High Cycle Fatigue of Powder Metallurgy Materials
Total Page：16
File Type：pdf, Size：1020Kb
VIII Congreso Nacional de Propiedades Mecánicas de Sólidos, Gandia 2002 195-204 HIGH CYCLE FATIGUE OF POWDER METALLURGY MATERIALS Herbert Danningera and Brigitte Weissb a Institute of Chemical Technologies and Analytics, Vienna University of Technology, A-1060 Wien, Austria b Institute of Materials Physics, University of Vienna, A-1090 Wien, Austria ABSTRACT: Powder metallurgy materials offer increased potential for manufacturing of special, often otherwise inaccessible, materials as well as for the mass production of complex-shaped components with high precision. Many PM products are subjected to fatigue loading in service, in part up to very high loading cycle numbers. In this paper the peculiarities of fatigue in PM materials and components are shown with focus to high cycle fatigue. It is stressed that for precision parts which are mostly ferrous the inherent porosity is the decisive factor; this porosity originates from the manufacturing process and is evenly distributed within the material. However, there is also the effect of singular defects such as large pores, pore clusters, slag inclusions, etc., and these defects become the more critical the lower the integral porosity is. For fully dense PM products such as powder forged components, tool steels, or extruded light alloys, the singular defects are the decisive feature in the material, and HC fatigue limit is controlled by the largest defect in the stressed volume. KEYWORDS: Powder metallurgy, high cycle fatigue; porosity; defects 1. INTRODUCTION Powder metallurgy implies production of solid metallic materials and components from predominantly metallic powder. This technology has encountered impressive growth in the last decades, and such indispensable products as cemented carbide tools are exclusively manufactured by powder metallurgy [1]. Also in the automotive industry, powder metallurgy components, mainly iron based precision parts, are increasingly being used [2]. Previously, PM automotive parts were geometrically precise but mechanically rather weak; however, this has drastically changed with the introduction of PM components for engines and transmissions. In this case, also fatigue and contact fatigue loading is quite common, and the fatigue behaviour of PM materials is therefore of high relevance for the applications. From the materials scientists’ viewpoint, the microstructure of PM materials is similar to that of cast and/or wrought counterparts. Form the engineer’s viewpoint however it has to be considered that there are much more manufacturing parameters relevant for production of PM materials. This also increases the number of microstructural parameters, e.g. pore related ones, to be taken into account most of which are also relevant for the fatigue behaviour. This makes the behaviour of PM materials more complex and difficult to understand [3]. PM materials can principally be split up into two groups according to manufacturing route and resulting microstructure: PM precision parts are produced by cold uniaxial pressing and subsequent pressureless sintering (to some extent also by powder forging). The residual pores are usually regarded to be the principal characteristic [4]. The most important parameters are total porosity and shape of pores/sintering contacts. Depending on total porosity - which is influenced primarily by the compacting pressure - and sintering conditions the sintering contacts can be isolated or interconnected [5]. In the matrix, the larger microstructural flexibility of PM materials is 195 Danninger and Weiss significant, e.g. distribution of the alloy elements - both homogeneous and inhomogeneous - can be adjusted more freely than with ingot metallurgy. Powder metallurgy full density products [6] are made by hot isostatic pressing (e.g. PM superalloys and tool steels), by extrusion (PM Al alloys) or by pressing with subsequent liquid phase sintering (hard metals, tungsten heavy alloys etc). Residual porosity should be low to negligible, and the microstructure is commonly homogeneous. The most important feature affecting the fatigue behaviour and resulting in marked differences to cast or wrought counterparts is the usually very fine and isotropic microstructure which gives uniform properties regardless of the orientation. On the other hand the materials are sensitive to inclusions the effect of which is further aggravated by the excellent properties of the basic material. Here, the fatigue behaviour of both types of materials is shown, and the common features as well as the differences are described. 2. INFLUENCE OF INTEGRAL POROSITY In pressed and sintered ferrous materials, e.g. for precision parts, the characteristic feature is the “integral” or “primary” porosity. This is in principle the porosity already present in the bulk powder, before compaction. Since the compaction of metal powders is commonly a cold working process, progressive work hardening occurs at the particle contacts and at some stage prevents further densification. Furthermore, the admixed organic pressing lubricant that is inevitable in uniaxial die compaction also absorbs considerable space in the compact and, being virtually incompressible, limits the densification of the metal skeleton. There remains an interconnecting network of pores, in part filled with lubricant, that is essential for the material during lubricant burnout in the initial stages of sintering (lubricant in isolated pores would inevitably result in blistering). During the sintering process, the metal powder particles weld together at the pressing contacts through diffusion processes and form stable metallic bonds. The typical microstructure of a sintered steel as depicted by metallographic techniques is shown in Fig.1 [7]. The sections however can be misleading: during sintering the pore structure remains interconnected in most cases, i.e. at the common density levels of up to 7.4 g.cm-3 for ferrous components (about 6% total porosity). Only through special pressing and / or sintering techniques or by powder forging density levels are attained where the interconnected pore network is dissolved into single isolated pores. Therefore, in most cases the sintered iron can be described through a “sponge” model, and only at rather high density levels the “swiss cheese” model, with isolated holes, applies. The structure of the pore network in sintered materials can be seen e.g. in resin replicas of the pore network, the iron skeleton having been etched away after resin impregnation [5]. The fact that the pores are mostly interconnected implies that the sintering contacts, which actually bear the load in the material (pores of course cannot be load-bearing), are isolated in these cases, and description of the microstructure has to focus at the sintering contacts. Furthermore, the pores form a very complex, 3-dimensional structure that is virtually impossible to depict in a single 2-dimensional metallographic section [8]. Therefore, it was shown rather early by Slesar [9] that fractographic techniques are more suited for describing the mechanical behaviour of sintered steels. If the fracture surfaces are obtained by a low deformation fracturing technique such as e.g. impact testing at 77K or high cycle fatigue testing, the fracture path runs through the weakest areas of the material, i.e. the thinnest parts of the sintering contacts, which are exactly those structural features that limit the mechanical resistance of the material [10]. Therefore, there should be a relationship between the total area of the sintering contacts in the weakest cross section – which can be defined as the “load bearing cross section” Ac – and the mechanical properties of the material. 196 VIII Congreso Nacional de Propiedades Mecánicas de Sólidos Figure 1. Schematic of the microstructure in ferrous PM components [7] If high strength sintered steels are e.g. impact tested at 77K, low-deformation fracture surfaces are obtained as typically shown in Fig.2a, c, e for varying total porosity levels. By suitable image analysing techniques [11], the area of the broken necks can be determined as a fraction of the total cross section, which yields the parameter Ac. In Fig.2b, d, f the broken contacts as analyzed by a specially developed routine are shown white. Here it stands out clearly that the load bearing cross section Ac is much lower than would be expected from the volume fraction of the metal phase. Even for the high porosity material in which the load bearing cross section is only about 0.15, i.e. only about 15% of the entire cross section consists of metallic contacts, still more than 70% of the volume is taken by the metallic phase. This of course results in comparatively low strength-to-weight ratio and indicates that increasing the relative density, i.e. lowering the porosity, is an essential measure to improve the mechanical behaviour of the material. In Fig.3a the fatigue endurance strength – push-pull mode at R = -1, determined by 8 ultrasonic techniques [12] at Nmax = 2.10 - of the Mo prealloyed sintered steel depicted in Fig.2 is shown as a function of the total porosity, and there is also the hypothetical function 1- P if Sw would follow the volume fraction of the metal phase. The much lower values for Sw compared to the function 1-P indicate the out-of-proportion effect of the porosity. This is still more noticeable if the specific endurance strength is given as in Fig.3b, once more in parallel to the absolute values; it is visible that the lower density of the porous specimens does not exert any significant effect on the trend of the fatigue endurance strength. 197 Danninger and Weiss Figure 2a/b: Ptot = 18.5% Figure 2c/d: Ptot = 12.3% Figure 2e: Ptot = 8.0% Figure 2. Fracture surfaces of high strength sintered steel Fe-1.5%Mo-0.7%C of varying porosity, heat treated. Impact testing at 77K: fracture surfaces analyzed through image processing system These results strongly confirm that it is not so much the total porosity that directly affects the mechanical properties but it must be a parameter that takes into account the real geometry of the microstructure. Here the load bearing cross section Ac seems to be well suited.