Investigation and Optimization of Sn/Gr Lubricants Effects on Cold Extrudability of Fe-TiC Nanocomposite Using Taguchi Robust Design Method ()
1. Introduction
Invention and development of advanced materials is a necessary requirement for technological progress. Accordingly, nowadays many researches are performed to minimize the restrictions concerning with their industrial production. Fe-TiC is a unique composite which has been under investigation since 1950 [1]. According to literature survey done by the authors there are many articles focused on the Fe-TiC compsites that can be categorized to 3 groups, synthesis, wear/mechanical properties and hot formability. In the first approach the investigators paid attention to the synthesis of Fe-TiC compsites [1,2]. Another approach focused on the wear and mechanical behavior of Fe-TiC compsites [3,4]. Another viewpoint concentrated on improvement of formability of Fe-TiC composites [5,6]. Until 2001, researches were invented the various synthesis methods of this composites, such as powder metallurgy, mechanical milling etc. But due to defect of limited formability of Fe-TiC composites, investigation was focused on formability improvement of this material [5,6]. In 2007, Peter Zwigl et al., create the super plasticity in ferrotic composite with performance a phase transformation [6]. Werner Theisen et al., performed the hot direct extrusion process on ferrotic composite. The achieved results indicated the anisotropy in microstructure and wear properties of Fe-TiC composites. They proved that both hot and cold working are not suitable for it. This is because due to its high hardness and strength, cold work forming causes to make some severe defects [5]. Because of these limitations, in the recent years, melting methods are utilized as an alternative for powder methods in Fe-TiC production, due to their ability in forming parts with complex shapes [1]. But high energy and specific equipments required for melting method production are still motivations to continue research on overcoming the difficulties of powder methods [1]. In powder metallurgy, the ability of adding powder lubricant to enhance extrudability makes it to be one of the most appropriate methods in forming such composite. To the best of our knowledge, and in spite of importance of role of lubricant on extrudabilty of Fe-TiC composite, the subject has not been under more attention. Thus, the main goal of this study is to study the effects of Sn/Gr lubricants on improvement of cold extrudability of Fe-TiC nanocom-posite.
2. Experimental Method
2.1. Materials
Iron powder with average size of 150 µm and titanium carbide powder with approximate size smaller than 36 nanometers were used as starting material to form Fe-TiC nanocomposite. Besides, Tin powder with average size smaller than 50 µm and graphite powder were used as powder lubricant. Figure 1 show the TEM micrograph taken from nano size titanium carbide powders.
2.2. Sample Preparation
Mixture of Iron, TiC, Tin and Graphite powder were milled by steel balls (10, 12, 15 mm) with ball to powder weight ratio (BPR) 10:1, in a low energy planetary ball mill with 250 rpm. For all of the samples, equal amount of powder (14 gr) were compressed by ZWICK universal machine in a 14.18 mm diameter cylindrical die with 300 MPa stress. This low pressure, avoids high work hardening of powders which makes more pressure required in extrusion step. Besides, high compress pressure leads to lubricant powders driven to porosities, decreasing the lubricant role in particles slip. But the insufficient strength of low pressure compressed powders makes extrusion impossible. So in order to obtain required strength for extrusion, all samples were pre-sintered by a digital oven with Ar atmosphere in temperature 450˚C for 1 hr with warming and cooling rate of 10˚C/min. According to previous studies [7], proper temperature required for sintering of iron-based composites, is in the range of 400˚C to 600˚C. Considering no high strength required and in order to avoid full sintering of powder particles, temperature 450˚C was selected for pre-sinter of samples. The extrusion die was made of heat treated and finished SPK. Area reduction of die was 20% with 8.5˚ semi die angle (Figure 2). Extrusion process was done in two conditions, with and without graphite as die wall lubrication. It was tried that all of the samples lubricated with same conditions. Extruded samples were rated according to their quality and analyzed by Taguchi robust design method of system optimization.
2.3. Main Parameters and Their Levels
In order to investigate the lubricant effect on extrudability of Fe-TiC nanocomposites, parameters and their levels mentioned in Table 1, were selected as Taguchi design factors.
Figure 1. TEM micrograph of TiC nanoparticles used for Fe-TiC nanocom-posite production.
Figure 2. Area reduction and semi die angle used in this research.
Table 1. Controlling factors and their levels.
2.4. Table of Experiment Design
Table 2 illustrates Taguchi orthogonal L16 matrix that considers experiments with 5 factors A, B, C, D, E in which every factor has 4 levels identified in Table 1. Each of the experiments repeated twice and quality of the same samples was very similar.