Tool wear characterizations in finish turning of AISI 1045 carbon steel for MQCL conditions
Introduction
The main cutting edge of the cutting tool in machining processes, especially in the case of hard materials, is subject to significant mechanical and thermal loads as well as aggressive chemical reactions [1]. The consequence of all those effects is resulting in wearing of the tool. Tool wear plays an important role in machining processes, especially during finishing, due to its effect on surface integrity and dimensional accuracy. The most reported wear patterns are flank wear and crater wear [2]. The possibility of predicting tool wear is thus necessary to avoid catastrophic tool failure, which affects the machining performance as well as the final part quality [1].
In the turning process, the most frequently observed types of wear are abrasive wear, especially when machining hard materials [1], adhesion wear due to interlocking of particles of the tribological pair materials under intermolecular forces and diffusion wear due to high temperatures and high stresses in hard turning [3]. Here, chemical interactions play a very important role, in particular oxidation that occurs intensively with high cutting speeds [3]. Chemical properties of the materials are also very important when machining with high cutting speeds since the temperature in the cutting zone accelerates the chemical reactions between the material of the tool and the machined material [4]. As an example, chemical wear of diamond tools occurs mostly due to the high affinity of ferrous materials to carbon [5].
In order to lower the temperature in the cutting zone, traditional flood emulsion cooling is applied. However, with the growing awareness of environmental effects and high costs of disposal of cooling lubricants [6], [7], studies on new improved cooling methods, limiting the negative impact of machining processes on the environment, are focused. The majority of coolants used in machining is not biodegradable and contain heavy chemicals that may be hazardous to the human health and the environment [8]. Dangerous bacteria can come into the skin contact or even in respiratory organ of the operators and may cause serious health effects. Additionally, when spoiled they cause bad smell that makes working conditions unpleasant. Also, the oils used can contain carcinogen elements that are extremely health problematic [8], [9].
Due to above emphasized problematic, studies on the use of environmentally friendly cooling methods that would prolong tool life with regard to dry machining and flood cooling, become inevitable. One direction for decreasing tool wear problems presents cutting tool coatings, e.g. multi-layer coatings [10], [11], [12]. Another way is the development of the minimum quantity lubrication (MQL) method [12], [13], [14], [15], based on cooling the cutting zone with oil mist, and the minimum quantity cooling lubrication (MQCL) method [6], [16], [17], where emulsion mist is the active agent. In order to increase the efficiency of cooling and lubrication in cutting zone, the active medium used in MQL and MQCL methods can be enriched with extreme pressure EP and anti-wear AW additives [16], [17], [18], [19], [20], or “eco- lubricating” is used in machining processes using graphite [21], [22] and/or molybdenum disulphide (MoS2) [23], [24] additives.
Analyzing different cooling lubrication methodologies, the current development on green manufacturing and the awareness of the negative impact of production on environmental, impose the idea of complete elimination of flood cooling method. Therefore, in this work no flooding way of cooling lubrication of the machining process will be analyzed. However, to correlate the idea of this work with conventional flood machining, the state-of-the art review on tool wear mechanisms, for flood cooling, dry machining and MQL method, has been made and summarized here. Tool wear research under dry cutting, flood cooling and methods based on minimum quantity cooling lubrication have been conducted by Da Silva et al. [25], Weinert et al. [26], Dhar et al. [27], Wakabayashi et al. [28]. Da Silva et al. [25] presented a comparative analysis of the impact of two machining conditions (dry and wet) on cemented carbide tool wear when finish milling the AISI1047 steel. Cooling/lubrication fluid was supplied to the cutting zone by three methods: flood cooling, reduced flow rate and MQL. The results demonstrated that a longer tool-life was obtained using reduced flow rate system. Furthermore, these cooling conditions, in the tool-chip interface, prevent the occurrence of cutting edge chipping. Dhar et al. [27] demonstrated that when turning AISI 4340 under MQL conditions, the tool wear has been observed to be lower than for the case of flood machining and/or dry machining. The cause of the tool wear reduction (VBmax) has been assigned to reduction of the temperature on the flank face. Therefore, it can be seen that the proper use of the MQL method can prolong tool-life on the account of the lower temperatures in the tool-chip interface.
To eliminate adhesion wear of cemented carbide tool inserts, Kümmel et al. [29] used different rake face textures. Laser surface texturing was applied on the rake face of the cutting tool with different features (dimples and channels) allowing changes in the adhesion tendencies of workpiece material to the cutting tool. Their research confirmed changes in adhesion conditions for built-up edge and the wear behavior even under dry conditions.
Chinchanikar and Choudhury proved that the use of MQL method reduces tool wear during the hard turning process by 20–25% compared with dry machining for coated inserts with nano-composite AlTiN, multi-layer nano-composite TiAlN/TiSiN and nanocrystalline AlTiCrN. The increased tool life when using MQL method is explained by obtaining lower temperatures in the cutting zone that are helping to reduce abrasion by retaining tool hardness and temperature-dependent diffusion type of wear [12].
Setti et al. [20] studied a nano-fluid used in MQL method based on water with Al2O3 and CuO additives during grinding. They proved that this fluid creates a thin tribofilm on the machined surface, which reduces the friction coefficient. The chip formation study also indicates effective cooling using nano-fluid, which reduces the wear area on wheel surfaces in the grinding process.
According to Weinert et al. [26], MQCL requires further research as the phenomena occurring during the application of this method are not thoroughly explained in comparison with MQL method. Improvements have to come from the lubrication process design in terms of delivery system, as well as from cooling performance side of view.
Many research publications in open literature demonstrate that the use of MQL method can slightly reduce the cutting tool wear during turning [13], [14] and that there are some reachability difficulties of lubrication fluid to the cutting zone [30]. In some of the machining processes, MQL [31] and MQL + EP/AW [32] methods can even show the opposite effect, i.e., increase in wear when using MQL method [32].
The aim of this study is to experimentally determine the effect of tribofilm formation on the surfaces of the tool in MQCL + EP/AW method that has the capability for tool wear reduction. Additionally, the study emphasizes the importance of basic input parameters of generated mist in MQCL + EP/AW method, such as air pressure and distance of the nozzle from the cutting zone. As a novelty in the field, for the first time the correlation between generating mist parameters and the effectiveness of EP/AW-based tribofilm formation, is introduced. This knowledge thus presents an innovative contribution to the relevant research filed of MQCL in machining processes.
The article thus presents a comprehensive analysis of the effect of MQCL (minimum quantity cooling lubrication) on the wear characteristic of a P25 cemented carbide cutting tool. The aim of the extensive research is to determine the cutting tool wear indicators (VBmax, KT) and types of wear (adhesion, adhesion-diffusion, diffusion, chemical) depending on the method of cutting zone cooling (dry cutting, MQCL and MQCL+EP/AW). During the study, the impact of formation of phosphate ester-based tribofilm used in MQCL+EP/AW method that has significant influence on reducing the wear of the cutting tool, is analyzed. Additionally, analysis is focused on the observations of influences of droplets distribution and their size on the tool wear. In this way, idea is to obtain results that would significantly contribute to the better understanding of operation and control of the mechanism of emulsion mist generation in MQCL method, which is substantial for their effective use in the industrial practice.
Section snippets
Experimental procedure
The cutting tool wear tests were conducted under dry machining conditions and with cutting zone cooling using MQCL and MQCL+EP/AW additives. EMULGOL emulsion concentrate based on highly refined mineral oil with additives such as ionic and non-ionic emulsifiers, corrosion inhibitors and other improvers were used for the generation of emulsion mist. Additionally, CRODAFOS EHA-LQ-(MH) based on phosphate ester was used as the EP/AW additive. The mixture was prepared using an electromagnetic stirrer
Flank wear
The impact of different lubrication and cooling strategies on the flank wear depending on the amount of material removed is shown in Fig. 3, Fig. 4, Fig. 5. The experimental studies were conducted at a constant cutting time of 20 min. In this work, every investigation has been four times repeated. Based on those data, the confidence limits for all experimental points and their mean values were calculated and are presented on the graphs. The graphs have been thus updated. On Fig. 6 the comparison
Conclusions
This article presents the impact of MQCL with phosphate ester-based EP/AW additive on the wear of cutting tools in comparison with dry and conventional cooling strategies. Various conditions of emulsion mist generation, which significantly affect the formation of tribofilm on the surfaces of the cutting tool and therefore the wear of the cutting tool point, were considered. Based on the experimental findings, the following conclusions can be made:
- I.
The lowest wear of the cutting tools was
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