated heat source. With proper combination o f alloying element and processing conditions, a surface may be tailored to accommodate most applications. Since the earliest attempts to produce surfaces with locally modified compositions using lasers, l a s e r surface a lloying (LSA) has received a good bit of attention.1 The interest arose as LSA enjoyed many advantages over conventional coating processes. After surface alloying, due to the small heat affected zone, bulk characteristics of the substrate are retained, scarce alloy materials are conserved, a high degree of adhesion is obtained between the substrate and coating, and the high cooling rates available open up the possibility of generating novel microstructures. Laser surface alloys are used in applications that are composition and structure sensitive, such as those relating to corrosion or wear. Thus, laser alloyed microstructures are of considerable metallurgical and engineering interest. Development o f laser alloyed structures is influenced by the nature and relative amounts of the alloyed species, their distribution, and the cooling rates that occur during laser processing.The precise combination of these factors is determined by processing conditions. A knowledge o f the effects o f the many process variables is thus desirable to gain a fundamental understanding o f laser alloyed structures. This information will help to tap the vast potential o f LSA for generating surfaces to suit diverse applications. The feasibility o f producing a highly alloyed, fairly uniform laser alloyed zone (LAZ) using high power CW CO2 lasers on ferrous substrates is well established.2'3'4 Salient features of LSA have been identified,2'4 and some information on the effect o f varying the traverse speed on the chroT. CHANDE, Graduate Research Assistant, Department of Metallurgy, and J. MAZUMDER, Assistant Professor, Department of Mechanical Engineering, are both with the University of Illinois, 1206 West Green Street, Urbana, IL 61801. Manuscript submittedMay 7 , 1982. METALLURGICAL
TRANSACTIONS B
mium content and the width and depth o f LAZ is available.3 A limited effort to optimize processing conditions during production of wear resistant coatings by LSA has also been mades (see Table I). Also, models are available to explain surface topographical e f f e c t s .6'7 A timely surveyI o f the state o f the art found no systematic study o f LSA to establish the effects of the many process variables on the dimensions of the LAZ, its solute content and microstructure, and the resultant metallurgical and mechanical properties. Reproducibility o f the process remains unassessed, while interactions between process variables have gone unexamined. A study
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