| In the field of automotive industry, replacing gray cast iron with lightweight aluminum alloy has become a cost-effective method for energy saving and emission reduction. However, the conventional Al-Si alloys in service are easy to wear. Plasma sprayed gray cast iron coating on aluminum alloy cylinder block surface can effectively improve the a nti-friction properties and reduce wear of the cylinder block. In the service process, the bonding strength of the coating is the key factor to determine the spalling failure of the coating. Based on formation characteristics of thermal sprayed coating, the microstructure and properties of the coating is mainly affected by the deposition behavior of individual splat; that is to say, the splat morphology and splat-substrate bonding formed mainly determine the microstructures and properties of the coating.In the present study, individual cast iron splat was deposited on polished aluminum surface by atmospheric plasma spraying technique(APS). The effects of the arc power, spray distance, and substrate preheating temperature on the morphology of the splat and splat-substrate bonding were studied. Moreover, the heat transfer process between the splat and the substrate was studied by a numerical calculation method to elucidate the formation mechanisms of the morphology and the splat-substrate bonding. Main research results are summarized as follows:The numerical calculation results show that increasing the splat thickness and the initial temperature of the substrate result in a reduction in the cooling rate of the splat, as well as an increase in the temperature at the splat-substrate interface. In addition, when the thickness of the cast iron splat is more than 1 μm, and the initial temperature of the aluminum substrate is above 250℃, the plastic deformation occurred in the substrate surface.Studies on splat morphology show that:(1) when the arc power ranged from 15~25 k W, and the spray distance ranged from 100~120 mm, most of the splats appeared radial or network splashing morphology; when the arc power increased to 30 kW, and the spray distance was reduced to 80 mm, the splashing of the droplets during flattening was greatly suppressed and the near disk-shaped splats with irregularly rippled periphery dominated. These results indicate that increasing the arc power and reducing the spray distance suppress the splashing.(2) When the substrate temperature was at room temperature, the splat showed an annular or network splashing morphology. However, the substrate preheating temperature increased to 130℃, the splashed splats number faction decreased to the lowest value of 18%, most of the splats were disk-shaped. As the substrate preheating temperature continued to be raised, the number of splashed splats increased again and presented a star-splashed shape with the central depression phenomenon. When the substrate temperature exceeded 290℃, the number fraction of splashed splats reached peak value of round 78%, and the layer splashing phenomenon appeared under this condition.Researches on the splat-substrate bonding show that:(1) arc power ranged from 15~25 k W, and spray distance increased from 100 to 120 mm, the splat-substrate bonding was weaker, resulting in the separation between the splat and the substrate. However, as the arc power increased to 30 k W, and the spray distance decreased to 80 mm, the splat-substrate bonding significantly improved with a mode of mechanical bonding.(2) With the substrate temperature was kept at room temperature, the splat-substrate bonding was weak. However, when the preheating temperature of the substrate increased to above 130 ℃, the splat-substrate bonding was improved. As the substrate temperature was above 190℃, the substrate surface exhibited the plastic deformation and central depression with metallurgical bonding. When the substrate was preheated to above 240℃, a barb-shaped structure in the splat periphery, which further enhances the splat-substrate bonding. |
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