Microstructural Analysis of the Laser-Cladded AISI 420 Martensitic Stainless Steel
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RODUCTION
AISI 420 martensitic stainless steel (SS) has excellent mechanical properties such as high tensile strength and moderate corrosion resistance. Properties of this steel grade can be altered by post-heat treatment.[1] AISI 420 SS is used in many diverse industrial applications, such as pressure vessels, mixer blades, cutting tools and medical components.[2] This stainless steel is considered to be one of the potential alloys for additive manufacturing of functional components using laser-cladding process. Laser cladding (LC), or direct energy deposition, is one of the additive manufacturing (AM) processes that utilizes laser power to melt the coaxial metallic powders (or wire) to coat part of a substrate or make a functional MOHAMMAD K. ALAM and AFSANEH EDRISY are with the Materials Engineering Graduate Program, Department of Mechanical, Automotive and Materials Engineering, University of Windsor, 401 Sunset Avenue, Windsor, ON N9B3P4, Canada. Contact e-mail: [email protected] JILL URBANIC is with the Industrial Engineering Graduate Program, Department of Mechanical, Automotive and Materials Engineering, University of Windsor, 401 Sunset Avenue, Windsor, ON N9B3P4, Canada. Manuscript submitted January 11, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
component using a layer-stacking strategy. LC is a complex metallurgical process that involves transient heat transfer and highly non-equilibrium solidification.[3] As the laser beam reaches the substrate, a significant amount of its energy is directly absorbed by the substrate and the powder particles, which then creates a melt pool on the substrate. The additional powder helps in conducting the heat from the meltpool.[4,5] Moreover, the surface tension gradient drives the fluid flow within the melt pool and penetrates into the substrate causing the energy transfer through a mass convection mechanism. During this process, the melted powder particles are swiftly mixed in the melt pool which combines three governing processes: (i) heat conduction, (ii) continuity, and (iii) momentum.[4,5] All three processes happen within a fraction of time so that the temperature and high-velocity fields in the melts create a highly non-equilibrium state during the rapid solidification. This can generate a very high cooling rate, up to 103–5 °C/s, due to (i) its controlled heat input, (ii) small- and thin-layer melt pool, and (iii) heat conduction to the bulk substrate.[6] Therefore, the microstructure generated in the laser-cladded coating is greatly influenced by the super-high cooling rate. Solid-state phase transformation, metastable phases or extended solid solutions are the characteristics of such rapid solidification.[7] Coatings with such microstructures generally
cannot be achieved by the conventional process. Hence, the use of laser cladding with 420 SS has been increasing recently for both coating and surface engineering applications in the pipeline as well as in the tool and die industries due to its excellent resistance to wear, corrosion, and high-temperature oxida
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