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Influence of the green density on the dewaxing behaviour of uniaxially pressed powder compacts
Materials Science and Engineering A 430 (2006) 277–284
Influence of the green density on the dewaxing behaviour of uniaxially pressed powder compacts S. Gim´enez a,∗ , A. Vagnon b , D. Bouvard b , O. Van der Biest a a
Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, Kasteelpark Arenberg 44, B-3001 Leuven, Belgium b Institut National Polytechnique de Grenoble, Laboratoire G´ enie Physique et M´ecanique des Mat´eriaux, CNRS, UMR 5010, ENSPG, BP 46, 38402 Saint Martin d’H`eres Cedex, France Received 26 January 2006; received in revised form 16 May 2006; accepted 17 May 2006
Abstract The effect of the green density on the dewaxing behaviour of two different uniaxially pressed iron based powder metallurgy alloys has been characterised by means of non-destructive mechanical and microstructural techniques, i.e. impulse excitation technique and synchrotron tomography. The elastic and damping properties of the green and dewaxed specimens pressed to two different densities have been evaluated. It was found that the as-pressed green specimens with lower density possessed higher stiffness and lower damping than the high density ones. This behaviour has been explained on a microstructural basis, considering the damage generated in the green specimens during the ejection step. The elastic and damping properties of the specimens after dewaxing strongly depended on their initial green density. Increased elastic properties (and lower damping) were observed for the low density specimens with respect to their green counterparts, while the opposite behaviour was observed for the specimens with higher green density (i.e. decreased stiffness and slightly higher damping after dewaxing). “In situ” monitoring of the evolution of the elastic and damping properties of the materials has been carried out by means of the high temperature impulse excitation technique to better understand the behaviour observed. Additionally, thermogravimetric analysis has been carried out to correlate the mass loss events with the relevant features of the HT-IET curves. It was demonstrated that the damage generated in the green specimens during ejection is amplified during dewaxing for the high density compacts, while some degree of pre-sintering is responsible for the increased properties of the lower density specimens. © 2006 Elsevier B.V. All rights reserved. Keywords: Powder metallurgy; Uniaxial pressing; Dewaxing; Impulse excitation technique; Synchrotron tomography
1. Introduction Powder metallurgy is a well-established manufacturing technology mainly devoted to the production of complex-shaped “near-net-shape” components. Its main advantages with respect to other manufacturing techniques are related to the low energy consumption. This is mainly due to the small number of steps in manufacturing and the highly efficient material use (up to 95%), which has also positive effects on the environment. The main market is related to iron based components for the automotive industry where powder metallurgy currently supplies around 18 kg of components per car in the United States and 8 kg in Europe. In 2002, Europe and the United States consumed a total of 500.000 t of iron powder [1].
∗
Corresponding author. Tel.: +32 16 321 192; fax: +32 16 321 992. E-mail address: [email protected] (S. Gim´enez).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.05.079
The basic procedure in the manufacture includes the following steps: powder production, mixing one or several powders with a suitable organic lubricant, loading the mixture into a die, which will define the desirable shape and application of pressure. This procedure provides a powder compact, which is referred to as the “green” part with its associated “green” properties. The green strength should be high enough to allow handling of the part. The compact is then heated, usually in a protective atmosphere at a temperature below the melting point of the main constituent, so that the powder particles weld together (sinter) and confer sufficient strength to the part for the intended use. The surfaces of the powder particles are generally covered by metallic oxides formed by the interaction of the powder particles and the atmosphere. Additionally, a film of the organic compaction lubricant covers the particles constituting the green compact. During the first stages of the sintering cycle, the compaction lubricant is eliminated, a process which is referred to as dewaxing or delubrication. The reduction of the metallic oxides and the elimination of surface contaminants, which normally
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occurs at temperatures higher than dewaxing is also mandatory since the powder particles must possess oxide-free surfaces to form the interparticle necks responsible for the increased properties of the finished “sintered” part. In order to improve the properties and dimensional accuracy of PM ferrous parts after sintering and minimize the secondary finishing operations (which constitute up to 75% of the production costs), increasing the green density of powder compacts has been targeted in the last years. Recently, pressing techniques like warm compaction [2] and high velocity compaction [3] have been established and at present green densities of 7.3–7.5 g/cm3 can be reached at industrial scale, leading to increased sintered properties. Consequently, the effect of higher green densities on the dewaxing and sintering behaviour of powder compacts has become an issue and received in the last years the attention of the powder metallurgy community [4,5]. The present work is focused on characterizing the effect of green density on the dewaxing behaviour of iron based powder compacts uniaxially pressed by warm compaction. The impulse excitation technique (IET) has been employed in order to “non-destructively” evaluate the elastic and damping properties of green and dewaxed green compacts and to “in situ” investigate their dewaxing behaviour. The technique is based on the mechanical excitation of a solid body by means of a light impact. For isotropic, homogeneous materials of simple geometry (prismatic or cylindrical bars), the resonant frequency of the free vibration provides information about the elastic properties of the materials. Moreover, the amplitude decay of the free vibration is related to the damping or internal friction of the material. At present, IET is a well-established non-destructive technique for the calculation of elastic moduli and internal friction in monolithic, isotropic materials. Standard procedures are described in ASTM E 1876-99 [6] and DIN ENV 843-2 [7]. IET can also be performed at high temperature (HT-IET) using a dedicated experimental set-up in a furnace. The technique has been successfully employed to characterise the high temperature behaviour of technical ceramics, the martensitic transformation in shape-memory alloys, etc. [8,9] and thus constitutes a valuable tool in the field of mechanical spectroscopy. In the present work, HT-IET has been used for the “in situ” characterisation of the dewaxing behaviour of powder metallurgy green compacts. In order to correlate the mechanical characterisation (elastic moduli, damping and strength) with microstructural information, 3D images of the microstructure of green compacts have been obtained by X-ray absorption micro-tomography. This non-destructive technique, which has been increasingly used in materials science in the past years, consists of recording radiographies of a specimen at different angular positions and then reconstructing the spatial distribution of the linear attenuation coefficient within the specimen. With the outstanding features of synchrotron radiation, materials can be characterised with a spatial resolution of the order of one micrometer [10]. X-ray micro-tomography already provided unique information about sintering mechanisms in powder compacts in terms of interparticle neck growth, pore shrinkage, particle rearrangement, etc. [11–14].
Table 1 Chemical composition of the base powders (wt.%) used for the manufacturing of the Densmixes and green compacts provided by H¨ogan¨as Base powder
Mo
Cu
Ni
Cr
Fe
Distaloy AE Astaloy CrM
0.50 0.50
1.50 0
4.00 0
0 3.00
Balance Balance
2. Experimental method Two metal base powders, manufactured by H¨ogan¨as AB (H¨ogan¨as, Sweden) by water atomisation, were used in the investigation: the diffusion-alloyed Distaloy AE (D) and the prealloyed Astaloy CrM (A). The composition of the base powders is listed in Table 1. The 0.65 wt.% lubricant and 0.5 wt.% carbon in the form of graphite were added to the base powders. Subsequently, beam-like green specimens (55 mm × 10 mm × 4 mm, round corners, curvature radius ≈3 mm) were warm-pressed at 130 ◦ C (powder, tool parts and filling devices) at H¨ogan¨as. Two different green densities were selected for each material: Astaloy CrM was pressed at 6.85 g/cm3 (A−) and 7.15 g/cm3 (A+) and Distaloy AE at 7.00 g/cm3 (D−) and 7.30 g/cm3 (D+). The specimens were dewaxed at 500 ◦ C during 1 h in a 75H2 –25N2 atmosphere, the heating and cooling rates were 10 ◦ C/min. Impulse excited resonant vibration analysis (IET) was carried out on green and dewaxed specimens at room temperature (RTIET) in order to characterise in a non-destructive way the elastic and damping properties of warm-pressed green specimens and the effect of dewaxing on the green properties. Three specimens were tested for each reported value. Tests were carried out by gently tapping on the antinodes of the selected vibration modes of the specimens. The resulting time signal of the vibration was recorded by means of a capacitive microphone. The RFDA software (IMCE, Diepenbeek, Belgium) was used to acquire the time signal, transform it to the frequency domain, and extract the resonant frequencies and damping of the excited vibration modes. The stiffness parameters were obtained by means of the formulae described in the ASTM standard [6]. More detailed information about this procedure has been reported in [15]. For the measurements carried out for the present work, the resonant frequencies of the “out-of-plane” (oop) and “in-plane” (ip) flexural modes, the torsion mode and the longitudinal (long) mode were used. These vibration modes provide information about the stiffness of the specimen along its length (E), shear modulus (G) and Poisson ratio’s (ν). The error due to the rounded-corner geometry of the bars in this study was estimated by comparing the resonant frequencies of two finite element models of a bar with and without the rounded-corner geometry. More details for this procedure are given in [16]. Additionally, the damping or internal friction (Q−1 ) was determined by the logarithmic free decay of the envelope of the measured amplitude signal, δ/π (Eq. (1)) [17]. This approximation is valid for anelastic materials at low strain amplitudes: Q−1 =
1 W δ = . 2π Wel,max π
(1)
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In this expression, W is the energy loss due to inelastic deformation and Wel,max refers to the energy stored due to elastic deformation. IET tests were carried out at high temperature (HT-IET) in an IMCE NV inert gas furnace (Tmax = 1750 ◦ C) (Diepenbeek, Belgium) in order to in situ characterise the evolution of the E modulus and damping of the green compacts during the dewaxing cycle. The specimen was loaded on the furnace sample support using a W wire compatible to testing up to 1500 ◦ C. The sample was heated up to 700 ◦ C at 2 ◦ C/min in a flowing 95% N2 –5% H2 atmosphere. IET tests were performed every 60 s, resonant frequency and damping were calculated from the amplitude–time signal recorded by the microphone as described in the previous paragraph. Thermogravimetry analysis (TGA) was carried out in a Q600 simultaneous TGA/DSC equipment (TA instruments; New Castle, Delaware). The thermal cycle and atmosphere conditions were identical to those used for the HT-IET tests. Three-point bending tests were carried out on green and dewaxed powder compacts. A universal Instron 4467 testing machine (High Wycombe Bucks, United Kingdom) was used. The span was 40 mm and the cross-head speed 0.1 mm/min. Three identical specimens were tested for each reported value. Green compacts of Distaloy AE pressed at different compaction pressures were selected for observation by X-ray absorption micro-tomography. Specimens with a suitable size for micro-tomography analysis (about 1 mm diameter and 1 cm height) were manufactured by turning three compacts obtained by warm die pressing under different pressures: 400, 600 and 800 MPa. The respective densities achieved were: 6.8 g/cm3 (D− −), 7.2 g/cm3 (D0 ) and 7.4 g/cm3 (D+ +). Fig. 1 shows a representative example of the machined specimens employed. Tests were conducted at the European synchrotron research facility (ESRF) (Grenoble, France). This non-destructive technique consists of recording digital radiographs of a specimen in several angular positions and next reconstructing with a mathematical algorithm the spatial distribution of the absorption coefficient within the specimen. The experiments have been performed at ID15 beam line of ESRF that provided an X-ray beam with energy between 40 and 60 keV. After passing through the specimen, the beam reaches a scintillator that converts it to visible light, which is then collected and focused by optics to a 1024 × 1024 CCD camera. For each data set 900 radiographies are recorded at different angles covering an interval of 180◦ . From the radiographies, the reconstruction procedure provides an image of 1024 × 1024 × 255 voxels. Each voxel is a cube of 1.57 m side with an effective absorption coefficient. Details about the experimental set-up and the data processing achieved to obtain 3D images have been reported elsewhere [13,14]. 3. Results Fig. 2a shows the Young’s modulus values along the length of the sample calculated from the flexural “oop”, “ip” and the longitudinal vibration modes, the shear modulus calculated from the torsion mode (upper part of Fig. 2a) and the Poisson’s ratio for both A and D materials pressed at both densities
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Fig. 1. Warm-pressed green specimens turned for micro-tomography observation.
“+” and “−” after correction for the rounded-corners geometry. The maximum stiffness is obtained for A− (E ∼ 75 GPa) and the minimum for D+ (E ∼ 68 GPa). The obtained trend was A− > D− > A+ > D+. A similar trend is obtained for the Poisson’s ratio. The shear modulus shows a different tendency probably due to the high error associated to the measurement of A+. In both materials E, G and ν decrease with increasing density (i.e. compaction pressure) contrary to the expected behaviour. On the other hand, similar values of the different E modulus values are obtained from the different vibration modes employed (Eoop , Eip and Elong ), the maximum relative difference (Eoop − Elong )/Eoop = 3% being obtained for D−. This result confirms a good homogeneity of the materials in the compaction direction. The values of internal friction (Fig. 2b) are relatively high (0.01–0.02) due to the presence of the viscous organic lubricant (0.65 wt.%). Other factors contributing to the internal friction are the inherent porous structure of the specimens (higher friction with air), the weak interparticle contacts produced by the uniaxial warm-pressing (in comparison to those for a sintered body) and the high dislocation density created by the plastic deformation of the powder particles during pressing. The observed trend for the internal friction (A− < D− < A+ < D+) is exactly opposite to that obtained for the elastic parameters as shown in Fig. 2b. Consequently, the higher the density, the higher the internal friction for both materials for the range of density
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Fig. 2. (a) E modulus calculated from the flexural “oop”, “ip” and longitudinal vibration modes and G modulus calculated from the torsion mode (upper part of the figure) for the warm-pressed green samples supplied by H¨ogan¨as. Poisson’s ratio calculated as ν = (Eoop /2G) − 1. (b) Damping values calculated from the flexural “oop”, “ip”, torsional and longitudinal vibration modes.
Fig. 3. (a) E modulus calculated from the flexural “oop”, “ip” and longitudinal vibration modes and G modulus calculated from the torsion mode (upper part of the figure) for the warm-pressed green samples after the dewaxing cycle. Poisson’s ratio calculated as ν = (Eoop /2G) − 1. (b) Damping values calculated from the flexural “oop”, “ip”, torsion and longitudinal vibration modes.
explored. The differences between the damping values measured for the different vibration modes can be explained by the different set-up configuration employed for each measurement. These configurations have been described in [16]. Since the measured friction is really a contribution of the internal friction of the material and an external friction of the specimen with the testing set-up, slightly different values are expected for different testing set-ups. The higher bias is generally observed for the torsion mode, which is consistent with the maximum wire-specimen contact area. Fig. 3a shows the elastic parameters Eoop , Eip , Elong , G and ν of both materials after the dewaxing cycle. Compared to the green properties (Fig. 2a), increased values are obtained for the lower density specimens A− and D− with an average stiffness of 96 and 84 GPa, respectively. Conversely, reduced values are obtained for the higher density specimens A+ and D+ (average stiffness of 53 and 57 GPa, respectively). Identical trends are obtained for the shear modulus (G) and the Poisson’s ratio (ν). Therefore, it is clear that the stiffness after the dewaxing cycle exhibits a strong dependence on the green density. The materials can be ranked as A− > D− > D+ > A+. On the other hand, the relationship between Eoop , Eip and Elong values for each individual specimen is also dependent on the initial green density. Low density specimens A− and D− are characterised by an approximate relationship Eoop = Eip = Elong revealing structural homogeneity while the high density A+ and D+ exhibit a trend Eoop > Eip > Elong , which is the typical relation for heterogeneous materials with a surface stiffer than the core. A similar
behaviour has been observed in soft magnetic composite (SMC) bodies after heat treatment in air [16]. As a general conclusion, it can be said that the initial structural homogeneity of the green specimens is maintained after the dewaxing cycle for the “−” specimens but a certain structural heterogeneity is developed for the “+” specimens. Analogously, the internal friction after the dewaxing cycle exhibits a strong dependence on the green density of the specimens (Fig. 3b). A significant reduction of the internal friction is observed for the “−” specimens compared to their green counterparts while similar average values are observed for the higher density specimens. Additionally, it is remarkable that the reproducibility for the elastic and damping parameters measured is considerably higher (smaller error bars) for the “−” materials. Fig. 4 summarises the results of the transverse rupture strength (TRS) obtained through three-point bending tests on green and dewaxed specimens. All the green strength values lie in the range 19–25 MPa. The higher the density, the higher the green strength, following the sequence D+ > A+ > A− > D−. The green strength values reported here are in good agreement with those previously published for Distaloy AE by Degnan et al. [18]; 23.7 and 18.9 MPa were reported for D+ and D−, respectively. After dewaxing, it is clearly observed that the “as-dewaxed” strength is strongly dependent on the initial green density of the specimens. The “+” specimens exhibit a slight decrease with respect to the green strength (14% and 8% for D+ and A+, respectively), while conversely the “−” specimens show a pro-
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Fig. 4. Transverse rupture strength values obtained by three-point bending tests on green and dewaxed specimens (ad). Average value and standard deviation from three specimens.
nounced increase (82% and 63% for D− and A−, respectively). This tendency is identical to that observed for the elastic properties (Fig. 3a). In general, the reproducibility is lower for the dewaxed specimens with respect to their green counterparts with the only exception of D+. The relative weight loss after dewaxing is shown in Table 2 for all the materials. It is clear that no significant differences are observed between “+” and “−” specimens and the values obtained are very close to the nominal amount of lubricant (0.65 wt.%). In order to better understand the dewaxing behaviour of the materials investigated, “in situ” HT-IET tests were also carried out. The evolution of E/E0 (E0 is the E modulus at room temperature before the HT measurement) and Q−1 with temperature during heating up to 700 ◦ C is shown for both materials and densities in Fig. 5. Identical trends can be observed for both materials independently of the green density. Initially, from room temperature up to around 100 ◦ C, a strong decrease of the E modulus is coincident with a pronounced increase of damping. The signal disappears from 100 ◦ C up to around 350 ◦ C as a consequence of the very high damping caused by the softening and melting of the lubricant. A pronounced increase of E/E0 (decrease of Q−1 ) takes place from 350 up to 450 ◦ C. A plateau of E/E0 and a slight increase of Q−1 can be observed from 500 ◦ C up to a certain temperature (550–650 ◦ C). From that temperature up to 700 ◦ C, a slight decrease of E/E0 is observed together with a continuous increase of damping. Comparing the influence of the initial green density on the evolution of damping and relative stiffness for both Distaloy AE and Astaloy CrM grades, no appreciable differences can be found up to 100 ◦ C, since both materials maintain approximately the same trend observed at RT (higher density leads to lower stiffness and higher damping). After recovering the signal at 320 ◦ C approximately, the differences between “+” Table 2 Weight loss after the dewaxing cycle for the green compacts Weight loss (%) D+ D− A+ A−
0.625 0.634 0.641 0.6227
± ± ± ±
0.001 0.008 0.007 0.0004
Fig. 5. Evolution of E/E0 and damping with temperature for (a) D+ and D− green specimens and (b) A+ and A− green specimens.
and “−” specimens become more pronounced but they progressively decrease as temperature increases. At 423 and 432 ◦ C for Astaloy CrM and Distaloy AE, respectively, the damping curves become coincident and the E/E0 curves intersect. From those intersection temperatures on, the higher relative stiffness correspond to the “+” specimens up to the maximum temperature (700 ◦ C). Fig. 6 shows the evolution of the specimen weight with temperature for both materials and green densities measured by TGA. Since the same lubricant is added to both base powders, the dewaxing kinetics of both materials is similar and only slight differences are observed between “+” and “−” specimens related to the fact that more dewaxing paths exist for the lower density specimens (higher porosity). Consequently, the “−” specimens exhibit a slightly higher mass loss at the same temperature compared to “+” specimens. Dewaxing starts at approximately 200 ◦ C (point 1 in Fig. 6). The dewaxing kinetics are slowed down at points 2 and 3 revealing that the elimination of the lubricant becomes progressively more difficult as its amount decreases. For Astaloy CrM, the transition between points 2 and 3 does not take place with a uniform slope, as it is the case for Distaloy AE. The temperature interval where the IET signal is not measurable is also indicated in Fig. 6. The lower temperature limit does not correspond to any relevant microstructural event identified by the employed techniques. The higher temperature of the interval corresponds to point 2, which sets the slowing down of the dewaxing kinetics. The results of the non-destructive microstructural characterisation of green compacts by synchrotron tomography are summarized in Fig. 7. This figure presents three virtual sections extracted from 3D images. The pressing direction is indicated
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Fig. 6. Evolution of the sample weight with temperature measured by TGA for D+, D−, A+ and A− green specimens.
by the vertical dotted line. Qualitatively, it can be seen that the higher the pressure, the smaller the pore size and the volume fraction of porosity. All the virtual microstructures contain cusped pores resulting from the initial particle packing. However the densest specimen (D+ +) shows a high number of thin elongated pores along the interparticle contacts predominantly oriented in the horizontal direction (perpendicular to the pressing direction). A few pores with the identical features appear in the material with intermediate density (D0 ). 4. Discussion 4.1. Green properties From the results presented in the previous section, it is clear that the green properties and the dewaxing behaviour of the powder compacts investigated are strongly dependent on the green
density. With respect to the green properties, the fact that higher values for E, G and ν are obtained for the lower density specimens “−” indicates that the volume fraction of porosity is not the main factor affecting the elastic properties. Simchi et al. [19,20] also showed a similar trend after measuring the electrical conductivity of green powder compacts of iron based alloys pressed at different compaction pressures. Lower electrical conductivity was obtained for green compacts pressed at 800 MPa compared to 450 MPa. They associated this behaviour to the relevance of the springback effect during high pressure compaction, which could lead to “micro-cracking” of the powder compacts. It is well known that the uniaxial die pressing of powder particles induces structural anisotropy due to the preferential flattening of the powder particles in the direction perpendicular to pressing. During the ejection step, this leads to an anisotropic springback effect (higher expansion in the pressing direction compared to the transversal direction) as shown by Kuroki and Hiraishi [21,22]. These authors identified this anisotropic effect with the separation of the particle contacts referred to as “micro-damage”. These particle contacts are preferentially oriented perpendicularly to the pressing direction. After sintering, anisotropic shrinkage was observed and Kuroki suggested that this effect should be related to the closure of the micro-damage generated during ejection. The microstructural verification of Kuroki’s analysis was provided by Lame et al. [13]. These authors identified this micro-damage in the microstructure of the green compacts and monitored their microstructural evolution during the sintering cycle by the non-destructive “in situ” synchrotron tomography. Then, they correlated the anisotropic sintering deformations observed with the closure of the microdamage (referred to as “contact porosity”) during sintering. More recently, the quantitative validation for Lame’s work has been provided by Vagnon et al. [14]. In that work, identical D+ and D− materials used in the present work were employed. The anisotropic deformations monitored during sintering by dilatometry were identified with the opening of the contact pores during the dewaxing step and closure during sintering. The microstructural characterisation of the green compacts carried out by synchrotron tomography in the present work (Fig. 7) is compatible with the previous findings. The higher the compaction pressure, the higher the volume fraction of particle contacts oriented perpendicularly to the pressing direction.
Fig. 7. Virtual slices extracted from micro-tomography images of Distaloy compacts obtained by warm-pressing under various pressures (the vertical dotted lines at the centre of each virtual slice show the direction of pressing).
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Consequently, the “micro-damage” induced during ejection is higher and the values measured for the elastic properties are lower (or values for damping higher). From the results obtained in Figs. 2 and 4 on the stiffness, damping and strength of the green compacts, it can be concluded that the elastic and damping properties constitute a good sensor for the “micro-damage” generated during ejection contrary to the strength. It is believed that a similar interpretation can explain the results of Simchi on the electrical conductivity [19,20]. Moreover, the relationship between the E moduli obtained from the different vibration modes: Eip > Elong > Eoop (Fig. 2a) is also consistent with the interpretation given above. The lowest value for E systematically obtained for Eoop agrees well with the fact that the “contact pores” are preferentially oriented perpendicularly to the pressing direction, since this is the vibrating plane in the out-of-plane mode. Moreover, the fact the highest stiffness value is systematically obtained for the in-plane mode could be explained by the contribution of two different factors. First, a lower extent of “micro-damage” is oriented in the inplane vibration plane. Second, a high friction exists between the die wall and the powder surfaces in contact with it. These surfaces are the maximum contributors to Eip and it has been suggested that the powder-die wall friction can lead to surface stiffening due to particle deformation and surface densification [16]. In the last cited Ref. [16], an identical trend was reported for the elastic and damping properties of the heat-treated SMC powder compacts of one of the materials investigated (i.e. higher density led to lower stiffness and higher damping). This behaviour was explained on the basis of microstructural heterogeneity due to surface oxidation during heat treatment. 4.2. Dewaxing behaviour The effect of the green density on the dewaxing behaviour observed in Fig. 3 is related to the available paths for the lubricant removal in the porous structure. The higher the green density, the lower the number of available dewaxing paths and the higher the gas pressure built up in the powder compact during dewaxing. If the gas pressure is too high, additional microstructural damage can be induced in the material [23]. The results of the elastic, damping and strength properties of the dewaxed compacts (Fig. 3) suggest that the higher density compacts experience gas overpressure during dewaxing. This interpretation can be correlated to the fact that the opening of the contact pores previously identified in [14] leads to mechanical damage in the “+” specimens, where the anisotropic swelling is more pronounced compared to the “−” specimens. Conversely, the fact that a higher number of dewaxing paths exists in the “−” specimens together with a lower amount of contact pores (Fig. 7) inhibits or minimizes the generation of damage during delubrication. Additionally, in these conditions some extent of pre-sintering takes place in the material as evidenced by the increase of strength measured (Fig. 4) and also suggested in a previous work [24], although SEM examination of the fracture surfaces of the dewaxed specimens did not reveal any evidence of necking. This pre-sintering effect can be correlated to the increase of stiffness
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(Fig. 3a) and the reduction of damping (Fig. 3b) observed in the dewaxed specimens. The decrease of mechanical properties for the “+” specimens is correlated to the relationship Eoop > Eip > Elong . This relationship evidences microstructural heterogeneity in the material indicating that the surface is stiffer than the core. The density gradients created during die pressing may explain some differences in stiffness although this is not considered to be the main reason. It is believed that this heterogeneity is related to the effect of the gas overpressure, since the deeper the gas is formed in the compact, the less degassing paths are available and the higher the overpressure experienced. Consequently, the induced damage is higher. Therefore, it can be concluded that the “micro-damage” generated during ejection is amplified for these “+” specimens and its distribution after dewaxing is heterogeneously distributed with the higher amount of damage in the inner region of the compact. Unfortunately, the high damping associated to the dewaxing process does not allow a complete tracking of the HT-IET signal during the thermal cycle (Fig. 5). However, the observed trends and the correlation with TGA provide valuable information to confirm the interpretation given in the previous paragraph. The coincidence between the recovery of the IET signal and point 2 in the TGA plots for both materials and densities indicates a sudden increase of the connectivity of the powder particles at this temperature. This is compatible with the decrease of the dewaxing rate marked by point 2 in Fig. 6. The increase of the E modulus in the interval 320–450 ◦ C is then interpreted as the increased connectivity between powder particles as dewaxing proceeds. The hypothesis of higher oxidation rate for the “−” specimens to explain the observed behaviour was dismissed since no significant weight differences between “+” and “−” specimens were measured (Table 2). The fact that a higher stiffness and lower damping is obtained for the “−” specimens after recovery of the IET signal (around 320 ◦ C) reinforces this interpretation. It clearly illustrates the degree of “micro-damage” or connectivity of the powder particles for both “+” and “−” specimens. The higher number of dewaxing paths for the “−” material leads to a lower extent of additional micro-damage during dewaxing. Consequently, higher stiffness and lower damping are obtained during dewaxing. Nevertheless, this trend is inverted as temperature increases. The intersection temperature for the E modulus is almost coincident for point 3 in Fig. 6, when practically all the lubricant has been eliminated. Therefore, the expected behaviour takes place from that temperature on: higher green density, higher mechanical properties. 5. Summary and concluding remarks Higher elastic properties have been obtained for the lower density specimens (A− and D−) with respect to their higher density counterparts (A+ and D+) contrary to the expected behaviour. An opposite trend was obtained for the damping behaviour. The microstructural analysis carried out by synchrotron tomography has confirmed that a higher volume fraction of contact pores in the “+” specimens is responsible for
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this behaviour in good correspondence with Kuroki’s analysis [21,22]. The relationship observed for the stiffness values from the different vibration modes (Eoop < Elong < Eip ) is also in good agreement with this interpretation. The green strength of the materials has shown to be less sensitive to the presence of these contact pores, increased strength was obtained for the high density specimens in good agreement with the classical behaviour. After dewaxing, decreased stiffness and strength (and higher damping) were obtained for the high density specimens (A+ and D+). These results together with the relationship Eoop > Eip > Elong evidenced amplification of the structural damage induced during ejection, which is distributed heterogeneously within the powder compact. “In situ” data provided by TGA and HT-IET are consistent with the fact that the contact pores generated during the ejection of the green compact open during dewaxing for the high density specimens. Conversely, a pronounced increase of stiffness and strength (and lower damping) was obtained for the lower density (A− and D−) specimens. These specimens do not experience such a pronounced pore opening as demonstrated in previous a work [14] and additionally, the increase of strength with respect to the green material indicates that pre-sintering takes place in the material. Acknowledgements The authors wish to thank Mr. Olof Anderson from H¨ogan¨as AB for the supply of the materials. Prof. Jef Vleugels and Dr. Georges Kapelski are acknowledged for fruitful discussions. This work was financed by the European Commission through the PM-MACH Growth project (contract no. G1RD-CT200200687). References [1] P. Lindskog, Met. Powder Rep. 59(2) (2004) 10–11. [2] Warm compaction, in: H¨ogan¨as Handbook, vol. 4, H¨ogan¨as AB, H¨ogan¨as, Sweden, 1998. [3] P. Skoglund, Powder Metall. 44 (3) (2001) 199. [4] S. Andersson, A. Ahlqvist. Proceedings of the 1998 PM World Congress on Powder Metallurgy, vol. 2, October 18–22, Granada, Spain, EPMA, pp. 261–265. ISBN 1-899072-09-8.
[5] H. Danninger, C. Xu, L. Blanco, M. Campos, J.M. Torralba. Proceedings of the 2004 World Congress on Powder Metallurgy, October 17–21, vol. 2, Vienna, Austria, EPMA, pp. 19–24. ISBN 1899072 15 2. [6] ASTM E 1876-99, Standard test method for dynamic Young’s modulus, shear modulus and Poisson’s ratio by impulse excitation of vibration, 2000. ASTM committee E-28 on mechanical testing, subcommittee E2803 on elastic properties. [7] ENV 843-2, Advanced technical ceramics—monolithic ceramics, mechanical properties at room temperature. Part 2. Determination of elastic moduli, CEN, 1996. [8] G. Roebben, B. Basu, J. Vleugels, J. Van Humbeeck, O. Van der Biest, J. Alloys Compd. 310 (2000) 284–287. [9] G.S. Firstov, J. Van Humbeeck, Y.N. Koval, Mater. Sci. Eng. A 378 (1–2) (2004) 2–10. [10] P. Cloetens, M. Pateyron-Salom´e, Y.P. Buffi`ere, G. Peix, J. Baruchel, F. Peyrin, M.J. Schlenker, Appl. Phys. 81 (1997) 5878. [11] D. Bernard, D. Gendron, J.M. Heintz, S. Bord`ere, J. Etourneau. Acta Mater. 53 (2005) 121. [12] M. N¨othe, K. Pischang, P. Ponizil, B. Kieback, J. Ohser. Proceedings of the 2004 World Congress on Powder Metallurgy, vol. 2, October 17–21, Vienna, Austria, EPMA, pp. 229–234. ISBN 1899072 15 2. [13] O. Lame, D. Bellet, M. Di Michiel, D. Bouvard, Acta Mater. 52 (2004) 977. [14] A. Vagnon, O. Lame, D. Bouvard, M. Di Michiel, D. Bellet, G. Kapelski, Acta Mater. 54 (2) (2006) 513–522. [15] G. Roebben, B. Bollen, A. Brebels, J. Van Humbeeck, O. Van der Biest, Rev. Sci. Instrum. 68 (1997) 4511–4515. [16] S. Gim´enez, T. Lauwagie, G. Roebben, W. Heylen, O. Van der Biest, J. Alloys Compd. (in press). [17] A.S. Nowick, B.S. Berry, Anelastic Relaxation in Crystalline Solids, Academic Press, New York, 1972. [18] C.C. Degnan, A.R. Kennedy, P.H. Shipway, Powder Metall. 46 (4) (2003) 365–370. [19] A. Simchi, H. Danninger, Powder Metall. 43 (3) (2000) 209–218. [20] A. Simchi, H. Danninger, B. Weiss, Powder Metall. 43 (3) (2000) 219–227. [21] H. Kuroki, Key Eng. Mater. 29 (1989) 365. [22] H. Kuroki, M. Hiraishi, Science of Sintering, Plenum, New York, NY, 1989, p. 91. [23] C. Binet, R. Zauner, D. Heaney. Proceedings of the 2004 World Congress on Powder Metallurgy, vol. 5, October 17–21, Vienna, Austria, EPMA, pp. 191–196. ISBN 1899072 15 2. [24] S. Gim´enez, G. Roebben, J. Vleugels, O. Van der Biest, Proceedings of the 2004 World Congress on Powder Metallurgy, vol. 5, October 17–21, Vienna, Austria, EPMA, pp. 423–428. ISBN 1899072 15 2.
Influence of the green density on the dewaxing behaviour of uniaxially pressed powder compacts S. Gim´enez a,∗ , A. Vagnon b , D. Bouvard b , O. Van der Biest a a
Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, Kasteelpark Arenberg 44, B-3001 Leuven, Belgium b Institut National Polytechnique de Grenoble, Laboratoire G´ enie Physique et M´ecanique des Mat´eriaux, CNRS, UMR 5010, ENSPG, BP 46, 38402 Saint Martin d’H`eres Cedex, France Received 26 January 2006; received in revised form 16 May 2006; accepted 17 May 2006
Abstract The effect of the green density on the dewaxing behaviour of two different uniaxially pressed iron based powder metallurgy alloys has been characterised by means of non-destructive mechanical and microstructural techniques, i.e. impulse excitation technique and synchrotron tomography. The elastic and damping properties of the green and dewaxed specimens pressed to two different densities have been evaluated. It was found that the as-pressed green specimens with lower density possessed higher stiffness and lower damping than the high density ones. This behaviour has been explained on a microstructural basis, considering the damage generated in the green specimens during the ejection step. The elastic and damping properties of the specimens after dewaxing strongly depended on their initial green density. Increased elastic properties (and lower damping) were observed for the low density specimens with respect to their green counterparts, while the opposite behaviour was observed for the specimens with higher green density (i.e. decreased stiffness and slightly higher damping after dewaxing). “In situ” monitoring of the evolution of the elastic and damping properties of the materials has been carried out by means of the high temperature impulse excitation technique to better understand the behaviour observed. Additionally, thermogravimetric analysis has been carried out to correlate the mass loss events with the relevant features of the HT-IET curves. It was demonstrated that the damage generated in the green specimens during ejection is amplified during dewaxing for the high density compacts, while some degree of pre-sintering is responsible for the increased properties of the lower density specimens. © 2006 Elsevier B.V. All rights reserved. Keywords: Powder metallurgy; Uniaxial pressing; Dewaxing; Impulse excitation technique; Synchrotron tomography
1. Introduction Powder metallurgy is a well-established manufacturing technology mainly devoted to the production of complex-shaped “near-net-shape” components. Its main advantages with respect to other manufacturing techniques are related to the low energy consumption. This is mainly due to the small number of steps in manufacturing and the highly efficient material use (up to 95%), which has also positive effects on the environment. The main market is related to iron based components for the automotive industry where powder metallurgy currently supplies around 18 kg of components per car in the United States and 8 kg in Europe. In 2002, Europe and the United States consumed a total of 500.000 t of iron powder [1].
∗
Corresponding author. Tel.: +32 16 321 192; fax: +32 16 321 992. E-mail address: [email protected] (S. Gim´enez).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.05.079
The basic procedure in the manufacture includes the following steps: powder production, mixing one or several powders with a suitable organic lubricant, loading the mixture into a die, which will define the desirable shape and application of pressure. This procedure provides a powder compact, which is referred to as the “green” part with its associated “green” properties. The green strength should be high enough to allow handling of the part. The compact is then heated, usually in a protective atmosphere at a temperature below the melting point of the main constituent, so that the powder particles weld together (sinter) and confer sufficient strength to the part for the intended use. The surfaces of the powder particles are generally covered by metallic oxides formed by the interaction of the powder particles and the atmosphere. Additionally, a film of the organic compaction lubricant covers the particles constituting the green compact. During the first stages of the sintering cycle, the compaction lubricant is eliminated, a process which is referred to as dewaxing or delubrication. The reduction of the metallic oxides and the elimination of surface contaminants, which normally
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occurs at temperatures higher than dewaxing is also mandatory since the powder particles must possess oxide-free surfaces to form the interparticle necks responsible for the increased properties of the finished “sintered” part. In order to improve the properties and dimensional accuracy of PM ferrous parts after sintering and minimize the secondary finishing operations (which constitute up to 75% of the production costs), increasing the green density of powder compacts has been targeted in the last years. Recently, pressing techniques like warm compaction [2] and high velocity compaction [3] have been established and at present green densities of 7.3–7.5 g/cm3 can be reached at industrial scale, leading to increased sintered properties. Consequently, the effect of higher green densities on the dewaxing and sintering behaviour of powder compacts has become an issue and received in the last years the attention of the powder metallurgy community [4,5]. The present work is focused on characterizing the effect of green density on the dewaxing behaviour of iron based powder compacts uniaxially pressed by warm compaction. The impulse excitation technique (IET) has been employed in order to “non-destructively” evaluate the elastic and damping properties of green and dewaxed green compacts and to “in situ” investigate their dewaxing behaviour. The technique is based on the mechanical excitation of a solid body by means of a light impact. For isotropic, homogeneous materials of simple geometry (prismatic or cylindrical bars), the resonant frequency of the free vibration provides information about the elastic properties of the materials. Moreover, the amplitude decay of the free vibration is related to the damping or internal friction of the material. At present, IET is a well-established non-destructive technique for the calculation of elastic moduli and internal friction in monolithic, isotropic materials. Standard procedures are described in ASTM E 1876-99 [6] and DIN ENV 843-2 [7]. IET can also be performed at high temperature (HT-IET) using a dedicated experimental set-up in a furnace. The technique has been successfully employed to characterise the high temperature behaviour of technical ceramics, the martensitic transformation in shape-memory alloys, etc. [8,9] and thus constitutes a valuable tool in the field of mechanical spectroscopy. In the present work, HT-IET has been used for the “in situ” characterisation of the dewaxing behaviour of powder metallurgy green compacts. In order to correlate the mechanical characterisation (elastic moduli, damping and strength) with microstructural information, 3D images of the microstructure of green compacts have been obtained by X-ray absorption micro-tomography. This non-destructive technique, which has been increasingly used in materials science in the past years, consists of recording radiographies of a specimen at different angular positions and then reconstructing the spatial distribution of the linear attenuation coefficient within the specimen. With the outstanding features of synchrotron radiation, materials can be characterised with a spatial resolution of the order of one micrometer [10]. X-ray micro-tomography already provided unique information about sintering mechanisms in powder compacts in terms of interparticle neck growth, pore shrinkage, particle rearrangement, etc. [11–14].
Table 1 Chemical composition of the base powders (wt.%) used for the manufacturing of the Densmixes and green compacts provided by H¨ogan¨as Base powder
Mo
Cu
Ni
Cr
Fe
Distaloy AE Astaloy CrM
0.50 0.50
1.50 0
4.00 0
0 3.00
Balance Balance
2. Experimental method Two metal base powders, manufactured by H¨ogan¨as AB (H¨ogan¨as, Sweden) by water atomisation, were used in the investigation: the diffusion-alloyed Distaloy AE (D) and the prealloyed Astaloy CrM (A). The composition of the base powders is listed in Table 1. The 0.65 wt.% lubricant and 0.5 wt.% carbon in the form of graphite were added to the base powders. Subsequently, beam-like green specimens (55 mm × 10 mm × 4 mm, round corners, curvature radius ≈3 mm) were warm-pressed at 130 ◦ C (powder, tool parts and filling devices) at H¨ogan¨as. Two different green densities were selected for each material: Astaloy CrM was pressed at 6.85 g/cm3 (A−) and 7.15 g/cm3 (A+) and Distaloy AE at 7.00 g/cm3 (D−) and 7.30 g/cm3 (D+). The specimens were dewaxed at 500 ◦ C during 1 h in a 75H2 –25N2 atmosphere, the heating and cooling rates were 10 ◦ C/min. Impulse excited resonant vibration analysis (IET) was carried out on green and dewaxed specimens at room temperature (RTIET) in order to characterise in a non-destructive way the elastic and damping properties of warm-pressed green specimens and the effect of dewaxing on the green properties. Three specimens were tested for each reported value. Tests were carried out by gently tapping on the antinodes of the selected vibration modes of the specimens. The resulting time signal of the vibration was recorded by means of a capacitive microphone. The RFDA software (IMCE, Diepenbeek, Belgium) was used to acquire the time signal, transform it to the frequency domain, and extract the resonant frequencies and damping of the excited vibration modes. The stiffness parameters were obtained by means of the formulae described in the ASTM standard [6]. More detailed information about this procedure has been reported in [15]. For the measurements carried out for the present work, the resonant frequencies of the “out-of-plane” (oop) and “in-plane” (ip) flexural modes, the torsion mode and the longitudinal (long) mode were used. These vibration modes provide information about the stiffness of the specimen along its length (E), shear modulus (G) and Poisson ratio’s (ν). The error due to the rounded-corner geometry of the bars in this study was estimated by comparing the resonant frequencies of two finite element models of a bar with and without the rounded-corner geometry. More details for this procedure are given in [16]. Additionally, the damping or internal friction (Q−1 ) was determined by the logarithmic free decay of the envelope of the measured amplitude signal, δ/π (Eq. (1)) [17]. This approximation is valid for anelastic materials at low strain amplitudes: Q−1 =
1 W δ = . 2π Wel,max π
(1)
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In this expression, W is the energy loss due to inelastic deformation and Wel,max refers to the energy stored due to elastic deformation. IET tests were carried out at high temperature (HT-IET) in an IMCE NV inert gas furnace (Tmax = 1750 ◦ C) (Diepenbeek, Belgium) in order to in situ characterise the evolution of the E modulus and damping of the green compacts during the dewaxing cycle. The specimen was loaded on the furnace sample support using a W wire compatible to testing up to 1500 ◦ C. The sample was heated up to 700 ◦ C at 2 ◦ C/min in a flowing 95% N2 –5% H2 atmosphere. IET tests were performed every 60 s, resonant frequency and damping were calculated from the amplitude–time signal recorded by the microphone as described in the previous paragraph. Thermogravimetry analysis (TGA) was carried out in a Q600 simultaneous TGA/DSC equipment (TA instruments; New Castle, Delaware). The thermal cycle and atmosphere conditions were identical to those used for the HT-IET tests. Three-point bending tests were carried out on green and dewaxed powder compacts. A universal Instron 4467 testing machine (High Wycombe Bucks, United Kingdom) was used. The span was 40 mm and the cross-head speed 0.1 mm/min. Three identical specimens were tested for each reported value. Green compacts of Distaloy AE pressed at different compaction pressures were selected for observation by X-ray absorption micro-tomography. Specimens with a suitable size for micro-tomography analysis (about 1 mm diameter and 1 cm height) were manufactured by turning three compacts obtained by warm die pressing under different pressures: 400, 600 and 800 MPa. The respective densities achieved were: 6.8 g/cm3 (D− −), 7.2 g/cm3 (D0 ) and 7.4 g/cm3 (D+ +). Fig. 1 shows a representative example of the machined specimens employed. Tests were conducted at the European synchrotron research facility (ESRF) (Grenoble, France). This non-destructive technique consists of recording digital radiographs of a specimen in several angular positions and next reconstructing with a mathematical algorithm the spatial distribution of the absorption coefficient within the specimen. The experiments have been performed at ID15 beam line of ESRF that provided an X-ray beam with energy between 40 and 60 keV. After passing through the specimen, the beam reaches a scintillator that converts it to visible light, which is then collected and focused by optics to a 1024 × 1024 CCD camera. For each data set 900 radiographies are recorded at different angles covering an interval of 180◦ . From the radiographies, the reconstruction procedure provides an image of 1024 × 1024 × 255 voxels. Each voxel is a cube of 1.57 m side with an effective absorption coefficient. Details about the experimental set-up and the data processing achieved to obtain 3D images have been reported elsewhere [13,14]. 3. Results Fig. 2a shows the Young’s modulus values along the length of the sample calculated from the flexural “oop”, “ip” and the longitudinal vibration modes, the shear modulus calculated from the torsion mode (upper part of Fig. 2a) and the Poisson’s ratio for both A and D materials pressed at both densities
279
Fig. 1. Warm-pressed green specimens turned for micro-tomography observation.
“+” and “−” after correction for the rounded-corners geometry. The maximum stiffness is obtained for A− (E ∼ 75 GPa) and the minimum for D+ (E ∼ 68 GPa). The obtained trend was A− > D− > A+ > D+. A similar trend is obtained for the Poisson’s ratio. The shear modulus shows a different tendency probably due to the high error associated to the measurement of A+. In both materials E, G and ν decrease with increasing density (i.e. compaction pressure) contrary to the expected behaviour. On the other hand, similar values of the different E modulus values are obtained from the different vibration modes employed (Eoop , Eip and Elong ), the maximum relative difference (Eoop − Elong )/Eoop = 3% being obtained for D−. This result confirms a good homogeneity of the materials in the compaction direction. The values of internal friction (Fig. 2b) are relatively high (0.01–0.02) due to the presence of the viscous organic lubricant (0.65 wt.%). Other factors contributing to the internal friction are the inherent porous structure of the specimens (higher friction with air), the weak interparticle contacts produced by the uniaxial warm-pressing (in comparison to those for a sintered body) and the high dislocation density created by the plastic deformation of the powder particles during pressing. The observed trend for the internal friction (A− < D− < A+ < D+) is exactly opposite to that obtained for the elastic parameters as shown in Fig. 2b. Consequently, the higher the density, the higher the internal friction for both materials for the range of density
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Fig. 2. (a) E modulus calculated from the flexural “oop”, “ip” and longitudinal vibration modes and G modulus calculated from the torsion mode (upper part of the figure) for the warm-pressed green samples supplied by H¨ogan¨as. Poisson’s ratio calculated as ν = (Eoop /2G) − 1. (b) Damping values calculated from the flexural “oop”, “ip”, torsional and longitudinal vibration modes.
Fig. 3. (a) E modulus calculated from the flexural “oop”, “ip” and longitudinal vibration modes and G modulus calculated from the torsion mode (upper part of the figure) for the warm-pressed green samples after the dewaxing cycle. Poisson’s ratio calculated as ν = (Eoop /2G) − 1. (b) Damping values calculated from the flexural “oop”, “ip”, torsion and longitudinal vibration modes.
explored. The differences between the damping values measured for the different vibration modes can be explained by the different set-up configuration employed for each measurement. These configurations have been described in [16]. Since the measured friction is really a contribution of the internal friction of the material and an external friction of the specimen with the testing set-up, slightly different values are expected for different testing set-ups. The higher bias is generally observed for the torsion mode, which is consistent with the maximum wire-specimen contact area. Fig. 3a shows the elastic parameters Eoop , Eip , Elong , G and ν of both materials after the dewaxing cycle. Compared to the green properties (Fig. 2a), increased values are obtained for the lower density specimens A− and D− with an average stiffness of 96 and 84 GPa, respectively. Conversely, reduced values are obtained for the higher density specimens A+ and D+ (average stiffness of 53 and 57 GPa, respectively). Identical trends are obtained for the shear modulus (G) and the Poisson’s ratio (ν). Therefore, it is clear that the stiffness after the dewaxing cycle exhibits a strong dependence on the green density. The materials can be ranked as A− > D− > D+ > A+. On the other hand, the relationship between Eoop , Eip and Elong values for each individual specimen is also dependent on the initial green density. Low density specimens A− and D− are characterised by an approximate relationship Eoop = Eip = Elong revealing structural homogeneity while the high density A+ and D+ exhibit a trend Eoop > Eip > Elong , which is the typical relation for heterogeneous materials with a surface stiffer than the core. A similar
behaviour has been observed in soft magnetic composite (SMC) bodies after heat treatment in air [16]. As a general conclusion, it can be said that the initial structural homogeneity of the green specimens is maintained after the dewaxing cycle for the “−” specimens but a certain structural heterogeneity is developed for the “+” specimens. Analogously, the internal friction after the dewaxing cycle exhibits a strong dependence on the green density of the specimens (Fig. 3b). A significant reduction of the internal friction is observed for the “−” specimens compared to their green counterparts while similar average values are observed for the higher density specimens. Additionally, it is remarkable that the reproducibility for the elastic and damping parameters measured is considerably higher (smaller error bars) for the “−” materials. Fig. 4 summarises the results of the transverse rupture strength (TRS) obtained through three-point bending tests on green and dewaxed specimens. All the green strength values lie in the range 19–25 MPa. The higher the density, the higher the green strength, following the sequence D+ > A+ > A− > D−. The green strength values reported here are in good agreement with those previously published for Distaloy AE by Degnan et al. [18]; 23.7 and 18.9 MPa were reported for D+ and D−, respectively. After dewaxing, it is clearly observed that the “as-dewaxed” strength is strongly dependent on the initial green density of the specimens. The “+” specimens exhibit a slight decrease with respect to the green strength (14% and 8% for D+ and A+, respectively), while conversely the “−” specimens show a pro-
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Fig. 4. Transverse rupture strength values obtained by three-point bending tests on green and dewaxed specimens (ad). Average value and standard deviation from three specimens.
nounced increase (82% and 63% for D− and A−, respectively). This tendency is identical to that observed for the elastic properties (Fig. 3a). In general, the reproducibility is lower for the dewaxed specimens with respect to their green counterparts with the only exception of D+. The relative weight loss after dewaxing is shown in Table 2 for all the materials. It is clear that no significant differences are observed between “+” and “−” specimens and the values obtained are very close to the nominal amount of lubricant (0.65 wt.%). In order to better understand the dewaxing behaviour of the materials investigated, “in situ” HT-IET tests were also carried out. The evolution of E/E0 (E0 is the E modulus at room temperature before the HT measurement) and Q−1 with temperature during heating up to 700 ◦ C is shown for both materials and densities in Fig. 5. Identical trends can be observed for both materials independently of the green density. Initially, from room temperature up to around 100 ◦ C, a strong decrease of the E modulus is coincident with a pronounced increase of damping. The signal disappears from 100 ◦ C up to around 350 ◦ C as a consequence of the very high damping caused by the softening and melting of the lubricant. A pronounced increase of E/E0 (decrease of Q−1 ) takes place from 350 up to 450 ◦ C. A plateau of E/E0 and a slight increase of Q−1 can be observed from 500 ◦ C up to a certain temperature (550–650 ◦ C). From that temperature up to 700 ◦ C, a slight decrease of E/E0 is observed together with a continuous increase of damping. Comparing the influence of the initial green density on the evolution of damping and relative stiffness for both Distaloy AE and Astaloy CrM grades, no appreciable differences can be found up to 100 ◦ C, since both materials maintain approximately the same trend observed at RT (higher density leads to lower stiffness and higher damping). After recovering the signal at 320 ◦ C approximately, the differences between “+” Table 2 Weight loss after the dewaxing cycle for the green compacts Weight loss (%) D+ D− A+ A−
0.625 0.634 0.641 0.6227
± ± ± ±
0.001 0.008 0.007 0.0004
Fig. 5. Evolution of E/E0 and damping with temperature for (a) D+ and D− green specimens and (b) A+ and A− green specimens.
and “−” specimens become more pronounced but they progressively decrease as temperature increases. At 423 and 432 ◦ C for Astaloy CrM and Distaloy AE, respectively, the damping curves become coincident and the E/E0 curves intersect. From those intersection temperatures on, the higher relative stiffness correspond to the “+” specimens up to the maximum temperature (700 ◦ C). Fig. 6 shows the evolution of the specimen weight with temperature for both materials and green densities measured by TGA. Since the same lubricant is added to both base powders, the dewaxing kinetics of both materials is similar and only slight differences are observed between “+” and “−” specimens related to the fact that more dewaxing paths exist for the lower density specimens (higher porosity). Consequently, the “−” specimens exhibit a slightly higher mass loss at the same temperature compared to “+” specimens. Dewaxing starts at approximately 200 ◦ C (point 1 in Fig. 6). The dewaxing kinetics are slowed down at points 2 and 3 revealing that the elimination of the lubricant becomes progressively more difficult as its amount decreases. For Astaloy CrM, the transition between points 2 and 3 does not take place with a uniform slope, as it is the case for Distaloy AE. The temperature interval where the IET signal is not measurable is also indicated in Fig. 6. The lower temperature limit does not correspond to any relevant microstructural event identified by the employed techniques. The higher temperature of the interval corresponds to point 2, which sets the slowing down of the dewaxing kinetics. The results of the non-destructive microstructural characterisation of green compacts by synchrotron tomography are summarized in Fig. 7. This figure presents three virtual sections extracted from 3D images. The pressing direction is indicated
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Fig. 6. Evolution of the sample weight with temperature measured by TGA for D+, D−, A+ and A− green specimens.
by the vertical dotted line. Qualitatively, it can be seen that the higher the pressure, the smaller the pore size and the volume fraction of porosity. All the virtual microstructures contain cusped pores resulting from the initial particle packing. However the densest specimen (D+ +) shows a high number of thin elongated pores along the interparticle contacts predominantly oriented in the horizontal direction (perpendicular to the pressing direction). A few pores with the identical features appear in the material with intermediate density (D0 ). 4. Discussion 4.1. Green properties From the results presented in the previous section, it is clear that the green properties and the dewaxing behaviour of the powder compacts investigated are strongly dependent on the green
density. With respect to the green properties, the fact that higher values for E, G and ν are obtained for the lower density specimens “−” indicates that the volume fraction of porosity is not the main factor affecting the elastic properties. Simchi et al. [19,20] also showed a similar trend after measuring the electrical conductivity of green powder compacts of iron based alloys pressed at different compaction pressures. Lower electrical conductivity was obtained for green compacts pressed at 800 MPa compared to 450 MPa. They associated this behaviour to the relevance of the springback effect during high pressure compaction, which could lead to “micro-cracking” of the powder compacts. It is well known that the uniaxial die pressing of powder particles induces structural anisotropy due to the preferential flattening of the powder particles in the direction perpendicular to pressing. During the ejection step, this leads to an anisotropic springback effect (higher expansion in the pressing direction compared to the transversal direction) as shown by Kuroki and Hiraishi [21,22]. These authors identified this anisotropic effect with the separation of the particle contacts referred to as “micro-damage”. These particle contacts are preferentially oriented perpendicularly to the pressing direction. After sintering, anisotropic shrinkage was observed and Kuroki suggested that this effect should be related to the closure of the micro-damage generated during ejection. The microstructural verification of Kuroki’s analysis was provided by Lame et al. [13]. These authors identified this micro-damage in the microstructure of the green compacts and monitored their microstructural evolution during the sintering cycle by the non-destructive “in situ” synchrotron tomography. Then, they correlated the anisotropic sintering deformations observed with the closure of the microdamage (referred to as “contact porosity”) during sintering. More recently, the quantitative validation for Lame’s work has been provided by Vagnon et al. [14]. In that work, identical D+ and D− materials used in the present work were employed. The anisotropic deformations monitored during sintering by dilatometry were identified with the opening of the contact pores during the dewaxing step and closure during sintering. The microstructural characterisation of the green compacts carried out by synchrotron tomography in the present work (Fig. 7) is compatible with the previous findings. The higher the compaction pressure, the higher the volume fraction of particle contacts oriented perpendicularly to the pressing direction.
Fig. 7. Virtual slices extracted from micro-tomography images of Distaloy compacts obtained by warm-pressing under various pressures (the vertical dotted lines at the centre of each virtual slice show the direction of pressing).
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Consequently, the “micro-damage” induced during ejection is higher and the values measured for the elastic properties are lower (or values for damping higher). From the results obtained in Figs. 2 and 4 on the stiffness, damping and strength of the green compacts, it can be concluded that the elastic and damping properties constitute a good sensor for the “micro-damage” generated during ejection contrary to the strength. It is believed that a similar interpretation can explain the results of Simchi on the electrical conductivity [19,20]. Moreover, the relationship between the E moduli obtained from the different vibration modes: Eip > Elong > Eoop (Fig. 2a) is also consistent with the interpretation given above. The lowest value for E systematically obtained for Eoop agrees well with the fact that the “contact pores” are preferentially oriented perpendicularly to the pressing direction, since this is the vibrating plane in the out-of-plane mode. Moreover, the fact the highest stiffness value is systematically obtained for the in-plane mode could be explained by the contribution of two different factors. First, a lower extent of “micro-damage” is oriented in the inplane vibration plane. Second, a high friction exists between the die wall and the powder surfaces in contact with it. These surfaces are the maximum contributors to Eip and it has been suggested that the powder-die wall friction can lead to surface stiffening due to particle deformation and surface densification [16]. In the last cited Ref. [16], an identical trend was reported for the elastic and damping properties of the heat-treated SMC powder compacts of one of the materials investigated (i.e. higher density led to lower stiffness and higher damping). This behaviour was explained on the basis of microstructural heterogeneity due to surface oxidation during heat treatment. 4.2. Dewaxing behaviour The effect of the green density on the dewaxing behaviour observed in Fig. 3 is related to the available paths for the lubricant removal in the porous structure. The higher the green density, the lower the number of available dewaxing paths and the higher the gas pressure built up in the powder compact during dewaxing. If the gas pressure is too high, additional microstructural damage can be induced in the material [23]. The results of the elastic, damping and strength properties of the dewaxed compacts (Fig. 3) suggest that the higher density compacts experience gas overpressure during dewaxing. This interpretation can be correlated to the fact that the opening of the contact pores previously identified in [14] leads to mechanical damage in the “+” specimens, where the anisotropic swelling is more pronounced compared to the “−” specimens. Conversely, the fact that a higher number of dewaxing paths exists in the “−” specimens together with a lower amount of contact pores (Fig. 7) inhibits or minimizes the generation of damage during delubrication. Additionally, in these conditions some extent of pre-sintering takes place in the material as evidenced by the increase of strength measured (Fig. 4) and also suggested in a previous work [24], although SEM examination of the fracture surfaces of the dewaxed specimens did not reveal any evidence of necking. This pre-sintering effect can be correlated to the increase of stiffness
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(Fig. 3a) and the reduction of damping (Fig. 3b) observed in the dewaxed specimens. The decrease of mechanical properties for the “+” specimens is correlated to the relationship Eoop > Eip > Elong . This relationship evidences microstructural heterogeneity in the material indicating that the surface is stiffer than the core. The density gradients created during die pressing may explain some differences in stiffness although this is not considered to be the main reason. It is believed that this heterogeneity is related to the effect of the gas overpressure, since the deeper the gas is formed in the compact, the less degassing paths are available and the higher the overpressure experienced. Consequently, the induced damage is higher. Therefore, it can be concluded that the “micro-damage” generated during ejection is amplified for these “+” specimens and its distribution after dewaxing is heterogeneously distributed with the higher amount of damage in the inner region of the compact. Unfortunately, the high damping associated to the dewaxing process does not allow a complete tracking of the HT-IET signal during the thermal cycle (Fig. 5). However, the observed trends and the correlation with TGA provide valuable information to confirm the interpretation given in the previous paragraph. The coincidence between the recovery of the IET signal and point 2 in the TGA plots for both materials and densities indicates a sudden increase of the connectivity of the powder particles at this temperature. This is compatible with the decrease of the dewaxing rate marked by point 2 in Fig. 6. The increase of the E modulus in the interval 320–450 ◦ C is then interpreted as the increased connectivity between powder particles as dewaxing proceeds. The hypothesis of higher oxidation rate for the “−” specimens to explain the observed behaviour was dismissed since no significant weight differences between “+” and “−” specimens were measured (Table 2). The fact that a higher stiffness and lower damping is obtained for the “−” specimens after recovery of the IET signal (around 320 ◦ C) reinforces this interpretation. It clearly illustrates the degree of “micro-damage” or connectivity of the powder particles for both “+” and “−” specimens. The higher number of dewaxing paths for the “−” material leads to a lower extent of additional micro-damage during dewaxing. Consequently, higher stiffness and lower damping are obtained during dewaxing. Nevertheless, this trend is inverted as temperature increases. The intersection temperature for the E modulus is almost coincident for point 3 in Fig. 6, when practically all the lubricant has been eliminated. Therefore, the expected behaviour takes place from that temperature on: higher green density, higher mechanical properties. 5. Summary and concluding remarks Higher elastic properties have been obtained for the lower density specimens (A− and D−) with respect to their higher density counterparts (A+ and D+) contrary to the expected behaviour. An opposite trend was obtained for the damping behaviour. The microstructural analysis carried out by synchrotron tomography has confirmed that a higher volume fraction of contact pores in the “+” specimens is responsible for
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this behaviour in good correspondence with Kuroki’s analysis [21,22]. The relationship observed for the stiffness values from the different vibration modes (Eoop < Elong < Eip ) is also in good agreement with this interpretation. The green strength of the materials has shown to be less sensitive to the presence of these contact pores, increased strength was obtained for the high density specimens in good agreement with the classical behaviour. After dewaxing, decreased stiffness and strength (and higher damping) were obtained for the high density specimens (A+ and D+). These results together with the relationship Eoop > Eip > Elong evidenced amplification of the structural damage induced during ejection, which is distributed heterogeneously within the powder compact. “In situ” data provided by TGA and HT-IET are consistent with the fact that the contact pores generated during the ejection of the green compact open during dewaxing for the high density specimens. Conversely, a pronounced increase of stiffness and strength (and lower damping) was obtained for the lower density (A− and D−) specimens. These specimens do not experience such a pronounced pore opening as demonstrated in previous a work [14] and additionally, the increase of strength with respect to the green material indicates that pre-sintering takes place in the material. Acknowledgements The authors wish to thank Mr. Olof Anderson from H¨ogan¨as AB for the supply of the materials. Prof. Jef Vleugels and Dr. Georges Kapelski are acknowledged for fruitful discussions. This work was financed by the European Commission through the PM-MACH Growth project (contract no. G1RD-CT200200687). References [1] P. Lindskog, Met. Powder Rep. 59(2) (2004) 10–11. [2] Warm compaction, in: H¨ogan¨as Handbook, vol. 4, H¨ogan¨as AB, H¨ogan¨as, Sweden, 1998. [3] P. Skoglund, Powder Metall. 44 (3) (2001) 199. [4] S. Andersson, A. Ahlqvist. Proceedings of the 1998 PM World Congress on Powder Metallurgy, vol. 2, October 18–22, Granada, Spain, EPMA, pp. 261–265. ISBN 1-899072-09-8.
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