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Numerical Investigation of a Hot Forging Process For Partially Particle-Reinforced Sintered Components
Abstract
The expanding range of applications for parts made of light metals (magnesium, aluminium or titanium) could lead to a replacement of parts made of steel by the ones manufactured from light metal parts. However, magnesium and aluminium parts in particular reach their technical limits when exposed to high tribological, mechanical or thermal stress. For this reason, often the so called metal-matrix-composites (MMC), which possess the advantages of light metal (low weight and high ductility) as well as of the reinforcing phase (high hardness, high strength and good wear resistance), are used. This paper provides the initial findings of a fundamental investigation of the specific forming behaviour and the mechanical material properties for production of partially particle-reinforced powder metal parts. Cylindrical raw parts consisting of aluminium powder and a ceramic powder are produced by powder pressing and further compacted in a subsequent sintering process. The produced raw parts form the basis for an examination for a reduction of the existing residual porosities by subsequent upsetting and extrusion processes. The effects of the different process parameters (pressing force and forming temperature) on the material flow of the partially particle-reinforced material system and the structural strength of the formed parts are investigated. Numerical simulations are performed to analyse the density development during the above mentioned forming processes in order to determine the influence of porosity on the deformation behaviour of the considered material. The findings will help to evaluate the dependence of the residual porosity for sinter-forged parts on the prevailing forming mechanisms.
... The powder metallurgy enables variable alloy formations and component design, not feasible with melt metallurgical processes [1]. It offers high material utilization, net-shape parts and low energy consumption [2,3]. Furthermore, PM processing is a very efficient way for mass production of small structural components and provides a wide range of different industrial applications. ...
The ever-increasing demand for weight reduction of manufactured parts is leading to the replacement of steel with light metals such as aluminum and magnesium. However, light metals are at times unable to withstand high tribological, thermal or mechanical loads. This leads to an application of metal-matrix-composites (MMC) that possess the advantages of light metals (low weight and high ductility), as well as the characteristics of the reinforcing phase (high hardness, high strength and good wear resistance). To manufacture the MMC components, metal powders were shaped in a pressing process and further densified during a subsequent sintering phase. The residual porosity within the produced parts can be reduced by means of a subsequent sinterforging operation. In this paper, cylindrical partially particle-reinforced specimens, with a radially layered structure, were manufactured by two-sided powder pressing and sintering. A subsequent forging operation was carried out to eliminate the minimal porosity. Different process parameters (forming temperature, true strain and dilation rate) were varied to investigate their effects on the density and structural bonding of the partially particle-reinforced material system. Therefore, the sinterforging process upsetting was carried out. Subsequently, the particle-reinforced parts were characterised by metallographic analysis and hardness measurements. The results show that cracks and defects in the layer system of partially particle-reinforced powder compacts can be repaired using sinter forging.
Powder metallurgy (PM) has received a significant importance in the metal processing industry during the recent years. Thus an efficient component manufacturing with reduced production and operation costs is vital. In the course of this work the interaction of a foreign material with steel powder during the compaction process is studied. The research work has been divided into two sections. In the first sections experiments are carried out to study the compaction behaviour of steel powder compacts and parametrize the Drucker-Prager cap model. The second section deals with the study of compaction and deformation behaviour of a powder-copper spiral compact. A copper spiral is placed in steel powder and compressed uniaxially from its top as well as bottom. The deformation behaviour of the spiral is numerically investigated in FE tool Abaqus. The findings will help optimize the integrated channels in the PM hot forging die being designed at the IFUM.
A computational approach based on a cell model of material offers real promise as a predictive tool for nonlinear fracture
analysis. A key feature of the computational model is the modeling of the material in front of the crack by a layer of similarly-sized
cubic cells. Each cell of size D contains a spherical void of initial volume fraction f
0. The microseparation characteristics of the material in a cell, a result of void growth and coalescence, is described by
the Gurson–Tvergaard constitutive relation; the material outside the layer of cells can be modelled as an elastic- plastic
continuum. The success of this computational model hinges on developing a robust calibration scheme of the model parameters.
Such a scheme is proposed in this study. The material-specific parameters are calibrated by a two-step micromechanics/fracture-process
scheme. This article describes the micromechanics calibration of void growth taking into account both the strain hardening
and the strength of the material. The fracture-process calibration is addressed in a companion paper.
Unit cell model analyses are carried out here for nonhardening elastic-plastic solids containing small voids, to obtain a more complete understanding of the effect of different stress states and void shapes on the growth of voids to coalescence. Both plane strain analyses for cylindrical voids and full three-dimensional analyses for a material containing a periodic array of spherical voids are carried out, and the results are related to previous results obtained by an axisymmetric model. It is shown that the predictions are reasonably well represented by a dilatant plasticity model, developed for a porous ductile material. Also, the cell model analyses fully capture the transition from an early stage of overall plastic yielding to the final stage of void coalescence by local necking of intervoid ligaments.
Widely used constitutive laws for engineering materials assume plastic incompressibility, and no effect on yield of the hydrostatic component of stress. However, void nucleation and growth (and thus bulk dilatancy) are commonly observed is some processes which are characterized by large local plastic flow, such as ductile fracture. The purpose of this work is to develop approximate yield criteria and flow rules for porous (dilatant) ductile materials, showing the role of hydrostatic stress in plastic yield and void growth. Other elements of a constitutive theory for porous ductile materials, such as void nucleation, plastic flow and hardening behavior, and a criterion for ductile fracture will be discussed in Part II of this series. The yield criteria are approximated through an upper bound approach. Simplified physical models for ductile porous materials (aggregates of voids and ductile matrix) are employed, with the matrix material idealized as rigid-perfectly plastic and obeying the von Mises yield criterion. Velocity fields are developed for the matrix which conform to the macroscopic flow behavior of the bulk material. Using a distribution of macroscopic flow fields and working through a dissipation integral, upper bounds to the macroscopic stress fields required for yield are calculated. Their locus in stress space forms the yield locus. It is shown that normality holds for this yield locus, so a flow rule results. Approximate functional forms for the yield loci are developed.
Sintered Gears -Achievable Loaded-carrying Capacities by Conventional and New Production Methods
- Ch Sander
- R Ratzi
- B Lorenz
SANDER, Ch., RATZI, R., LORENZ, B. and TOBIE, T. Sintered Gears -Achievable Loaded-carrying Capacities
by Conventional and New Production Methods. In International Conference on Gears. München, Germany, 2002.
Numerical and experimental investigations of aluminium powder compaction and sintering. Material Science and Engineering Technology
- B.-A Behrens
- A Bougecha
BEHRENS, B.-A. and BOUGECHA, A. Numerical and experimental investigations of aluminium powder
compaction and sintering. Material Science and Engineering Technology. 2012. vol. 43, no. 6, pp. 511-519.
Einfluss von Werkzeugschwingungen auf das Verdichtungsverhalten metallischer Pulver beim Matrizenpressen, Dissertation
- E Gastan
GASTAN, E. Einfluss von Werkzeugschwingungen auf das Verdichtungsverhalten metallischer Pulver beim
Matrizenpressen, Dissertation, Universität Hannover, Berichte aus dem IFUM, 2012.