Hard turning using HiPIMS-coated carbide tools: Wear behavior under dry and minimum quantity lubrication (MQL)

Highlights

• HiPIMS nanocrystalline AlTiCrN tool has encouraging potential to hard turning.

• Improvement in tool life almost by 20–25% when machining under MQL condition.

• Chipping off at the nose and clearance face: dominant wear forms in hard turning.

• High hardness high toughness coated cutting edge: highly resilient to edge fracture.

• Abrasion and adhesion as prominent wear mechanisms during hard turning.

Abstract

In the present work, experimental investigations carried out to assess the applicability of HiPIMS (High Power Impulse Magnetron Sputtering)-coated carbide tools to hard turning (55 HRC) and to address the widely debated topic about the use of coolants in hard turning are presented. Tool wear progressions and hence, tool life, different tool wear forms and wear mechanisms observed for tools coated with HiPIMS coating technique, namely, nanocomposite AlTiN, nanocomposite multi-layer TiAlN/TiSiN and nanocrystalline AlTiCrN are presented along with the images captured by digital and electron microscope. Characterization results of all the coated tools in terms of their average coating thickness (measured using Calotest and Fractographs), adhesion strength of the coating(s) (determined using Scratch test), composition and microhardness (using EDAX and Vickers microhardness test, respectively) are presented. Experimental observations indicate higher tool life with nanocrystalline AlTiCrN coated carbide tools which shows encouraging potential of these tools to hard turning. Improvement in tool life of almost 20–25% has been observed under minimum quantity lubrication (MQL) due to better cooling and lubricating effects. However, this effect was more prominent at higher cutting speed of 150 m/min.

Introduction

Among users in recent years, demands for extremely tough and hard steels are continuously increasing, which create ever new challenges for machining operations and place very high demands for high performance cutting tools. The field of cutting tool coating also continues to develop with new materials and stimulates interactively with the latest developments in thin films and coating deposition technologies. In recent years, research efforts have been oriented to the development of nanostructure and particularly hard nanocomposite coatings for cutting tools. The interest towards these particular systems comes from the flexibility in depositing different compositions and in engineering their mechanical and chemical properties. Another important feature of such materials, given by the nanometric size of the particles, is that high hardness is combined with high toughness [1].

For dry and hard machining, high hardness and high oxidation resistance at elevated temperature of more than 1000 °C are of utmost importance [2], [3], [4], [5]. Numerous studies have shown that the cubic fcc-TiN structured coatings with high aluminum content (such as TiAlN or AlTiN) can provide higher hardness, higher thermal stability (900 °C) as well as higher oxidation resistance than aluminum-free nitride or carbide coatings (such as CrN, TiN and TiC) [6], [7], [8], [9], [10], [11]. Therefore, during last few years, industrial application of monolayer and multilayer coatings based on TiAlN, deposited using PVD methods have considerably increased [12]. It is reported that the combination of the most important add-on materials such as Ti, Cr, Al and Si brings relevant advantages in machining performance in comparison to conventional cutting tool coatings [13]. The inclusion of chromium (Cr) to TiAlN or AlTiN (when Al content is higher than 50%) coating results in better thermal stability, mechanical strength and hardness of the coating [14]. The addition of silicon (Si) also leads to a significant improvement of TiN coatings [15], [16]. Kim et al. [17] observed superior properties with addition of elements, Al and Si, into TiN coatings compared to those of (Ti, Al)N and of (Ti, Si)N coatings. Ichijo et al. [2] also reported that cutting tools coated with (Ti, Al, Si)N coating are more wear resistant than tools coated with (Ti, Al)N coating.

Recent years have seen a massive interest in pulsed sputter PVD coatings among users. High Power Impulse Magnetron Sputtering (HiPIMS) represents the latest developments in the field of pulse technology that provides high-energy power pulses in the megawatt range to the sputtering metal source (target) causes a high-density plasma (10¹⁹ m⁻³) to form in front of the target, resulting in an ionization of the sputtered material up to three orders of magnitude higher than for a conventional direct current Magnetron Sputtering (dcMS) technique [18]. It is reported that cutting tools coated with HiPIMS technique (HiPIMS-coated) showed significantly improved mechanical, tribological properties compared to conventional coatings [19], [20]. Moreover, cutting tools coated by this technique are able to process hard-to-machine materials such as nickel-based alloys and stainless austenitic steels at lower costs with significantly improved machining parameters and considerably less tool wear [21], [22]. The main advantage of HiPIMS coatings include a denser coating morphology with high thermal stability and oxidation resistance, an increased ratio of hardness to Young’s modulus (which is a measure of toughness), and a very uniform and smooth coatings free from droplets compared to conventional PVD coatings.

As cutting fluids pose a serious problem to the preservation of the environment and to human health [23], numerous studies have been attempted to assess the machining performance under dry cutting [24], [25], [26], wet cutting and cutting with minimum quantity lubrication (MQL). Most of the researchers observed better performance under MQL condition in comparison to cutting under flood coolant and dry condition [27], [28], [29], [30], [31]. However, some of the researchers observed that MQL technique did not significantly reduce the tool wear [23], [32]. Attanasio et al. [33] experimental study also concluded that although, MQL gives some advantage during turning operation, but it presents some difficulty in exactly reaching the lubricant at the cutting zone. In recent years, carbide inserts coated with new generation coatings using HiPiMS technique, which are suggested for machining of hardened steel up-to 70 HRC, are made available by leading tool manufacturers. However, almost no attempt has been made to assess the applicability of these HiPIMS-coated cemented carbide tools to hard turning. Also, there is disagreement between the researchers about the use of coolants in hard turning, which needs further investigations, as almost no attempts have been made to evaluate the performance of these new generation coated tools under dry and minimum quantity lubrication (MQL) cutting environment during hard turning.

With this view, in the present work, experimental investigations carried out using HiPIMS-coated nanocomposite AlTiN, nanocomposite multi-layer TiAlN/TiSiN and nanocrystalline AlTiCrN carbide tools during hard turning of AISI 4340 steel (54–57 HRC) under dry and minimum quantity lubrication (MQL) conditions are presented. Characterization results of all the coated tools in terms of their average coating thickness (measured using Calotest and Fractographs), adhesion of the coating(s) with the substrate (determined using Scratch test), composition and microhardness of the deposited coating(s) (using EDAX and Vickers microhardness test, respectively) are presented. Tool wear progressions and hence, tool life, different tool wear forms and wear mechanisms observed for these tools under dry and MQL cutting conditions are compared and presented in view of evaluating the benefit of MQL during hard turning.