Processing and Characterization of Fe-Mn-Cu-Sn-C Alloys Prepared by Ball Milling and Spark Plasma Sintering

JMEPEG (2018) 27:1475–1483 https://doi.org/10.1007/s11665-018-3181-5 The Author(s). This article is an open access publication 1059-9495/$19.00 Processing and Characterization of Fe-Mn-Cu-Sn-C Alloys Prepared by Ball Milling and Spark Plasma Sintering Elz_ bieta Ba˛czek, Janusz Konstanty, Andrzej Romański, Marcin Podsiadło, and Jolanta Cyboroń (Submitted August 22, 2017; in revised form December 31, 2017; published online January 29, 2018) In this work, Fe-Mn-Cu-Sn-C alloys were prepared by means of powder metallurgy (PM). Powder mixtures were ball-milled for 8, 30 and 120 h and densified to < 1% porosity using spark plasma sintering (SPS) at 900 °C and 35 MPa. After consolidation, all samples of the Fe alloys were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), hardness and flexural strength tests. Resistance to abrasive wear was evaluated in both three-body abrasion and two-body abrasion tests. The SEM observations revealed an evident dependence of grain size and microstructural homogeneity on milling time. The XRD analysis showed a marked increase in austenite content in the as-sintered specimens with milling time. Although the proportion of deformation-induced martensite was small, the strengthening effect of abrasion on the subsurface layer of the investigated alloys was clearly indicated by Knoop hardness measurements. Keywords ball milling, diamond-impregnated composite, Fe alloys, powder metallurgy, spark plasma sintering, wear resistance 1. Introduction Sintered diamond-impregnated tools consist of diamond crystals embedded in a metallic matrix most often composed of: cobalt, copper, nickel, tin, iron, tungsten and tungsten carbide. Cobalt has long been the most valued matrix material used for professional tools, due to its excellent diamond retention characteristics and sinterability of commercial Co powders. As the price of cobalt is highly unstable and increasingly contributes to the overall tool cost, the recent industrial trend is toward replacement of cobalt-containing matrices with other, preferably iron alloys produced by PM (Ref 1-10), by various routes (Ref 11, 12). In our previous publications, we have identified and reported on such a promising alloy Fe—12% Mn—6.4% Cu—1.6% Sn—0.6% C (Ref 6-9). Mixed powders were ball-milled and hot-pressed in a graphite mold. Before evaluating diamond retention by expensive industrial-type tests, mechanical and tribological properties of the prospective matrix material are explored. It is important to find the best processing route, and therefore, spark plasma sintering (SPS) technique is now considered. It is a variant of pressure-assisted sintering which has the merit of high heating rates, broad range of sintering temperature and short sintering time (Ref 13, 14). A El_zbieta Ba˛czek, Marcin Podsiadło, and Jolanta Cyboron, The Institute of Advanced Manufacturing Technology, Wroclawska 37A Str., 30–011 Kraków, Poland; and Janusz Konstanty and Andrzej Romański, AGH University of Science and Technology, Al. A. Mickiewicza 30, 30–059 Kraków, Poland. Contact e-mail: . Journal of Materials Engineering and Performance typical SPS equipment is fitted with a vacuum chamber, wherein a graphite die is positioned, and a hydraulic pressing system for the application of pressure. First, the diamond-metal powder mixture is axially compacted inside the graphite die until the desired pressure is reached. Then the pressurized powder is heated up to the sintering temperature by passing a high-frequency pulsed current through the graphite die and, in the case of conductive materials, also through the sintered powder. The transition of current pulses through the powder bed leads to the formation of plasma discharges/sparks between adjacent powder particles. The aforementioned phenomena and very fast heating rates (up to 800 K/min) make SPS viable for commercial use (Ref 13, 15). The technique can be successfully used for synthesis and processing of ceramics, metals, cermets, intermetallics, etc. (Ref 16-19). The pulsed current, which differentiates SPS technique from the conventional hot pressing, has also been tested as an alternative method of heating in ultrahigh-pressure synthesis of superhard materials (Ref 20, 21). It can also offer new ways for the manufacture of materials having properties tailored for special cutting or grinding tools. In the present work, we discuss the feasibility of SPS technique in densification of ball-milled Fe-Mn-Cu-Sn-C alloy powders intended for the fabrication of diamond tools used to process abrasive materials. 2. Materials and Methods 2.1 Materials The ball-milled Fe-Mn-Cu-Sn-C alloy powder, which contained 12 wt.% Mn, 6.4 wt.% Cu, 1.6 wt.% Sn and 0.6 wt.% C, was prepared from commercially available powders. Spongy iron, ground ferromanganese and water-atomized tin-bronze powders were provided by Höganäs, ESAB and NeoChimie, respectively. The morphology and chemical composition of the starting powders are shown in Fig. 1 and Table 1, respectively. Volume 27(3) March 2018—1475 Fig. 1 SEM images of the starting powders: (a) spongy iron grade NC 100.24, (b) tin-bronze grade NAM40-80/20, (c) ground ferromanganese grade XH1210, (d) ground ferromanganese grade XH1218 Table 1 Chemical composition and particle size of the starting powders Chemical composition, wt.% Powder Powder grade Fe Mn Cu Sn C Mean particle size, lm (a) Iron Ferromanganese Ferromanganese Tin-bronze NC100.24 XH1210 XH1218 NAM40-80/20 100 bal bal … … 80 80.5 … … … … 80 … … … 20 … 7.0 1.5 … 85 134 143 23 balbalance (a) Sieve analysis 2.2 Ball Milling Prior to milling, the powders were mixed in the required proportions for 10 min in a Turbula-type mixer. The mixture was subsequently divided into three equal parts, which were ball-milled in a roll mill for 8, 30 or 120 h in air. The milling container was filled with powder and 12 mm 100Cr6 steel balls to 50% of its volume and rotated at 70% critical speed. The ball-to-powder weight ratio was 10:1. After milling, the particle shape and size were examined by SEM and sieve analysis, respectively. In order to determine the effect of milling time on the phase composition of Fe-Mn-Cu-Sn-C alloy powder, an xray phase analysis was performed. 2.3 Spark Plasma Sintering (SPS) The ball-milled powders were sintered using the HPD 5 type SPS system. Each powder was placed in a graphite die, 30 mm in diameter, axially pressed under 35 MPa, heated to 900 C at 100 K/min and held at temperature for 5 min in argon. A 0.5mm-thick graphite foil was used to isolate the sintered powder 1476—Volume 27(3) March 2018 from graphite punches in order to facilitate extraction of the sintered body. The graphite die was additionally wrapped with a carbon blanket to minimize heat losses during sintering. Typical SPS curves are presented in Fig. 2. 2.4 Characterization of Sintered Specimens The as-sintered densities were measured using the ArchimedesÕ pri (...truncated)