Consolidation by MF-ERS of mechanically alloyed Al powder

CONSOLIDATION BY MF-ERS OF MECHANICALLY ALLOYED Al POWDER E.S. Caballero1, F. Ternero1, R. Astacio1, F.G. Cuevas2, J.M. Montes1, J. Cintas1 1 Escuela Técnica Superior de Ingeniería. Universidad de Sevilla. Camino de los Descubrimientos s/n, 41092 Sevilla (Spain). 2 Escuela Técnica Superior de Ingeniería. Universidad de Huelva. Campus El Carmen, Avda. Tres de Marzo, s/n, 21071 Huelva (Spain). ABSTRACT The aim of this work is to study the viability of producing, by medium-frequency electrical resistance sintering (MF-ERS), compacts from mechanically alloyed aluminium powders. The MF-ERS process was carried out using different values of current intensity (6, 8, 10 and 11 kA) and dwelling (heating) times (400, 700 and 1000 ms). Results were compared with compacts processed by the conventional cold pressing and sintering route (850 MPa and 650 ºC-1h). Depending on the processing route different properties were obtained. The final porosity of the MF-ERS compacts (23.6 - 7%) can be as low, under the tougher tested sintering conditions, as that of the conventionally produced compacts (6%). The compacts obtained by MF-ERS are less ductile, with lower compression strength than that obtained by the conventional route. Furthermore, a similar electrical resistance and higher microhardness can be reached by the MF-ERS process, despite the duration of the consolidation process is only a fraction of that of the conventional process. Keywords: mechanical alloying; aluminium; electrical resistance sintering; MF-ERS; FAST; ECAS; hot pressing; sintering; powder metallurgy 1. INTRODUCTION Traditionally, powder metallurgy (PM) route has consisted in two steps: cold pressing to get a green compact with the necessary strength to by handled, followed by a sintering process to obtain the final product with the required properties [1-3]. Although this method is widely used in the industrial sector, being one of its main advantages not producing waste material, other PM techniques are in a continuous development in order to achieve more environmental friendly processes. In this way, the use of electricity to consolidate powders has been Licencia Creative Commons CC BY-NC-ND experimentally approached from time ago, with the advantage of requiring just a tiny portion of the time required in traditional processing. One of the different available modes of electrical sintering [4] is known as electrical resistance sintering (ERS). This technique basically consists in applying, to a powder mass contained in an electrically insulating die, a low voltage and high intensity current and, at the same time, a low consolidation pressure [5, 6]. ERS process typically lasts around one second. In such period of time, the applied voltage and pressure induce powder densification due to the heating by Joule effect. With the additional use of medium-frequency technology (developed for the resistance welding filed) it is possible to achieve lighter and smaller welding transformers, better control of current intensity and, in general, more efficient processes. The same advantages are transferred to the MF-ERS process as compared to the low frequency technology of the conventional ERS. In the last decades, the structural applications of aluminium and its alloys have attracted much interest, mainly, in the automotive and aerospace industries [7-10]. In this way, it is interesting to achieve an improvement of their properties, either by the reinforcement with a wide variety of ceramic particles [11, 12], or by grain size reduction [13, 14]. Both aspects can be achieved by means of mechanical milling (MM) [15-17]. During MM, the metal powder particles are subjected to severe plastic deformation due to collisions with the various parts of the mill (walls, balls, etc.). Consequently, strong plastic deformation occurs at high strain rates (~103-104 s-1) inside the particles, and the average grain size can be reduced to a few nanometres after prolonged milling. At first, MM leads to a rapid decrease in the average grain size to 40-50 nm (which constitutes a new contribution to the hardening of the material). Further refinement occurs slowly at about 15 nm after extended milling. The mean deformation at the atomic level reaches values of up to 0.7%. When the metal particles are plastically deformed, most of the mechanical energy spent in the deformation process is converted into heat, but the rest is stored in the metal, in the form of dislocations and other defects that strongly harden the metal. This was first investigated in detail for a series of high melting temperature metals with bcc and hcp crystalline structures. Metals with fcc structure are inherently more ductile and often show a stronger tendency to adhere to container walls and balls and weld to larger particles, often several millimetres in diameter, during the milling process. To avoid this, a process control agent (PCA) is added, essential to avoid excess soldering and guarantee the subsequent refining of the grain. Thus, even though the milling process is carried out with a pure powder, carbon, oxygen and nitrogen atoms from the PCA are incorporated in solid solution hardening the material, and could originate dispersed particles during the sintering process. The improvements made to the powder would be of little interest if they did not survive the consolidation process. Often, the high temperature exposure during sintering degrades the improvements made to the powder, e.g., increasing grain or dispersoids size. The choice of sintering conditions is therefore not an easy task. It is here that the extraordinary speed of electrical consolidation techniques can play an important role, reducing high temperature exposure to seconds, compared to 30 to 60 minutes for conventional sintering. In the present work, MF-ERS technique has been used to prepare compacts from aluminium powder milled for 5 hours in vacuum. After consolidation, compacts densification was measured, and microstructural studies were carried out. Likewise, microhardness, electrical resistivity and compression strength were determined, and results were compared with those obtained following the conventional consolidation route. 2. MATERIALS AND EXPERIMENTAL PROCEDURE As starting material, as-received elemental aluminium powder (AS 61, Eckart, Germany) was used. The powder was mechanically alloyed for 5 hours in vacuum. A 3 wt% of ethylene bisstearamide (EBS) micropowder organic wax was used as PCA to control the balance between the welding and fracture processes of the Al powder during milling. The 1400 cm3 water-cooled stainless steel vessel was filled with 72 g of powder and 3600 g of balls (charge ratio in wt.%: 50:1). Milling process allows reducing Al particle size from D[4,3] of 80.5 µm to 29 µm (Figure 1), as well as incorporating into the powder particles both the superficial aluminium oxide (Al2O3) layers and the carbon coming from the wax used as PCA. (a) (b) Figure 1. SEM micrographs of (a) as-received and (b) milled aluminium powder. Milled powder were submitted to a degassing process consisting in a vacuum (5 Pa) heat treatment at 350 ºC for 30 min, followed by furnace cooling. This is a necessary process to avoid that, during the heating stage of the MF-ERS process, gasses can be released once the powders form a material with closed porosity, therefore avoiding the material densification. Experiences carried out directly with as-milled powders reach a low densification, even lower for higher intensity and heating times, which are the proper conditions for both quickly reach a closed porosity and releasing a higher amount of gases. The consolidation process using MF-ERS was carried out in a medium-frequency resistance welding machine (Serra Soldadura, S.A., Spain) appropriately adapted for this purpose. The consolidation pressure was 100 MPa for all the experiences, in an alumina die wall –lubricated with graphite, and a powder mass of 1.8 g that were initially vibrated up to reach their tap density [18]. Different current intensities (6, 8, 10 and 11 kA) and dwelling (heating) times (400, 700 and 1000 ms) were employed to test the influence of these parameters on the powder consolidation. MF-ERS experiments start with a cold-pressing period (lasting 1000 ms) under constant pressure and no current passing through the powder. Then, electrical current is applied in the heating and pressing period. The consolidation finishes by only applying pressure for 300 ms during the cooling/forging period. For comparative purposes, cylindrical compacts of 1.8 g were also produced following the conventional PM route of cold pressing and furnace sintering. Milled powders were vibrated to reach their tap density and uniaxially cold pressed at 850 MPa in a hardened steel die, and sintered in vacuum (5 Pa) at 650 ºC for 1 hour. Powders morphology was observed by scanning electron microscope (SEM, FEI TENEO), while an optical microscope (Nikon Epiphot 200) was used for microstructural studies. Because of the compacts symmetry, Vickers Microhardness HV1 (Struers DURAMIN-A300) was only measured in one quadrant (Figure 2) of a longitudinal section of the samples. Figure 2. Diametrical section of cylindrical compacts with the distribution of hardness points. For the electrical resistance measurements at room temperature, a four points probe and a Kelvin bridge with a measuring range between 0.01  and 1000 Ω were used. The thermoelectric effects were eliminated by changing the probes polarity in the measurements, and the mean value was considered for each specimen. Compression tests were performed in a universal testing machine (Instron 5505), up to compacts breaking or until achieving a strain of 50%, whichever happening first. 3. RESULTS AND DISCUSSION The viability of applying the MF-ERS process to MM Al powder has been first analysed according to the porosity reached, although a detailed analysis of the different areas of the compact has been carried out because of the non-homogeneous structure typical of electrical consolidation processes. Once known the microstructure, the actual degree of sintering, i.e., whether particles really show metal-metal contacts has been checked by measuring both the microhardness of the prepared materials and their electrical resistivity, both properties very much related to the microstructural characteristics of the material. Finally, the macroscopic behaviour of the compacts was analysed by studying their compression behaviour. 3.1. Final porosities Final porosities for all the compacts were determined from their final dimensions, once determined the absolute density of the powder by picnometry, with a value of 2.56 g/cm3. The final porosity of the different compacts obtained by MF-ERS is shown in Table 1 as a function of the current intensity (I) and the heating time (t). As expected, porosities decrease for higher current intensities and heating times. Thus, it would be possible to estimate the final porosity of compacts, from the sintering parameters (I, t). Table 1. Values of the compacts final porosities (F) in %, for the different MF-ERS experiments. Heating time [ms] 400 700 1000 Intensity [kA] 6 23.6 20.0 17.6 8 19.0 14.3 12.5 10 15.2 12.5 10.5 11 12.3 11.1 7.0 On the other hand, compacts prepared by the conventional PM route (pressing and sintering) achieved a porosity of 6.5% after the pressing step, which is reduced down to 6% after being sintered. It is remarkable that the porosity in compacts obtained by MF-ERS can be relatively closer to that achieved for compacts produced by the conventional route, even though pressing and sintering times are very different. Conventional route needs a pressing and a sintering stages lasting more than 1/2 hour. On the contrary, the whole MF-ERS process lasts less than 1 second. 3.2. Microstructural study A microstructural study was carried out in order to evaluate the porosity distribution obtained for both methods. As can be seen in Figure 3, a spherical, small and uniformly distributed porosity is present in the sample obtained by the conventional route. Figure 3. Porosity distribution of a diametrical section of a conventionally consolidated compact: macrograph of the whole section (top) and micrographs at the corner and centre (bottom). However, the porosity distribution in MF-ERS compacts is non-uniform (Figure 4). The lack of uniformity is mainly due to a non-homogeneous temperature distribution during the MFERS process. As observed by Lenel [19], temperature is higher in the centre, and lower in the bases and sides of the compact. As shown in diametrical section of Figure 4, the periphery is more porous than the centre (darker in the images). Figure 4. Porosity distribution in MF-ERS Fe compacts. Lighter areas (more reflecting) indicate a lower porosity content (higher densification). Figures 5 and 6 show detailed micrographs of the inner structure of MF-ERS compacts, the first in the corner of the compact and the second in the section centre. Both sets of images clearly show that the porosity degree deceases for tougher electrical consolidation conditions (higher current intensities and/or heating times). Figure 5. Micrographs showing the porosity distribution in a corner of diametrical sections of MF-ERS compacts consolidated under different conditions. Figure 6. Micrographs showing the porosity distribution at the centre of diametrical sections of MFERS compacts consolidated under different conditions. According to the micrographs, the intensity seems to be some more determining than the heating time with regard to the obtained porosity. Only with 10 or 11 kA the corner of the compact acquires a relatively low porosity, even being quite dense for 11 kA – 1000 ms. The centre, however, reaches a low porosity for any of the compacts prepared with 10 or 11 kA and even for 8 kA – 1000 ms. 3.3. Microhardness distribution Mean microhardness of a conventionally consolidated compact, after measuring in the five point of Figure 2, resulted 102 ± 2 HV1. On the other hand, Table 2 gathers the mean values, and standard deviation from the five tested points, for the MF-ERS specimens. Because of the non-uniform distribution of the porosity, consequence of the different temperatures reached on different zones of the compact, hardness also show a clear non-uniformity inside the compacts.