R. Bidulský

Effect of High-temperature Sintering and Severe Plastic Deformation on the Porosity Distribution

The paper focused on the effect of high-temperature sintering along with the back pressure equal channel angular pressing (ECAP-BP) on the porosity distribution in powder metallurgy (PM) aluminium alloys. Analyses of the dimensional and morphological parameters indicate that high-temperature sintering couples to ECAP-BP lead to porosity reduction and improve pore morphology. ECAP-BP influences the porosity distribution in terms of the severe shear deformation involved and therefore influenced the pore morphology along with pore distribution.

Stainless Steels Sintered Form the Mixture of Prealloyed Stainless Steel and Alloying Element Powders

The aim of the presented paper is to describe the sintered duplex stainless steels manufactured in sinter-hardening process and their structural and mechanical properties. Duplex stainless steels were obtained through powder metallurgy starting from austenitic 316L or ferritic 410L prealloyed base powders by controlled addition of alloying elements powder. Prepared mixes were compacted at 700MPa and sintered in a vacuum furnace with argon backfilling at temperature of 1240°C for 1h. After sintering different cooling cycles were applied: rapid cooling (6°C/s) using nitrogen under pressure and slow cooling (0.1°C/s) with furnace in argon atmosphere. Produced sintered duplex stainless steels were studied by scanning and optical microscopy and EDS chemical analysis of microstructure components as well as X-ray analysis. Mechanical properties were studied through tensile and three-point bending tests and Charpy impact test. It was demonstrated that austenitic-ferritic microstructures with regular arrangement of both phases and absence of precipitates can be obtained with properly designed powder mix composition as well as sintering cycle with rapid cooling rate. Produced sintered duplex steels show good mechanical properties which depend on austenite/ferrite ratio in the microstructure and elements partitioning (Cr/Ni) between phases. The optimal mechanical properties were obtained for compositions based on ferritic 410L powder where the balanced distribution of α and γ is present and the tensile strength can reach value about 500MPa with 16% of elongation and impact energy about 120J. The precipitations of hard intermetallic σ-FeCr phase take place when sintering with slow cooling cycle what cause substantial decrease of plastic properties, including reduce of elongation to 7% and in particular decrease of impact energy to 68 J.

The Production of Cracks Evolution in Continuously Cast Steel Slab

Abstract The prediction of evaluation a U- and V-shaped cracks on the surface and subsurface of a continuously cast steel slab, is investigated during hot rolling. The numerical simulation is carried out by means of FE-code DEFORM 3D. Therefore, an algorithmic decision tree was developed by the C 4.5 program and applied in prediction of surface and subsurface defects behaviour during the numerical simulation of hot rolling. Cracks were selected as a transversal and longitudinal to the rolling direction. In addition to the transversal and vertical U- and V-shaped cracks on the surface; subsurface defect, referred as “circle hole”, on the side part of workpiece were evaluated. In terms of surface defects evolution, U-shaped cracks show a deteriorating influence during hot rolling. On the basis of the algorithmic decision tree established, the prediction of cracks evolution (defined before plastic deformation process) during hot rolling is examined.

Effect of Severe Plastic Deformation on the Properties and Structural Developments of high purity Al and Al-Cu-Mg-Zr Aluminium Alloy

Demands of industry producers are to find new forms and facilities for appropriate properties of structural parts suitable for different miscellaneous structural applications in the civil, automotive and aircraft industries. With respect to these facts, aluminium alloys find a wide variety of uses due to their remarkable combination of characteristics such as the low density, the high corrosion resistance, high strength, easy workability and high electrical and heat conductivity. The traditional process is to obtain the improvement in the mechanical properties of aluminium alloys through the precipitation of a finely dispersed second phase in the matrix. This is accomplished by a solution treatment of the material at a high temperature, followed by quenching. The second phase is then precipitated at room or elevated temperatures. For aluminium alloys this procedure is usually referred to as age hardening and it is also known as precipitation hardening (Michna et al., 2007); (Mondolfo, 1976). Conventional forming methods are ineffective in the achieving of favourable properties area of produced parts, adequate to structural properties; moreover through them only limited levels of structural and strength-plastic characteristics can be obtained. The solution may be non-conventional forming methods (Kvackaj et al., 2005), (Kvackaj et al., 2004), (Kvackaj et al., 2010 a) as well as Severe Plastic Deformation (SPD), such as more preferable are equal channel angular pressing ECAP, (Valiev & Langdon, 2006), (Valiev et al., 2000) to obtain results structured at the nm level. A combination of high strength and ductility of ultrafine polycrystalline metals, prepared by SPD, is unique and it indeed represents interesting cases from the point of view of mechanical properties (Chuvil’deev et al, 2008); (Zehetbauer et al., 2006); (Han et al., 2005) ;(Ovidko, 2005); (Meyers et al., 2006); (Kopylov & Chuvil’deev, 2006); (Zehetbauer & Estrin, 2009). In the past decade, the research focused on to strengthen Al alloys without any ageing treatment, via SPD (Kvackaj et al., 2010 b). The finite element method (FEM) is a proven and reliable technique for analyzing various forming processes (Kvackaj et al., 2007); (Kocisko et al., 2009); (Li et al., 2004); (Leo et al., 2007); (Cerri et al., 2009), (Figueiredo et al., 2006); (Mahallawy et al., 2010); (Yoon & Kim, 2008), in order to analyze the global and local deformation response of the workpiece with

Response of the Cr-alloyed PM Steels on Vacuum Sintering and Heat Treatment

Abstract The present paper deals with the evaluation of different vacuum heat treatments on the fracture surfaces of a low alloyed sintered steel in correlation with the mechanical and plastic properties achieved. The heat treatment conditions consist of the sintering process in vacuum furnace at 1393 K for 1800 s with an integrated final tempering at 473 K for 3600 s. The average cooling rates were calculated in the range of 1393 K to 673 K and were 0.1 K/s, 0.235 K/s, 3 K/s and 6 K/s, respectively. Vacuum heat treatment performed is supporting bainite/martensitic microstructure with increasing cooling rates; it provides a marked increase in strength coupled to a decrease in ductility. The typical microstructure appearances on the fracture surfaces show that microstructure-fracture developments are closely associated with strength/plastic properties. The fracture surface mode at higher magnification revealing two main micromechanisms of fracture: brittle (transgranular cleavage fracture) and ductile (dimples and plastic deformation by slip). Finally, the cooling rate of the vacuum furnace in order of 3 K/s is sufficient to obtain suitable properties of strength and plasticity which can however be accepted for several applications.

Wear Characteristics of Vacuum Sintered Steels

The present paper is focused on the wear characteristic of vacuum sintered Cr-Mo-[Mn]-[Cu] steels. The effect of chemical composition and the processing conditions in a vacuum furnace were evaluated. In such furnaces the cooling rate is generally determined by the pressure of the gas (N2) introduced into the chamber, the average cooling rates were calculated in the range of 1240°C to 400°C. The wear characteristics were analyzed as function of the processing and microstructures of the tested alloys through pin on disk test. Sintering of specimens in vacuum together with rapid cooling resulted in the formation of dominant martensitic microstructures with some small bainitic areas. The effect of both surface hardness and microstructure on the wear behaviour of the investigated steels shows the relation between the hardness and the wear rate. The influence of processing condition on the amount of martensite is also presented.

Microstructure, Fracture and Mechanical Properties of ECAPed Aluminum P/M Alloy with Respect to the Porosity

The main aim of this paper is to investigate the effects of different processing conditions on the behavior of a P/M (Powder Metallurgy) aluminum alloy with respect to the microstructure, fracture and mechanical properties. Moreover, the evolution of porosity as a consequence of pressing, sintering and ECAPing processes was investigated. A commercial Al-Mg-Si-Cu-Fe powder was used as material to be investigated. Different compacting pressures (400, 500, 600, 700 MPa) were applied. Specimens were dewaxed in a ventilated furnace at 400 °C for 60 min before sintering. Sintering was carried out in a vacuum furnace at 610 °C for 30 min. The specimens were ECAPed for 1 pass. The 2-dimensional quantitative image analysis was carried out by means of SEM and OM for the evaluation of the dimensional and morphological porosity characteristics. The detailed microstructure revealed the main features of sintering processes as well as secondary pores at the prior alloying particle sites. The tensile fracture surfaces in both studied processing condition (as-sintered and ECAP) show limited ductility, with fracture occurring on a plane normal to the tensile stress axis. Examination at higher magnifications revealed predominantly transparticle ductile features. In terms of mechanical properties, ECAP is almost doubling the tensile strength of the as-sintered materials

The Influence of Thermo-Plastic Processes on Materials Recovery

The influence of thermo-plastic processes through methods of severe plastic deformations (SPD) and rolling carried out at ambient and cryogenic temperatures on recovery of two materials was investigated. The aim of this study was to insert strains to materials having middle and high stacking fault energy (SFE) in ambient and cryogenic temperature conditions, respectively and subsequently, through DSC method, to observe an influence of the storage energy on structural recovery of materials. As experimental materials were used oxygen free high conductivity copper (OFHC Cu) and C-Si steel which represent materials with middle and high stacking fault energy (SFE), respectively. The OFHC Cu was subjected to equal channel angular rolling (ECAR) by seven passes. ECAR is a method belonging to a SPD group. It was shown, five ECAR passes have a significant effect on material properties. The rolling performed at cryogenic temperatures using a laboratory duo rolling mill was carried out only once. This study implies that a recovery process (characterized by the mobility of structural defects) starts as follows: for OFHC Cu without ECAR and processed by 5th ECAR passes: 0.31·Tmelt and 0.19·Tmelt, respectively, for C-Si steel processed by cryorolling: 0.095·Tmelt.

Effect of Severe Plastic Deformation on the Characteristics of a PM Aluminum Alloy

The paper deals with the influence of severe plastic deformation on the typical powder metallurgy (PM) microstructural characteristics, such as porosity, of a PM aluminum alloy. A commercial ready-to-press aluminum based powder was used as material to be investigated. After applying different compacting pressures (400, 500, 600 and 700 MPa), specimens were debinded in a ventilated furnace at 400 °C for 60 min. Sintering was carried out in a vacuum furnace at 610 °C for 30 min. The specimens were ECAPed for 1 pass. The dimensional and morphological porosity of investigated materials were measured individually for each pore. Results show that ECAP generates shearing stress breaking down the oxide film; this, coupled to particles deformation under local constraints, enables strong bonding and stability. Therefore, ECAP supports next progressive decreasing of pore size as well as strongly influences both dimensional and morphological porosity characteristics, considering that small pores evolve easily to a circular form. Moreover, ECAP cause strong bonding between adjacent particles, which results in a significant increase of mechanical properties.

Wear of TiCN Coated Sintered Fe-1.5Cr-0.2Mo-0.7C Steels

The wear behaviour on prealloyed sintered steel parts TiCN PVD coating was analysed. Cylindrical specimens (uf066 40x30 mm3) were compacted to the green density of ~7.0 g.cm-3. The specimens were sintered in pusher furnace at the temperature of 1180°C for 40 minutes in an atmosphere of 25H2+75N2. Part of sintered specimens was then coated by Physical Vapour Deposition. Wear tests were carried out by means of a pin-on-disk tribometer. The results of profilograms (the values of surface roughness) and the wear curves (mass loss versus sliding distance) showed positive effect of coating on the wear resistance of the tested specimens.