| In the practical production, the failure of mechanical components is closely related to the surface properties of the components. Surface modification treatment is a promising solution to improve the surface property of the materials. Laser treatment, as a new surface treatment method, owns many merits compared to the traditional surface treatment methods due to its high power density, fast healing rate and cooling rate, minor heat influence on the substrate. As a novel materials system beyond traditional metallurgical design, the high entropy alloys (HEAs) consist of multiple principal elements in equiatomic or close-to-equiatomic ratios. In HEAs, simple solid solutions with FCC or BCC structure tend to form with the absence of intermetallic compounds and elaborate phases during solidification.In this paper, the low-carbon steel and HI3 steel were modified using high entropy alloys by laser surface alloying and laser cladding techniques, respectively. A Nd:YAG pulsed solid laser device was used in the study and the laser parameters were optimized. The cross-sectional microstructure, phase assemblages, micro-hardness and abrasive resistance of the laser alloying layer and cladding layer were characterized with optical microscopy, scanning electronic microscopy with EDS, X-ray diffractometer, micro-hardness tester and high temperature and high speed abrasion tester.The multipass lap fabrication parameters for the laser alloying of low-carbon steel were obtained:current 275A, scanning rate 5.5mm/s, defocusing amount-14mm, whereas for the laser cladding of low-carbon steel and H13steel:current 275A, scanning rate 3mm/s, defocusing amount-14mm. The high entropy alloy coating with good microstructure and property was prepared on the low-carbon steel and H13 steel. The coating is compact with a few cracks and well bonded with the substrate.The laser alloying layer and cladding layer own the typical microstructure of laser rapid solicitation. The laser alloying layer is a hybrid structure composed with planar, columnar and dendritic crystals; planar crystal exists at the interface of the alloying layer and substrate; the bottom and middle side of the alloying layer are mainly composed of columnar and dendritic crystals which grow perpendicularly to the bonding face, in the form of "band" structure. The surface of alloying layer is dendritic crystals which are in disordered direction. The laser cladding layer of the low-carbon steel is mainly composed with fine dendritic crystals. The laser cladding layer of the H13 steel is also a hybrid structure composed with planar, columnar and dendritic crystals. Planar crystals exist at the interface of the cladding layer and substrate. The bottom and middle side of the cladding layer are mainly composed of columnar and dendritic crystals which grow perpendicularly to the bonding face. The surface of cladding layer is dendritic crystals which are in disordered direction. The laser alloying layer of the low-carbon steel is composed of Fe and Cr7C3. The existence of O7C3 is the main contributor to the enhancement of the hardness of the alloying layer. The highest hardness of the alloying layer is about 650HV, about four times of that of the low-carbon steel substrate. The laser cladding layer of the H13 steel is mainly composed of solid solution with BCC and FCC structure. The hardness of the cladding layer is enhanced mainly through the refining strengthening and solidification strengthening mechanism. The highest hardness of low-carbon steel and H13 steel are 639HV and 476HV, which are four times of that of the of low-carbon steel and two times of that of the HI3 steel substrate. The results of high-temperature abrasion test demonstrated that the abrasive resistance of alloying layer and cladding layer was greatly enhanced compared with the substrate. The weight loss of the low-carbon steel substrate is 2.0 times of that of the alloying layer and 2.7times of that of the cladding layer after 4000r (16min) of test. The weight loss of the low-carbon steel substrate is 1.60 times of that of the alloying layer and 1.65times of that of the cladding layer after 8000r (32min) of test. Meanwhile, the weight loss of the H13 substrate is 1.90 times of that of the cladding layer. The abrasion mechanisms of the low-carbon steel and H13 steel substrate is adhesive wear, whereas the main abrasion mechanism of alloying layer and cladding layer is abrasive wear. The formation of Cr7C3 hard particles and solid solution is the main reason for the enhancement of the abrasive resistance of the alloying layer and cladding layer, respectively. |
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