Papers by Keyword: Hot Roll Sintering

Abstract: Mechanically milled austenitic stainless steel powder is applied to hot roll sintering (HRS) process. Microstructure and mechanical properties of the HRS material are investigated in detail. The mechanically milled powder has a bimodal structure with a severely deformed powder surface domain which is named as “Shell”, and an inner domain which is named as “Core”. The shell and core microstructure in the milled powder can be maintained even after sintering. As the result, microstructure of the HRS materials consists of a shell and core bimodal microstructure. Because severe plastic deformation mainly concentrates to the shell domain, a nano grain structure forms in the shell, while a coarse (meso) grain structure forms in the core. Such a nano / meso harmonic structured material demonstrates not only superior strength but also a large elongation. The mechanical properties of the HRS materials are strongly influenced by the nano / meso harmonic microstructure, such as grain size of the shell / core and the shell volume fraction. The shell has role of strength and the core has role of ductility. Thus, the nano / meso harmonic microstructure has been proved to be very effective to improve mechanical properties of structure materials.

Abstract: Grain refinement is well known to influence the mechanical properties of materials, especially the strength characteristics. The promising method for grain refinement is a SPD process and it produces the homogenized nano grain material which exhibits very high strength and limited ductility. Recently the grain refinement technique by the SPD in powder metallurgy (PM) field has received much attention. The SPD-PM process is one of new processes combining mechanical milling (MM) or alloying (MA), heat treatment and sintering processes. Microstructure of the SPD-PM materials is easily controlled by the MM condition, and hence we can intentionally make a heterogeneous microstructure. In the present study, commercially pure titanium, Ti-6Al-4V alloy and SUS316L stainless steel powders are applied to the SPD-PM process. These MM powders are sintered by Hot Roll Sintering (HRS) process. These SPD-PM materials demonstrate a heterogeneous microstructure and high strength and advanced plastic strain. The microstructure of materials consists of a shell and core hybrid microstructure, that is, a shell structure with nano grains and a core structure with work-hardened coarse grains. All of the materials fabricated by these processes demonstrate not only superior strength but also enough elongation. The mechanical properties are strongly influenced by the shell / core microstructure. The nano / meso hybrid microstructure by these processes has been proved to be very effective to improve mechanical properties.

Abstract: Mechanical milled SUS316L stainless steel powder is applied to hot roll sintering (HRS) process. Microstructure and mechanical properties of the HRS material is investigated. Microstructure of material produced by HRS process consists of the shell and core hybrid microstructure, that is, a shell structure with nano grains and a core structure with work-hardened coarse grains. The fine grain structure corresponding to the shell area has (austenite + sigma) nano duplex structure. Work-hardened structure in the core area composes of an austenite phase. The HRS material demonstrates not only superior strength but also enough elongation. Mechanical properties are strongly influenced by the shell/core structure, such as grain size, shell/core size and/or the volume fraction. The shell and core nano-duplex hybrid microstructure by the HRS process has been proved to be very effective to improve mechanical properties.

Abstract: An SUS316L and a Ti-6Al-4V alloy powders are treated by Mechanical Milling (MM) process, which is one of SPD processes, and then sintered by Hot Roll Sintering (HRS) process. The HRS process consolidates powder by hot rolling of an evacuated metal pipe filled with the powder at elevated temperatures. Those MM powders have a heavy deformed microstructure at the surface region and have a work hardened microstructure in the core region of the powder. In the surface region, a nano grain structure forms after the MM treatment in both materials. In case of the SUS316L powder, such a nano grain structure consists of an equiaxed nano ferrite (􀁄) grains which has transformed from nano austenite (􀁊) grains. Volume fraction of the 􀁄 phase decreases with distance from the surface of powder. During HRS the (􀁄 + 􀁊) nano-duplex structure changes to (sigma (􀁖) + 􀁊) nano-duplex structure by transformation of the 􀁄 to the 􀁖 phase. Thus, the SUS316L HRS material consists of a hybrid structure. That is, a (􀁖 + 􀁊) nano-duplex structure at the powder shell region, and a work hardened 􀁊 structure in the powder core region. In case of the Ti-6Al-4V MM powder, though no remarkable transformation occurs, a heavy deformed shell and work hardened core hybrid structure is also produced in the powder. By HRS the Ti-6Al-4V MM powder demonstrates a hybrid structure consists of an equiaxed nano grain structure and a coarse martensite structure. These two HRS materials indicate superior mechanical properties. Mechanical properties are improved by the HRS process. The proof stress and tensile strength in the SUS316L HRS material are x3.8 and x2.1 of the SUS316L conventional material, respectively. In the Ti-6Al-4V HRS material, they are x1.7 and x1.5 compared to the Ti-6Al-4V conventional material.