Mechanical and tribological characterization of additive manufactured Co-alloyed tool steels aimed for cutting tool bodies

Sammanfattning: A drill tool body operates under tough cutting conditions where it is subjected to severe rubbing at the drill flute margin by the sliding chips. The tool wear caused by the contact stresses of the high-speed sliding chips is one of the common damage mechanisms observed in drill tool bodies. The present investigation is aimed at evaluating the wear properties, mechanical properties and tribological characteristics of the selective laser melting (SLM) produced drill bodies of Co-alloyed tool steels. Two different maraging steel powders, Modified 18Ni300 and Osprey MAR were the materials of interest in this work. A ranking was obtained after a detailed examination to select the most suitable cutting tool body material for drilling application. The microstructures of SLM-produced materials after aging were carefully characterized and analyzed. The result shows that both the materials are composed of fine dendritic cellular structures and even exhibit melt pool boundaries. The microhardness values taken on the as-polished samples indicate that Modified 18Ni300 is marginally harder than Osprey MAR. The material pick-up tendency, friction characteristics and wear properties of the two material samples at room temperature were also evaluated using a scratch tester and a pin-on-disc tribometer. The wear volume for the scratch test and the pin-on-disc test was quantified by a 3D optical profilometer. The overall coefficient of friction (COF) and the wear volume was considerably higher for Osprey MAR than Modified 18Ni300 in scratch testing. The increase in COF for Osprey MAR can be attributed to the build-up edge adhered to the moving stylus. Wear characterization of the scratched surface shows secondary plowing which validates the adhesion tendency of Osprey MAR. The results from the pin-on-disc test conform to the scratch results displaying marginally lower COF and wear volume for the Modified 18Ni300 disc specimen. The higher volume loss of pin manufactured from workpiece material SS2541 used against Modified 18Ni300 disc substantiates the better wear characteristics of this material. Characterization of wear on the surface and cross-section of disc samples suggests that the total wear is the result of adhesion by delamination and build-up edge, abrasion, and cellular fracture at the interacting interface. Machining application tests were also conducted to study the chip wear characteristics and facilitate the ranking of the materials concerning wear resistance. Chip breaker wear test was performed with chip breakers of two different surface topography, milled and ground, to evaluate the effect of surface texture and roughness on the wear behavior. The result shows adhesion on the chip breaker surface for both the materials of interest. The wear is higher in Osprey MAR than Modified 18Ni300 in both milled and ground conditions. A series of actual drill tests from SLM-produced drill bodies indicates wear at the drill flute margin. The drill body material at the chip flute margin is abraded by the sliding chips for Osprey MAR exhibiting greater wear than Modified 18Ni300 in which case the flute margin is intact. Adhesion is also seen at the flute surface for both materials. EDS analysis undertaken at the site of wear confirms the adhered material is of the workpiece SS2541. The results from all the aforementioned tests suggest that Modified 18Ni300 has better wear resistance than Osprey MAR. It also indicates that the cellular microstructure of SLM-produced maraging steels is not suitable for sliding wear resistance.