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

Premature failures of die-casting dies used in the metal casting industry o ccur because of the damage caused by thermal fatigue, erosion, stress corro sion, and soldering on the die surfaces. In this work, the effects of two l aser surface-treatment methods for the prevention of die failures were inve stigated. A 1500-W CO2 laser with round and line beam-shapes was employed t o glaze H-13 steel substrate or alloy the substrate with TIC of various par ticle sizes (30 and 2 mum, and 300 nm). Laser parameters for the glazing an d surface alloying processes were optimized, the criteria being a specified surface finish and integrity. The corrosion and erosion properties of lase r-treated samples in aggressive casting conditions were evaluated by testin g them in molten aluminum alloy A390. Laser-glazed and -alloyed specimens w ith mum-sized particles exhibited hardness 30-100% higher than that of heat -treated H-13 steel substrates. However, the hardness of specimens that wer e 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 obtai ned by the use of optical and scanning electron microscopy, and X-ray diffr action. However. it is hypothesized that some titanium dissolves in steel, promoting the formation of ferrite in preference to austenite at high tempe ratures, thereby decreasing the hardness. Laser glazing and alloying improv ed the resistance of H-13 steels to both corrosion and erosion, but a marke d improvement occurred in the specimens alloyed with nanocrystalline powder s. The beneficial effects of nanocrystalline alloying are attributed to smo oth, crack-free, and tough surface layers, as well as to uniform and homoge neous microstructures. Laser surface processing of nanocrystalline material s is potentially important in the casting industry for improving die life a nd reducing downtime. (C) 2001 Elsevier Science B.V. All rights reserved.