Sepehr Hatami

Surface Preparation of Powder Metallurgical Tool Steels by Means of Wire Electrical Discharge Machining

The surface of two types of powder metallurgical (PM) tool steels (i.e., with and without nitrogen) was prepared using wire electrical discharge machining (WEDM). From each grade of tool steel, seven surfaces corresponding to one to seven passes of WEDM were prepared. The WEDM process was carried out using a brass wire as electrode and deionized water as dielectric. After each WEDM pass the surface of the tool steels was thoroughly examined. Surface residual stresses were measured by the X-ray diffraction (XRD) technique. The measured stresses were found to be of tensile nature. The surface roughness of the WEDM specimens was measured using interference microscopy. The surface roughness as well as the residual stress measurements indicated an insignificant improvement of these parameters after four passes of WEDM. In addition, the formed recast layer was characterized by means of scanning electron microscopy (SEM), XRD, and X-ray photoelectron spectroscopy (XPS). The characterization investigation clearly shows diffusion of copper and zinc from the wire electrode into the work material, even after the final WEDM step. Finally, the importance of eliminating excessive WEDM steps is thoroughly discussed.

Flow properties of tool steel powders for selective laser melting – influence of thermal and mechanical powder treatments

ABSTRACT In powder bed fusion additive processes the flow properties of the powder influence the quality of the final component and the efficiency of the process. In this investigation an attempt is made to identify flowability indicators which can describe the flow performance of the powder during the powder layering (i.e. recoating) step; common to all powder bed fusion processes. To this end, shear tests were performed by means of a powder rheometer. Bulk density, flow function and degree of cohesion were measured. The results suggest that there is a good correlation between the aforementioned parameters and the flowability of the powder during SLM processing. In addition, it was found that thermal treatments and tumbling enhance flowability. Thermal treatments were performed at 150, 200 and 250°C for a period of 10 min and in air.

Tempering of 3Cr–0.5Mo sintered steel: influence on mechanical properties

Abstract Prealloyed 3Cr–0·5Mo steel powder (FL-5305) is considered as an outstanding sinter hardenable grade. Although the FL-5305 powder, at conventional sintering conditions, renders excellent tensile strength, yield stress and hardness, it suffers from low toughness. In this study, with the aim of increasing toughness without a considerable loss of hardness, different tempering treatments have been evaluated. To this end, tempering treatments have been performed at 200, 400, 550 and 650°C for different times. The influence of tempering on hardness, tensile properties as well as impact energy has been investigated. As compared to the hardness (20 HRC) and impact energy (14 J) of a conventionally sintered material, sinter hardening plus tempering at 650°C for 30 min resulted in a significant increase of impact energy (17 J), while the hardness remained the same. Tempering at 400°C for 2 h led to temper embrittlement and hence the lowest impact properties.