| Hot forge dies contacted with workpiece frequently during forge service.They suffered action of pressure stress and thermal stress,which caused the failure of wear,facture and plastic deformation on the surface of the dies.Laser cladding technology is an effective way to improve the properties and reduce the production cost of enterprises by means of modifying the wear properties of forge dies.In this paper,TiB2+TiC reinforced Fe-based composite coatings were fabricated on the surface of 5CrNiMo die steel by laser cladding processing.Meanwhile,ultrasonic vibration and constant magnetic field were introduced to assist the laser cladding process to further improve the properties of the substrate.Microstructure evolution and properties of cladding coatings under different external-field assisted conditions were studied,and the influence of each external-field during laser cladding process was analyzed.The laser processing parameters were optimized according to the principle of producing a crack-free,continuous and relatively smooth appearance of the coating.The laser power,scanning speed and laser beam diameter used in the experiment were 1400W,7 mm/s and 4 mm,respectively.In addition,pre-placed coatings with an approximately 1.0 mm thickness were used in the text.The results showed that metallurgical bonding between coatings and substrate and relative smooth performance of the coatings were obtained by employing those optimized laser parameters.The phases of the cladding coatings are mainly composed of ?-Fe,Cr7C3,TiB2,TiC and Fe-Ni.In-situ formation TiB2 and TiC are dispersed in the cladding coatings,which effectively improve the hardness and wear resistance of cladding coatings.However,micro-cracks were found in the cladding coating due to the rapid melting and solidification of laser processing.It is found that phase structure of the cladding coatings have not change obviously after employing ultrasonic vibration during laser cladding.However,the distribution and size of structure of matrix and reinforcements are improved,as well as the mechanical properties of the cladding coatings are improved significantly by ultrasonic vibration.The results show that the cavitation,sonic flow and mechanical effect of ultrasonic vibration plays a role in breaking grains,changing solidification conditions and stirring molten pool,which causes the matrix structure and ceramic reinforced particles refined,and the distribution tends to be uniform,as well as forming a defect-free(such as porosity and cracks)coating.The effect of ultrasonic vibration enhanced with the increase of ultrasonic output power.However,micro-defects are found in the coating while ultrasonic output power beyond a certain value.The severe thermal effect promotes the cracks appear again in the cladding coating.The overall performance of the cladding coating tended to best with the ultrasonic power of 300W,and the highest hardness is 1201.1HV0.2,which is 13.35%higher than that without ultrasonic vibration.The wear mechanism of cladding coatings is abrasive wear dominated by micro-cutting.The wear trace is shallow,and the wear resistance is about 2.5 times that of the matrix.It shows that grains can be refined after introducing the constant magnetic field during laser cladding.Meanwhile,the refinement effect of magnetic increases with the increasing of constant magnetic strength.Besides,the distribution of reinforcements is also homogenized with the increase of thermo-electromagnetic convection.However,the constant magnetic field produce an obvious electro-magnetic braking effect on the molten pool at high magnetic field strength.As a result,the hardness and wear resistance of the cladding increase first and then decrease with the increase of magnetic field strength.When the magnetic field strength is 15 mT,the cladding coating possesses microhardness of 1253HV0.2,which is 19.71%higher than that of the cladding coating without assisting of constant magnetic field.Under the same wear test condition,the weight loss of the coating with magnetic assisting is 63.64%of the coating without constant magnetic field,and the wear resistance is 1.7 times of that of the substrate. |
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