MATERIAL CHARACTERIZATION OF DIRECTED ENERGY DEPOSITION OF H13 TOOL STEEL AND FUNCTIONALLY GRADED H13-COPPER

Abstract

The aim of this study is to determine the laser directed energy deposition (DED) parameters for an H13-Cu functionally graded material (FGM). Monolithic steel tools have poor thermal conductivity, reducing the cycle times. This could be improved with the high thermal conductivity of Cu in a FGM structure. Research began with wrought H13 heat-treated at varying tempering temperatures to examine the influence on the microstructure and mechanical properties. DED H13 samples were produced by varying the scanning speed and powder feed rates to examine geometrical and microstructural influences. Several techniques were employed to characterize the materials which included surface roughness, optical and scanning electron microscopy, hardness, X-ray diffraction, energy dispersive X-ray spectroscopy, and scratch hardness. The top surface roughness of the samples was found to be greater than the side surface roughness. The addition of a draft angle to a vertical surface was shown to reduce the side surface roughness. The deposited material undergoes rapid solidification, producing a highly refined microstructure with near-theoretical density (99.7%). The as-printed multi-layer samples show a cellular dendritic structure with tempered martensite and alloy carbides in the lower layers. The top layers consist of fine lath martensite and retained austenite. Tempering the multi-layer samples results in a homogenized microstructure of tempered martensite and alloy carbides. There was variation in the Vickers hardness of the asprocessed samples due to the cyclic heating. Applying a heat treatment to the DEDprocessed materials reduced the hardness and variation. Scratch hardness was highest in the as-printed condition with a heat treatment reducing the scratch resistance. No trend was observed in the scratch response with changing the scanning speed, powder feed rate, or number of layers. Varying H13-Cu blends were printed on wrought H13 as well as a multi-layered FGM. Single layer and 3-layer tracks resulted in detrimental porosity and vertical cracking characteristic of solidification cracking. The multi-layered FGM samples showed transverse cracking and porosity. The microstructure consisted of tempered martensite, martensite, alloy carbides, retained austenite, and a Cu-rich matrix with scattered H13 particles. There is clear separation of the H13- and Cu-rich liquids due to the miscibility gap, aided with differential scanning calorimetry analysis. Printing onto C110 was unsuccessful using varied laser powers resulting in inconsistent beads.