Riassunto analitico
For metals, surface modification by laser remelting changes the materials’ phase composition and, consequently the functional material properties. Laser remelting is characterized by the melting and overheating of the materials, followed by the formation of a ledeburitic structure during solidification that contains various matrix and hard carbide phases (white solidification). The hardness of the remelted zone is increased, leading to enhanced wear properties and component lifetime. The main objective of this thesis is the correlation between microstructures and the final mechanical properties of the analyzed materials, in particular, hardness and wear resistance. Three different cast iron materials, LP7, VP6, and KV1, re-melted by a diode laser with three different pre-heating temperatures each (Room T, 250°C and 500°C), are analyzed. Furthermore, additional stress-relief heat treatment was performed on the specimens. Microstructural qualitative analysis of all samples with light microscopy, SEM and XRD were performed. Carbide content and secondary dendrites arm spacing measurements have been carried out through image analysis. Hardness measurements were performed to correlate the microstructure features of the laser melted zones (LMZ) to the final properties of the materials. Finally, abrasive wear tests were performed to check the correlation between hardness and wear resistance. Results show that the main influence on hardness is given by the matrix of the LMZ, while carbide content becomes relevant for the wear resistance. LP7’s LMZ shows a martensitic matrix when not pre-heated, while at 250°C and 500°C, martensite disappears and is replaced by pearlite and ferrite. Consequently, the hardness of the LMZ decreases as the pre-heating temperature increases. Likewise, wear resistance shows the same trend. Due to its alloying content, VP6’s LMZ shows a martensitic structure at all the three pre-heating temperatures tested. The hardness and wear resistance values are similar at room temperature, 250°C, and 500°C. Finally, KV1’s LMZ shows a pearlitic-ferritic matrix at all the three pre-heating temperatures, with some martensite at room temperature. The hardness decreases as the pre-heating temperature increases. The same trend is followed by wear resistance. Among the three materials, before stress-relief, VP6 is the hardest and most resistant to wear, followed by LP7 and, lastly, KV1. However, among the latter two, KV1 has a slightly higher wear resistance because of the higher carbon content. Stress-relief heat treatment is carried out to reduce the internal stresses caused by laser remelting. KV1 was heat-treated at 540°C, while LP7 and VP6 at 640°C, both for two hours. Microstructural qualitative analysis showed the presence of graphite in all the LMZ of LP7 and VP6, because of the higher heating temperature. Moreover, it was observed that martensite transformed into ferrite and secondary cementite. Therefore, hardness and wear resistance of these two materials drop to lower values than KV1. On the other hand, KV1 shows the same microstructure as before stress-relief, due to the lower heating temperature. Nevertheless, all three materials show a decrease in hardness, wear resistance and carbide content. As stress-relief cannot be avoided, a pearlitic matrix appears to be the most suitable to obtain increased final mechanical properties.
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