Nanocrystalline TiC powder alloying and glazing of H13 steel using a CO2 laser for improved life of die-casting dies

摘要

Premature failures of die-casting dies used in the metal casting industry occur because of the damage caused by thermal fatigue, erosion, stress corrosion, and soldering on the die surfaces. In this work, the effects of two laser surface-treatment methods for the prevention of die failures were investigated. A 1500-W CO2 laser with round and line beam-shapes was employed to glaze H-13 steel substrate or alloy the substrate with TIC of various particle sizes (30 and 2 mum, and 300 nm). Laser parameters for the glazing and surface alloying processes were optimized, the criteria being a specified surface finish and integrity. The corrosion and erosion properties of laser-treated samples in aggressive casting conditions were evaluated by testing them in molten aluminum alloy A390. Laser-glazed and -alloyed specimens with mum-sized particles exhibited hardness 30-100% higher than that of heat-treated H-13 steel substrates. However, the hardness of specimens that were laser-alloyed with 300-nm particles was lower, approximately 25% of that of the substrate. The anomalous effects of nanocrystalline powder alloying could not be explained satisfactorily by the microstructural evidence obtained by the use of optical and scanning electron microscopy, and X-ray diffraction. However. it is hypothesized that some titanium dissolves in steel, promoting the formation of ferrite in preference to austenite at high temperatures, thereby decreasing the hardness. Laser glazing and alloying improved the resistance of H-13 steels to both corrosion and erosion, but a marked improvement occurred in the specimens alloyed with nanocrystalline powders. The beneficial effects of nanocrystalline alloying are attributed to smooth, crack-free, and tough surface layers, as well as to uniform and homogeneous microstructures. Laser surface processing of nanocrystalline materials is potentially important in the casting industry for improving die life and reducing downtime. (C) 2001 Elsevier Science B.V. All rights reserved. [References: 21]