Abstract
This study is based on a series of turning tests of UNS M11917 magnesium alloy, using low cutting speeds to emulate the conditions of repair and maintenance operations. More specifically, intermittent turning process, using both dry machining and minimum quantity lubrication system, is analysed. Cutting operations are done to evaluate the chip morphology obtained in order to assess their suitability for the cutting process and chip ignition probability of occurrence. From the point of view of machining, favourable chips are identified in all the tests done, getting chips of short length. However, in terms of ignition risk, short chips are considered unfavourable. Main results show how uncoated tools (HX) provided worse results in terms of short chips, while coated tools (TP2500) provided chips of size higher than 1 mm in all the cutting operations performed. Besides, a detrimental effect of the use of the minimum quantity lubrication system in the ignition risk is also identified.
Introduction
Magnesium is used in different industrial sectors such as aeronautical, electronic, medical or sportive, mainly due to its low density1–3 and good machinability.3,4 However, its use has some drawbacks to deal with, such as its reactivity with water to form hydrogen, which is flammable and potentially explosive,5,6 and the occurrence of sparks that represents an important ignition risk when temperature reached is over 450 °C. 7 Moreover, an imminent explosion danger is identified when particles with a size under 500 µm are generated. 7
Traditionally, continuous cutting has been widely used to study machining processes. However, nowadays, discontinuous cutting is a common type of operation in modern machining, and it has been used to generate good quality complex sculptured surfaces. 8 Discontinuous cutting process is characterised by the coexistence of cutting and non-cutting cycles due to the machining process 9 or as a result of the workpiece geometry.10–12
Friction between tool and workpiece and high temperatures generated during the cutting process lead to tool wear or, even, to tool failure. Therefore, a worse quality of the machined surface is expected. In order to diminish the effects of friction and temperature, cutting fluids have been widely used.13,14 In the case of magnesium cutting, the selection of a cooling system is a critical issue because of its reactivity with water.
In machining processes, the final shape is reached by removing material in the form of chips from the initial workpiece using cutting tools due to shear deformation.15,16 The chip is generated at the shear zone and then it moves along the rake face until it curls off and gets broken. 17 Chip formation process influences the efficiency of machining operations, and it can affect surface finish, workpiece accuracy and tool life. 18 The formation of favourable chips that can be easily and reliably evacuated from the working zone is very important because it can contribute to the improvement of the reliability of the process, the generation of high-quality surfaces, the increase of productivity and the protection of machines and tools. 19
Chip morphology is studied by Noordin et al. 20 during dry turning of stainless tool steel, identifying a clear influence of feed rate. The increase of feed rate tends to provide smaller chips. In the case of steels, Suresh et al. 16 state that in soft steels the generation of long continuous chips can be expected. Influence of minimum quantity lubrication (MQL) system in chip formation in the turning of AISI 1040 steel is analysed by Dhar et al. 21 Its use lets achieve favourable chips due to the more effective cooling. Besides, the size of the chips collected is smaller than when using dry and wet machining. From their side, Arai et al. 22 studied the chip formation process during skiving operations of magnesium alloys, acknowledging the importance of depth of cut and feed rate in order to obtain stable tubular helical chips.
This study is based on a series of turning tests of UNS M11917 magnesium alloy, using low cutting speeds, and both dry machining and MQL system. Thus, it is intended to emulate the conditions of repair and maintenance operations of complex and singular pieces, 23 analysing the ignition risk associated with that kind of operations. Because cutting conditions used in this kind of operations benefit the formation of small chips, it is important to know the conditions more favourable to avoid, or at least diminish, the risk of ignition during repair and maintenance operations of magnesium.
Experimental procedure
The experimental procedure established to analyse chip morphology during intermittent turning at low cutting speeds is based on the following steps:24,25
Previous activities to turning tests. The activities comprise the identification of the needed resources and the preparation of the protocols to register the observations of the turning process.
Turning tests. According to the experimental design, turning tests are done following the established order and under the cutting conditions defined for each test.
Chip collection. A sample of the chips produced in each test is collected, identified and saved in order to have it perfectly identified and accessible to be analysed after the realisation of the experiments.
Process monitoring. All the turning tests are recorded by video and the chips obtained are photographed with a high-resolution camera.
Chip designation. The samples of the chips collected are designated as favourable or unfavourable, according to their form and length, from the point of view of the machining process and in terms of ignition risk.
Applications
Workpiece material used was UNS M11917 magnesium alloy, whose composition is shown in Table 1. Three types of specimens were used in the tests (Figure 1) defined by a diameter of 53 mm and a length of 125 mm: one continuous and two discontinuous geometries characterised by the slot width (15 and 30 mm).
Composition (mass %) alloy UNS M11917.
Types of workpieces tried.
A parallel lathe (PINACHO model L-1/200) equipped with an external MQL system (Accu-Lube Micro-Lubrication system) was used. A specific lubricant for magnesium (r.rhenus Nor SSL) was used as cutting fluid in the MQL tests. Two types of tools with identical geometry (SECO manufacturer code: DCMT 11T308-F2), but different coatings, uncoated (HX), and coated (TP2500), Ti(C,N)+Al2O3 DURATOMIC®, were used.
Factors chosen for the experiment are as follows: depth of cut (d), feed rate (f), type of tool (t), spindle speed (n), type of interruption (i) and MQL flow rate (q). Depth of cut is fixed at one low level (0.25 mm) that corresponds to finishing operations. Table 2 shows the factors chosen and their levels.
Factors and levels.
Turning experiments were performed following the order established in the experimental design, 26 where different cutting conditions were analysed. During the realisation of the experimental tests, photographs and videos of the chips were systematically taken using a Nikon D5000 high-resolution camera.
Results and discussion
For practical reasons, a small sample of the chips collected in the experimental tests are shown in Figures 2 (HX tools) and 3 (TP2500 tools), conveniently arranged to let appreciate the chips according to the type of interruption and the cutting conditions tested.
Chips obtained using HX tools.
Chips obtained using TP 2500 tools.
Chip morphology
In its G annex, ISO 3685:1993 27 standard provides a classification of chips formed during cutting processes. In Table 3, the types of chips and the main options of these types according to the referred ISO standard are identified.
Classification adapted from ISO 3685:1993. 27
Chips collected during the different tests done are mainly of short length. According to ISO 3685:1993, predominant forms of the chips include arc chips, connected and loose, elemental chips and, in some cases, conical helical chips. From the point of view of machining, segmented chips are preferable to continuous and long chips. 28 So, according to the results obtained, chips can be identified as favourable because no long and continuous chips are formed.
Ignition risk
Magnesium reactivity with water,5,6 temperature reached during machining and the size of particles 7 were highlighted in the ‘Introduction’ section as possible sources of fire and/or occurrence of explosions in magnesium machining.
In this study, ignition risk is assessed in terms of the size of particles because high temperature in small particles could lead to ignition and/or explosions. Ignition risk should be assessed considering as guidance that particles with a size under 500 µm (0.5 mm) present an imminent explosion danger. 7 Although during all of the tests done no chip ignition or, even, sign of ignition appeared, an analysis of chips in terms of ignition risk can provide indications of conditions close or likely close to ignition.
In terms of ignition, chips are considered favourable (F) when the length of the majority of the chips collected is clearly higher than 1 mm. Besides, although ignition risk is higher when chips have lengths below 0.5 mm, all the chips that are not clearly above 1 mm are considered unfavourable (U). Thus, according to the samples collected, a classification of the chips is done in Table 4.
Classification of chips as favourable (F) or unfavourable (U) from the point of view of the ignition risk.
From that table, it is possible to recognise the influence of the tool used in the chip formation process. In general, HX tools provide smaller chips than TP2500 tools. HX tools give in 10 of the 18 cases analysed unfavourable chips, while TP2500 tools give favourable chips in all the tests performed. Because tool geometries are the same, differences in chip morphology have to be associated with the tool coating, acknowledging a better ability to break the chips at an earlier stage in the case of uncoated tools.
Cutting parameters play an important role in the chip formation. Zhou 29 indicates that there is a critical depth of cut and a critical feed rate that determine chip breakability. However, the analysis of the influence of the cutting parameters (depth of cut, feed rate and spindle speed) cannot be suitably done in this research because of the experimental design selected, and further research is needed to do it conveniently. However, considering that feed rate was fixed within a small range of low values, it is expected to get shorter chips if higher feed rates are used.20,29
From another side, the influence of interruption can be analysed by comparing the results obtained using the same cutting conditions for each of the types of tools tested. Analysing these results, it is possible to recognise the absence of a clear trend, appearing of unfavourable chips when cutting both continuous and interrupted workpieces.
In the case of the use of the MQL system, although the flow rates tested are within a small range, between 0 and 9 mL/h, it is possible to recognise a detrimental effect on the size of the chips when machining with the MQL system. In particular, two cases of unfavourable chips appeared in the dry machining case, while eight cases of unfavourable chips are identified in the MQL tests. The negative influence of the MQL system in terms of chip morphology can be explained by acknowledging that the cooling system helps to the breakage of chips during the chip formation process.30,31 This result agrees with the one presented in the work by Noordin et al. 20 that identifies a beneficial effect of the MQL system in order to obtain smaller chips. Besides, although dust emission was not studied in the research, according to Balout et al., 31 it is expected that the use of the cooling system would lead to a lower dust emission in magnesium machining.
Conclusion
Chip morphology obtained in the intermittent turning of UNS M11917 magnesium alloy at low cutting speeds lets evaluate the suitability of the process in terms of machining operation and ignition risk, highlighting the next conclusions:
In terms of the process, during intermittent turning of UNS M11917 magnesium alloy, discontinuous and short chips are obtained during the tests done. So, in terms of machining, chips obtained can be considered as favourable. Besides, according to the ISO 3685:1993 standard, chips can be classified as arc chips (connected and loose), elemental chips and, in some cases, conical helical chips.
In terms of ignition risk, short chips may be considered unfavourable when their size is below 500 µm. In the research, uncoated tools (HX) provided worse results in terms of short chips, while coated tools (TP2500) provided chips of size above 1 mm. No clear trend in chip morphology was found when assessing the type of interruption. Besides, related with cutting parameters, although MQL flow rate values used are small, a detrimental effect of the use of the MQL system on chip morphology in terms of ignition risk is also identified.
Footnotes
Acknowledgements
The authors would like to thank the support given by the research groups ‘Machining & Tribology (MACTRIB)’ (University of Aveiro) and ‘Industrial Production and Manufacturing Engineering (IPME)’ (National University of Distance Education – UNED). In addition, authors would like to thank Grupo Antolín Magnesio S.L. for the transfer of part of the materials used in this work.
Declaration of conflicting interests
The authors declare that there is no conflict of interest.
Funding
The authors would like to thank the Spanish Ministry of Science and Innovation and the Industrial Engineering School-UNED for funding the Projects DPI2011-27135 and REF2013-ICF03.