Influence of a micropatterned insert on characteristics of the tool–workpiece interface in a hard turning process

Highlights

• We fabricated the micropatterns on CBN insert by the layer-by-layer EDM machining.

• We performed AISI 52100 hard turning experiments with the micropatterned CBN insert.

• We reduced the resulting force by 2.5 ∼ 10.9% using the micropatterned insert.

• We decreased the flank tool wear by 9.7 ∼ 11.4% compared with the non-patterned insert.

Abstract

A micropatterned insert leads to decreases in cutting force, the coefficient of friction, and tool wear. This study prepared a pattern on the tool rake surface using layer-by-layer electrical discharge machining. Hard turning was investigated by measuring the cutting forces and chip morphologies. Friction was calculated by modeling continuous and saw-chip formation with various feed rates and surface velocities. Tool wear was measured using the increase in the material removal rate. The micropatterned insert decreased the force by 2.7 ∼ 10.9% compared with the non-patterned insert because the friction was reduced by 9.5 ∼ 34.5% with decreases in the feed rate and surface velocity. In comparison, the flank wear improved by 9.7 ∼ 11.4% for the micropatterned insert compared with the non-patterned insert as the surface velocity decreased. Air gaps on the micropatterned insert cause the friction reduction due to additional shear deformation, escaping wear particles into apertures, reducing the contact area of the tool-chip, and uniform contact stress.

Introduction

Turning processes involving hardened steel are referred to as hard turning. Hard turning yields significant benefits, including a reduction in the machining time, simple process steps, low costs, effective machinery setup, and delamination of cutting fluid (Jeon et al., 2013). König et al. (1990) reported that the hard turning shortened production sequence.

Cubic boron nitride (CBN) is the best cutting tool material for hard turning. CBN tools are suitable for high-temperature environments, have high toughness, and resist external impact during machining. CBN is also stable chemically and has a longer tool life than other materials (Sobiyi et al., 2014). However, the fabrication of CBN tools is expensive and requires numerous manufacturing processes. Consequently, extending CBN tool life is important. Many researchers have optimized the machining parameters and determined wear mechanisms to increase tool life in dry machining (Bartarya and Choudhury, 2012). Lima et al. (2005) conducted practical experiments of tool materials including PCBN, coated carbide, and mixed alumina with different workpiece materials and hardnesses: AISI 4340 (42, 48HRC) and AISI D2 cold steel (58HRC). Choudhury and Srinivas (2004) developed a mathematical model of the diffusive coefficient of flank wear. Bouacha et al. (2014) optimized the hard turning parameters using analysis of variance, response surface methodology (RMS), and the Grey–Taguchi method. He et al. (2014) found that the wear of TiAlN coated tools treated cryogenically was less than that of conventional machining of AISI 52100. Huang and Liang (2003) proposed modeling tool wear and mechanisms during hard turning. Poulachon et al. (2001) demonstrated the tool wear, chip formation, and cutting forces in various machining conditions. Karaguzel et al. (2014) reported that a non-conventional turning system increases tool life in machining difficult-to-cut materials. Kong et al. (2014) pointed out that laser-assisted machining (LAM) dramatically reduces tool wear compared with conventional machining.

Hardened steel can be machined after thermal softening of the workpiece by the heat generation at the tool-workpiece interface during the hard turning process. However, this mechanism reduces the life of CBN tools, so it is important to control the friction in dry machining.

A few researchers have demonstrated that the tool edge geometry influences machining performance. The curvilinear geometry of an insert can protect the cutting edge from chipping and a built-up edge (BUE), and enhances impact resistance, with better heat transfer from the cutting zone (Dogra et al., 2010). A chamfer on the insert edge strengthens and improves the resistance to chipping and breakage. Chamfered edges increase the strength of tools and decrease tool wear (Hirao et al., 1982). An edge honed with micro-geometry can reduce tool wear (Özel et al., 2008). Denkena and Biermann (2014) surveyed the influence of the tool edge micro-geometry on machining performance. The cutting edge geometry affects chip formation, forces, wear behavior, and surface integrity.

In the current work, the surface texture enhanced the tribological performance. Etsion (2004) found that laser surface texturing (LST) reduced friction, compared with a non-textured surface. Yuan et al. (2011) reported that microgrooved surfaces reduce friction due to hydrostatics, lubricant supply, and contact stress effects. Other researchers found that microtextures on a WC/Co disk surface filled with molybdenum disulfide solid lubricants decreased friction during the dry sliding test (Wu et al., 2012).

Several researchers have reported that adhesive effects are diminished with a cutting tool that is patterned (textured or grooved) on the rake face. Kawasegi et al. (2009) prepared microscale and nanoscale textures on tool rake faces using a femtosecond laser, and found that machinability was improved with both microscale and nanoscale textures. Sugihara and Enomoto (2009) developed a face-milling cutting tool with diamond-like carbon (DLC)-coated textures on the nano/microscale, and found that these textures improved the anti-adhesive effects during wet cutting. Chang et al. (2011) reported that a microstructured milling tool had the least flank wear. Obikawa et al. (2011) fabricated DLC-coated tools with four types of microtexture. The coefficient of friction and force were reduced effectively with parallel and dot-type microtextures. Furthermore, the edge distance is an important parameter. Xie et al. (2013) observed the effects of grooves produced using a process of a mechanical grinding on a tool rake surface: the grooved tool reduced friction and forces during the machining of titanium alloy. The cutting temperature was also reduced markedly to ∼500 °C, while the standard tool reached ∼1322 °C. Koshy and Tovey (2011) fabricated a textured cutting tool using electrical discharge machining (EDM). The textured tool reduced machining force. Thus, the effects of patterned cutting tools have been shown to decrease friction, wear, force, and temperature in many examples. However, to our knowledge, no research to date has explored the influence of micropatterned inserts in hard turning processes. Moreover, it is very difficult to machine micropatterns on CBN inserts because of the high hardness of the tool material. Thus, there is a need for a distinct approach to solve this problem.

In this study, a micropatterned insert was used to improve the tribology properties at the tool–workpiece interface during a hard turning process. A micropatterned insert was fabricated by a layer-by-layer EDM process considering electrode wear. Machining experiments in a hard turning configuration were conducted to investigate the influence of a micropatterned CBN tool under various machining conditions. Cutting force, chip morphologies, and tool wear were evaluated and friction was computed using the chip-formation equation. CBN tool wear was also assessed with increasing material removal rates (MRR, mm³/min) with non-patterned and micropatterned inserts. This research focused on the machinability and tool life of the micropatterned insert.