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Selective laser melting (SLM) is an additive manufacturing technique that can be used to make the near-net-shape metal parts. M2 is a high-speed steel widely used in cutting tools, which is due to its high hardness of this steel. Conventionally, the hardening heat treatment process, including quenching and tempering, is conducted to achieve the high hardness for M2 wrought parts. It was debated if the hardening is needed for additively manufactured M2 parts. In the present work, the M2 steel part is fabricated by SLM. It is found that the hardness of as-fabricated M2 SLM parts is much lower than the hardened M2 wrought parts. The characterization was conducted including x-ray diffraction, optical microscopy, scanning electron microscopy, and energy dispersive x-ray spectroscopy (EDS) to investigate the microstructure evolution of as-fabricated, quenched, and tempered M2 SLM part. The M2 wrought part was heat-treated simultaneously with the SLM part for comparison. It was found the hardness of M2 SLM part after heat treatment is increased and comparable to the wrought part. Both quenched and tempered M2 SLM and wrought parts have the same microstructure, while the size of the carbides in the wrought part is larger than that in the SLM part. [ABSTRACT FROM AUTHOR]

In this study, TiCrAlCN/TiAlN multilayer coatings were deposited on M2 high speed steel substrates by the Closed-Field Unbalanced Magnetron Sputtering system. The chemical composition, microstructure, morphology, mechanical and high temperature wear resistance properties of the coatings were characterized, analyzed and compared to the substrate. The high temperature wear tests were carried out under a load of 2 N at the lap (wear test distance) of 50 m and in dry sliding condition at Room Temperature (RT), 150, 300, 450, and 600 °C on atmospheric conditions. It has been found that the TiCrAlCN/TiAlN multilayer coatings have a higher wear resistance than the M2 substrate. The stable friction behavior and low friction tendency was determined at 600 °C. When the test temperature increased, the wear rates decreased. Narrow and smooth wear tracks and also the lowest wear rate were obtained at 600 °C. [ABSTRACT FROM AUTHOR]

Cryogenic treatment is an effective process to enhance the wear resistance of work pieces. Influences of cryogenic temperature, holding time and cooling rate on high-temperature wear behavior of M2 steel were studied, using a computerized ball-on-disc wear tester (HT1000). The results of the experimental tests showed that lowering the cryogenic temperature and prolonging the holding time, the wear resistance increased until being saturated. The optimal treatment condition was identified as cryogenic treatment=77 K, holding time=24 h, and the cooling rate = 2 K/min. However, there is no obvious difference in the friction coefficient among samples experienced different treatments. The specimen of M2 steel experienced traditional heat treatment more involved adhesive wear, whereas that experienced cryogenic treatment showed more oxidative wear during high-temperature wear testing. Microstructure features of worn surface were analyzed for understanding underlying mechanisms. [ABSTRACT FROM AUTHOR]

Abstract: Nanocomposite coatings of Cr–Si3N4 exhibiting low friction and high toughness were prepared on plasma nitrided AISI M2 steel substrates using an unbalanced magnetron sputtering system. The surface morphology and cross-sectional microstructure of the Cr–Si3N4 nanocomposite coatings were studied using field emission scanning electron microscopy (FESEM) techniques. Cr–Si3N4 nanocomposite coatings prepared at 48 at.% Cr exhibited a dense microstructure with nanoindentation hardness and toughness values of 18 GPa and 2.0 MPam½, respectively. Nanoscratch measurements indicated that Cr–Si3N4 nanocomposite coatings exhibited good adhesion with a maximum critical load of 150 mN. Ball-on-disc reciprocating tests at a load of 2 N showed that Cr–Si3N4 nanocomposite coatings prepared at 48 at.% Cr exhibited an average friction coefficient of 0.30. FESEM studies of the wear tracks indicated that there was no significant wear loss and the Cr–Si3N4 nanocomposite coatings exhibited only mild wear due to oxidation. [Copyright &y& Elsevier]

Abstract: Effects of cooling slope angle and the temperature of molten metal on the globular structure of M2 high speed steel after holding at the semisolid state have been investigated. The globular structure was achieved by pouring the molten metal at 1595°C on the ceramic cooling slope with the length of 200mm and the angle of 25°. The globular structure of M2 high speed steel in the form of rolled–annealed and as cast condition after holding at semisolid state has been achieved. The size of globular grains of cooling slope sample was smaller than that of the rolled–annealed and as cast samples. Solid particles of rolled–annealed sample after holding at semisolid state had better roundness compared with cooling slope sample. Dissolution of carbides in the austenite phase at grain boundaries leads to formation of globular particles in the semisolid state. MC-type and M6C-type eutectic carbides reprecipitate during cooling cycle along grain boundaries. [Copyright &y& Elsevier]

The globular structure of M2 high speed steel in the rolled - annealed and as cast conditions was investigated in the semisolid state. Metallographic observations resulted in globular austenite particles that were surrounded by a liquid phase. Dissolution of various carbides in the austenite phase at semisolid temperatures led to grain boundary liquation and formation of near-spherical solid grains in a liquid matrix. Therefore, at the semisolid state, the solid particles were free from carbides. MC- type and M6C- type eutectic carbides re- precipitated at the grain boundaries during cooling of the samples from the semisolid temperature. The variation of shape factor versus holding time and holding temperature was examined. A transition value for shape factor changes in high speed steels was achieved. The growth rate constants of the Ostwald ripening and the coalescence mechanisms were calculated by using the experimentally determined rate constant. It was observed that less liquid droplets were enclosed inside the solid particles compared with non-ferrous alloys. Besides, it has been shown that at high solid fraction, the Ostwald ripening mechanism plays a prominent role in the coarsening phenomenon in comparison with the coalescence mechanism. Grains can rotate and arrange low misorientation with each other at high liquid contents, therefore low energetic grain boundaries form between these grains. These grain boundaries play an important role in the coalescence mechanism. [ABSTRACT FROM AUTHOR]

Abstract: In order to develop coatings to be used in dry machining aluminum alloys, it is necessary to have a better understanding of the factors that influence transfer and adhesion of aluminum to the surfaces of coatings that are potential candidates for this function. In this work, the material transfer and adhesion phenomena between 319 Al alloy samples and different coatings deposited on M2 type steel substrates were examined using a pin-on-disc type wear machine. The coatings included titanium diboride (TiB2), CrN, titanium nitride based compounds, (TiN, TiCN, TiAlN) and uncoated M2 steel discs. The effects of sliding speed, sliding distance, test temperature and working atmosphere were studied. Sliding distances were kept short because the focus of the work was on the initial transfer behavior. The effect of surface roughness was examined using two TiB2 coatings; one with an R a value of 16nm and the other with a large R a of 83 nm. The morphology of sliding tracks was examined using a scanning electron microscope (SEM). With the aid of image analysis software, the SEM images were used to rank the coatings according to the amount of aluminum transferred and the loose debris generated during the tests. In general, the TiB2 and TiCN coatings exhibited the least amount of aluminum transfer on their surfaces compared to the other coatings. TiN and CrN exhibited the largest amount of aluminum transfer, but the amount of loose debris generated on the surfaces of these two coatings was small. Increasing the sliding speed from 0.12 to 0.6 m/s led to a decrease in the amount of aluminum transferred to the surfaces of all the coating tested. Increasing the test temperature to 160 °C did not significantly change the amount of aluminum transferred to TiB2 and TiCN coatings. However TiN, TiAlN, CrN and M2 steel picked up significantly less aluminum compared to that at room temperature. When tested under argon atmosphere, all coatings (except TiB2) exhibited a sharp decrease in the amount of aluminum adhered to their surfaces suggesting that the presence of oxygen and water vapor promoted aluminum adhesion to these coatings. The amount of aluminum transfer to the TiB2 coating was a weak function of the inertness of the environment, which was attributed to the high chemical stability of TiB2. Surface roughness played a critical role in aluminum transfer, especially in the early stages of sliding of TiB2. [Copyright &y& Elsevier]