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Influence of tool steel microstructure on initial material transfer in metal forming—In situ studies in the SEM
Wear 302 (2013) 1249–1256
Contents lists available at SciVerse ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Influence of tool steel microstructure on initial material transfer in metal forming—In situ studies in the SEM J. Heinrichs a,n, M. Olsson a,b, S. Jacobson a a b
˚ Angstr¨ om Tribomaterials group, Uppsala University, Sweden Materials Science, Dalarna University, Sweden
a r t i c l e i n f o
abstract
Article history: Received 25 January 2013 Received in revised form 30 January 2013 Accepted 31 January 2013 Available online 8 February 2013
Metal forming constitutes a group of industrially important processes to form metallic components to net shape. When forming aluminium and other materials that tend to stick to the tools, problems associated with material transfer, e.g. galling, may occur. In a previous study by the present authors, in situ observations of aluminium transfer during sliding contact in the SEM revealed that the surface topography and chemical composition of the tool steel counter surface have a strong impact on the initial material transfer tendency. Even if carefully polished to a very smooth surface (Rao 50 nm), transfer of aluminium was found to immediately take place on a very fine scale and preferentially to the surface irregularities presented by the slightly protruding M(C,N) particles (height 15 nm) in the tool steel. In contrast, the less protruding M6C carbides, as well as the martensitic steel matrix exhibited very little initial transfer. The mechanism behind the preferential pick-up tendency displayed by the M(C,N) particles was not fully understood and it was not possible to determine if the decisive mechanism operates on the microstructural scale, the nanoroughness scale or the chemical bonding scale. In the present study, these mechanisms have been further investigated and analysed by comparing the very initial stages of material transfer onto different types of tool steels in sliding contact with aluminium in the SEM. The tool steels investigated cover conventional ingot cast and powder metallurgy steel grades, selected to possess a range of different types, amounts and sizes of hard phase particles, including MC, M(C,N), M7C3 and M6C. The transfer mechanisms are investigated using high resolution SEM, and the differences between the different microstructures and carbide types are carefully analysed. The implications for real metal forming are discussed. & 2013 Elsevier B.V. All rights reserved.
Keywords: Surface topography Microstructure Material transfer Scanning electron microscopy In situ studies
1. Introduction High friction and transfer of work material to tool surfaces constitute important problems in many industrial forming operations, such as in cold forming of aluminium. The high friction results in high forming forces that limit the forming possibilities; for example the cavities of dies will not be completely filled when the shapes become too complex or their radii too small. The transferred work material roughens the surfaces and results in increased friction forces when forming the following piece and also impairs its surface finish. Thick and uneven transferred material will result in scratches and other types of imperfections on the surface formed. This process is commonly called galling. The hardened aluminium can be removed from the tools by wet chemical etching. However, this causes production stops that are
n
Corresponding author. Tel.: þ46 184 717 236; fax: þ 46 18 471 35 72. E-mail addresses: [email protected] (J. Heinrichs), [email protected] (M. Olsson), [email protected] (S. Jacobson). 0043-1648/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2013.01.114
highly undesired and further the etching agent might etch the tool, although to a lesser degree. The area has been researched for many years, and the forming processes (tool materials, coatings, surface preparation processes, lubricants, etc.) have gradually been developed so that today more advanced shapes and more units can be formed before the tool must be exchanged or reconditioned [1–6]. The transfer of aluminium to harder (tool) surfaces and its influence on friction has been investigated using several approaches. Despite the extensive research, there is still a lack of understanding of the physical and chemical mechanisms involved. Such understanding would be highly useful when developing tool materials, tool coatings, tool surface preparation techniques or lubricant systems. One outstanding problem is to describe the decisive mechanisms behind the initiation of transfer. Usually, the very initial stages of transfer have not been studied at all. Rather, the surfaces have been investigated first when a lot of material has been transferred, and therefore covered any signs of where it originated. Furthermore, the evaluation techniques or the instrument
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settings selected have typically not allowed detection of thin transfer layers [7–9]. Recently, the present authors have developed and employed a technique to evaluate the very first sliding contact between tool material surfaces and aluminium and stainless steel [10–12]. The technique involves a sharp tip of work material put into contact with a tool material under a controlled load, followed by a relative movement of the two surfaces. The test rig is adapted in a Scanning Electron Microscope (SEM) to allow direct observation of the dynamics of the tribological contact. With this well controlled and limited contact, the observed movement and passages of surface features can be directly related to load variations, friction responses and material transfer. The passage of the tip involves a sliding distance determined by the diameter of its flattened end (typically 100–200 mm). This allows observations of transfer and other changes to the tool material flat during very limited sliding distances, not easily attainable by other experimental techniques. The contact surfaces are further studied in the SEM, using low acceleration voltage to be able to also detect such thin transfer layers that would be transparent when using the typical, higher acceleration voltages. In Ref. [10], the tested power metallurgical tool steel (Vancron 40) immediately showed transfer of aluminium, even if carefully polished. The transfer took place at different scales: on the scale of surface damage (filling up scratches, building up in front of ridges and protrusions around scratches) and on the scale of microstructure of the steel (on top of or in front of slightly protruding hard phase particles). The effect was very strong despite of the very limited roughness on the tool surface. In Ref. [11], the tool steel was compared to one of the most successful DLC coatings. The difference in transfer onto the polished surfaces was dramatic. The DLC showed almost no transfer even after 10 passages of the aluminium tip, while the tool steel picked up aluminium already at the first passage, most notably on top of the slightly protruding carbonitride particles. On subsequent passages more aluminium was transferred, now mostly building up in front of the initial transfer. The friction force increased correspondingly for every passage, while the surface became
rougher and rougher. It could therefore be concluded that it is this surface, roughened by transferred lumps, rather than the initial surface that determines the initial ‘‘steady state’’ friction level. Contrastingly, for the DLC, almost no aluminium was transferred and the friction stayed at the initial very low level. It was concluded that the great advantage of the DLC coating over the tool steel primarily seems to be its more advantageous topography after polishing, probably in combination with very weak adhesion tendency towards the aluminium oxide. The objective of the present study was to further investigate the generality of these findings, by including more types of tool steels, with different amounts of hard phase, different hard phase sizes and different hard phase compositions. One main challenge is to be able to separate the influence from local surface roughness and local surface chemistry, in determining the transfer tendency. After fine polishing, these differences also result in different roughness on a nanometre scale. The hard phases protrude the surface to different heights. The contact between aluminium and four different tool steels is investigated and analysed by comparing the very initial stages of material transfer, and the corresponding change of the friction coefficient. The tool steels investigated cover ‘‘carbide free’’ matrix, conventional ingot cast and powder metallurgy steel grades, selected to possess a range of different types, amounts and sizes of hard phase particles, including MC, M(C,N), M7C3 and M6C.
2. Experimental The initial conditions and material transfer in a forming operation have been studied in a contact situation where the work material (aluminium) has been represented by a relatively sharp tip that has been slid against a polished tool steel surface under such a load that the softer work material has deformed plastically. The contact area becomes very limited, initially determined by the load and the hardness of the tip. In these tests the initial contact area has been around 100–200 mm in diameter. 2.1. Materials
Table 1 Chemical composition (in wt%) and hardness of the aluminium alloy 6082 [13]. Si
Mg
Mn
Fe
Cr
Cu
Zn
Others Al
1.2 0.78 0.50 0.33 0.14 0.08 0.05 0.15
Hardness, HV (kg/mm2)
Bal 80
Fig. 1. Sketch showing the cylindrical aluminium pin (work material) with the final tip shape.
2.1.1. The work material The work material used was an industrially important aluminium alloy (6082), including the main alloying elements magnesium and silicon, see Table 1. The work material cylinders were manufactured from sheet metal by turning in a lathe, followed by a rough shaping of the tip. The final tip shape, see Fig. 1, was formed with SiC grinding paper, first 1000 grit followed by 4000 grit. The cylindrical part was 2.9 mm in diameter and about 15 mm long. The conical end part was 5 mm long with a tip angle of about 351. The tips were made as sharp as possible, which with this simple technique typically left a relatively flat end surface, less than 50 mm in diameter. During the first static loading, the sharper tips become plastically flattened, leaving a flat surface of diameter 50–100 mm. Due to this deformation, the differences from manufacturing are evened
Table 2 Chemical composition (in wt%) and hardness of the tool steels investigated. Steel gradea
C
Si
Mn
Cr
Mo
W
N
V
Fe
Hard phase content (vol%)
Hardness, HV3 (kg/mm2)
Dievar Sverker 21 Vancron 40 Vanadis 10
0.35 1.55 1.10 2.90
0.2 0.3 0.5 0.5
0.5 0.4 0.4 0.5
5.0 11.8 4.5 8.0
2.3 0.8 3.2 1.5
– – 3.7 –
– – 1.80 –
0.6 0.8 8.5 9.8
Bal Bal Bal Bal
MC, 1–2 M7C3, 13 M6C, 5 MCN, 19 MC, 16 M7C3, 7
6407 20 7307 20 7907 30 8107 30
a
Uddeholms AB designation.
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Fig. 2. Examples of surface irregularities and microstructure features for the four tool steels. AFM (a, c, e, g) and SEM (b, d, f, h) were used to image the same area for each steel (about 10 mm 10 mm). (a) Topography map of MATRIX. (b) Microstructure of MATRIX, showing lack of large hard phase particles. (c) Topography map of CAST. (d) Microstructure of CAST, showing a large M7C3 carbide, slightly darker than the matrix. (e) Topography map of PM1. (f) Microstructure of PM1, showing M(C,N) appearing black and M6C appearing white. (g) Topography map of PM2. (h) Microstructure of PM2, showing M7C3 appearing grey and MC appearing dark grey.
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Table 3 Topography data for the four steels. The data were achieved from measurements in the AFM. Typical values
Steel type MATRIX
CAST
– –
2 nm (M7C3)
0.59 nm
0.95 nm
PM1
PM2
Hard phase Protrusion Depression Ra (10 mm 10 mm)
6–8 nm (M(C,N)), 1–2 nm (M6C) 2 nm (MC), o1 nm (M7C3) 1.3 nm
0.53 nm
depressions are given in Table 3, together with the average surface roughness value Ra, measured in the AFM. 2.2. Experiments in the in situ manipulator in the SEM
Fig. 3. Schematic drawing of the in situ manipulator in the SEM.
out. The hardness of the alloy depends on the degree of work hardening, and is estimated to be about 80 HV in the contact area.
2.1.2. The tool materials Four different tool steels: a matrix steel grade, Dievar, a conventional high alloyed ingot cast steel grade, Sverker 21 and two PM steel grades, Vancron 40 and Vanadis 10, were included in the tests, see Table 2. In the following sections these grades are denoted MATRIX, CAST, PM1 and PM2. The microstructures of the tool steel grades in the hardened and tempered condition consist of a martensitic matrix (MATRIX) and a martensitic matrix with a hard phase consisting of relatively large M7C3 carbides (CAST), fine M6C carbides and M(C,N) carbonitrides (PM1) and fine MC and M7C3 carbides (PM2). The sizes of the carbide particles are up to 50–100 mm in the ingot cast steel, around 1–2 mm for the carbides in the PM steels and around 0.7 mm for the carbonitrides in the PM1 steel. Prior to the test all steel samples were grinded and carefully polished using 1 mm diamond in the last step. The polished tool steel surfaces were studied using Atomic Force Microscopy (AFM; PSIA XE150) and Scanning Electron Microscopy (SEM; LEO 1550) after the final polishing step, see Fig. 2. The hard phase particles, visible in the SEM due to compositional contrast, were affected differently by the polishing compared to the matrix steel. This caused some protrusion, alternatively depression, of the flat surface in connection to the hard phase particles; compare Fig. 2a, c, e and g with b, d, f, h, respectively. The height of the protrusions and depth of the
The tests were performed in an in situ manipulator comprising a work material tip in contact with a tool steel flat, as schematically illustrated in Fig. 3. The equipment is installed in an SEM (Leo 440) and operates under vacuum, so that the sliding contact can be directly observed using the electron beam and a secondary electron detector. The working distance is fixed at about 40 mm and to achieve decent resolution at this long distance, the lowest acceleration voltage is limited to 20 kV. The tip is mounted orthogonal to the flat surface. To offer a good viewing angle in the SEM, the tip on flat arrangement is tilted to 601. During testing, the tip is held stationary and loaded using a spring while the flat is moved using electric motors. Also the spring loading and tip positioning mechanisms are driven by electric motors. During the sliding test the normal load and friction force are continuously monitored. All tests comprised sliding of the aluminium tips against the prepared tool steel surfaces. The normal load was set to 3 N in all tests, the sliding distance was 4 mm for each passage and the sliding speed was roughly 3 mm/min. The tests were run unlubricated, to represent the critical moments in actual forming when the lubricant film present becomes too thin to separate the surface or is locally missing. Before testing all samples were ultrasonically cleaned in acetone and ethanol for 3þ 3 min. The aluminium tips were initially relatively sharp, but flattened on the first static contact, as mentioned. The contact area then rapidly grew during the sliding contact, and more during high friction conditions. Both plastic deformation and wear were found to contribute to the contact area growth. Typically, the flat end of the tips exhibited diameters in the 100–200 mm range after testing. The tests comprised 1, 5 and 10 passages along three parallel tracks over the tool surface. After each passage, the tip was lifted out of contact, the flat was moved back and the tip again lowered at the original starting position. The passages were studied in the SEM, which facilitated direct observation of tip deformation and transfer of material to the tool surface. This allows for immediate correlation between the work material transfer and changes in tool steel surface appearance as well as influence on friction. 2.3. Post-test characterisation The direct in situ studies were supplemented by highresolution SEM studies of both flats and tips using a Zeiss Ultra 55 FEG-SEM and a LEO 1550 FEG-SEM. The element composition of the surfaces was analysed by energy dispersive X-ray spectroscopy (EDS) using an Oxford Inca Energy system as well as an Oxford Aztec X-max system. Since the transferred material was typically very thin, a low primary electron energy of 3 keV (resulting from using an acceleration voltage of 3 kV) was preferably used in order to limit the interaction depth. When imaging, this results both in a shallow
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‘‘depth of sight’’, and a higher resolution, due to the limited broadening of the area emitting secondary electrons.
3. Results 3.1. In situ observations During testing, only occasional micrometre sized transfer could be observed on the polished steel surfaces, irrespective of number of passages and type of tool steel grade. While sliding over the polished MATRIX and CAST steel surfaces the aluminium tip shows very limited macroscopic deformation. This is also true while sliding over the two polished PM steel surfaces during the first passage. However, with increasing number of passages against the two PM steel surfaces the aluminium tip surface starts to shear towards the rear edge. After 10 successive passages this has resulted in a significant distorted tip geometry.
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3.2. Friction behaviour Fig. 4 shows the friction development during 10 passages in the same track for the four steel grades investigated. As can be seen, significant differences in the friction behaviour can be revealed. For aluminium sliding against the MATRIX grade the coefficient of friction was very low for all 10 passages, starting at around 0.12 during the first passage and then decreasing for each passage in the same track, ending at about 0.08 during the 10th passage. The CAST grade displays a similar decrease in friction coefficient with increasing number of passages but in this case from around 0.25 during the first passage to 0.20 during the 10th passage. The two PM grades display an opposite behaviour where the coefficient of friction is low, around 0.15, during the first passage and then increases with increasing number of passages reaching values in the range 0.45–0.50 for the PM1 grade and 0.35–0.45 for the PM2 grade. For both PM grades the coefficient of friction reaches its maximum in the very start of the track after which it slightly decreases during the sliding event. This tendency is most pronounced for passages 2–5 and especially for the
Fig. 4. Friction recordings for each of ten passages along the same track by the aluminium tips over the different tool steel surfaces. (a) MATRIX, (b) CAST, (c) PM1, and (d) PM2.
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PM2 grade. It should be noted that the obtained friction curves are in good agreement with the in situ observations of the aluminium tip as discussed above, i.e. that the pronounced shearing and deformation of the aluminium tip observed during sliding against the PM surfaces is due to high friction coefficients (high tangential forces).
smoother than both the carbonitrides and the matrix. As a result, the final surface topography of the different steel grades may differ significantly on a nanometre scale.
4. Discussion 3.3. Transfer of work material During testing, only occasional micrometre sized transfer could be observed on the polished surfaces. However, the combination of higher magnifications and lower acceleration voltages in the FEG– SEM makes it possible to resolve thinner transfer layers and finer scratches and the initial transfer mechanisms can be studied in detail, see Figs. 5 and 6. For the MATRIX steel, being almost free from carbides, transfer is very rare and when observed restricted to fine scratches in the sliding track. For the CAST steel containing relatively few but large carbides transfer is initiated and more or less restricted to regions with small pores in connection to the larger carbides or fine scratches caused by carbide fragments being detached during the polishing process. For the two PM grades transfer is initiated in connection to the hard phase particles. However, the hard phase particles show different tendencies to pick up material. For the PM1 grade MCN carbonitrides show a higher material pick-up tendency as compared with the M6C carbides while for the PM2 grade the MC carbides show a higher material pick-up tendency as compared with the M7C3 carbides. For the CAST, PM1 and PM2 grades the constituent phases differ with respect to composition, mechanical properties (hardness and stiffness) and topography. The difference in topography is caused by the polishing process which affects the phases differently. The finescale topography caused by the polishing is best imaged with atomic force microscopy (AFM), as illustrated in Fig. 2. For example, for the PM1 grade, the carbonitrides protrude about 6–8 nm from the matrix after the fine polishing, while the M6C carbides protrude only about 1–2 nm. Similar results have been reported by Heikkil¨a [14]. It can further be noted that the polishing here made the carbides much
Although finely polished to a very smooth surface, i.e. Rao10 nm, the different tool steel grades display different friction characteristics in sliding contact with the aluminium tip. Post-test high resolution SEM of the steel samples revealed that the friction characteristics correlate well with the tendency to transfer aluminium to the tool steel surfaces. For the carbide containing tool steels, i.e. CAST, PM1 and PM2, the first transfer occurs on a very fine scale and is localised to individual surface irregularities in the surface. For the PM steels these surface irregularities were found to be individual hard phase particles while for the CAST steel the surface irregularities were represented by small pores in connection to the larger carbides or fine scratches caused by carbide fragments being detached during the polishing process. In contrast, the MATRIX steel containing a very small amount of carbides displays a surprisingly low friction in combination with a low tendency to aluminium pick-up. The reason for the obtained results is not fully understood and it is still difficult to separate the influence from local surface roughness and local surface chemistry in determining the transfer tendency. In a previous study by the present authors it was concluded that the most probable reason for the initial pick-up of aluminium on polished tool steel (PM1) was due to the combined action of protruding ‘‘rough’’ carbonitride particles that damage the native aluminium oxide film, expose the highly reactive aluminium to the particle surface resulting in welding and tear of aluminium. However, in the present study the polished PM2 surface shows a similar initial pick-up tendency without showing any protruding hard phase particles which makes the overall picture even more complex. Common for the two PM steel grades is a very high density of hard particles at a slightly (nm scale) different level as the surrounding steel matrix and that this surface morphology may promote initial material transfer.
Fig. 5. Initial aluminium transfer to the polished tool steel surfaces after 1 passage. (a) MATRIX, (b) CAST, (c) PM1 and (d) PM2. Sliding direction from left to right.
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Fig. 6. General appearance of the fine polished tool steel surfaces after 10 passages of the aluminium tip. (a) MATRIX, (b) CAST, (c) PM1 and (d) PM2. Sliding direction from left to right. SEM tilted view.
The reason for the very low friction coefficient displayed by the aluminium–MATRIX steel sliding contact is not clear. However, it should be noted that the tested surfaces, although ultrasonically clean in acetone and ethanol and handled with care, are far from atomically clean. The presence of contaminant films resulting from exposure to the atmosphere in addition to the oxide films may, in the case of smooth surfaces and under mild sliding contact conditions, prevent direct metal–metal contact and material transfer. Analysis of the aluminium tip shows that the contact area after sliding against the fine polished MATRIX surface is smooth and also shows an enrichment of carbon and oxygen, indicating the pick-up of surface contaminants from the tool steel surface. Despite a nominal hardness significantly higher than the aluminium pin the MATRIX surface shows shallow scratches, typically some 100 nm in width, in the sliding path. The origin of these is not fully understood since no protruding particles were found on the aluminium tip surface. However, since the Al-alloy used in the experiments contain small inclusions of Si as well as AlFeMnSi of sub-mm size these may have been responsible for the mild plastic deformation associated with these scratches. Also, the presence of small alumina particles must be considered. The fact that the aluminium tip surface is not subjected to any wear (due to the smooth counter surface and very small transfer tendency) makes the surface as well as the action of inclusion particles stable. No signs of small chips, due to abrasion, could be detected on the tribo surfaces. It should be noted that the CAST surface also shows some shallow scratches in the sliding path although to a lower number. In contrast, scratches in the PM surfaces are uncommon. The relative ranking of the tool steels with respect to their material pick-up tendency correlates well with their topographies on a nanometre scale as well as their microstructures. Obviously, the presence of a large number of protrusions/depression as associated with the fine hard phase particles in the PM tool steels will increase the tribological interaction with the counter surface and the tendency to initial material transfer and consequently a matrix steel should be
selected if material transfer must be avoided. However, the latter steel type may exhibit a higher wear which with increasing time may result in significant roughening of the surface and as a result an increased material transfer tendency.
5. Conclusions Based on the observations from the present study of an aluminium alloy pin sliding over tool steel surfaces of different microstructures it can be concluded that:
The nanoscale surface topography of the fine polished tool
steel surfaces is dependent on the distribution, size and composition of hard phase particles. Of the tool steel grades investigated, the matrix steel shows very little initial transfer while the two PM steels show a high initial transfer tendency. For both PM steels initial transfer occurs on a very fine scale and is located to surface irregularities presented by the hard phase particles. The friction coefficient is strongly dependent on the transfer of aluminium to the tool steel surface. For both PM steels the accumulation of transferred aluminium during each passage results in a gradually increasing friction coefficient for each passage until it reaches a saturation level. In contrast, the matrix steel and the cast steel show a slight decrease in friction coefficient with increasing number of passages.
Acknowledgements The authors would like to acknowledge the Swedish Foundation for Strategic Research for financial support via the programme Technical advancement through controlled tribofilms.
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References [1] J. Heinrichs, S. Jacobson, Laboratory test simulation of aluminium cold forming—influence from PVD tool coatings on the tendency to galling, Surface and Coatings Technology 204 (2010) 3606–3613. [2] P. Carlsson, M. Olsson, PVD coatings for sheet metal forming processes—a tribological evaluation, Surface and Coatings Technology 200 (2006) 4654–4663. [3] P. Harlin, P. Carlsson, U. Bexell, M. Olsson, Influence of surface roughness of PVD coatings on tribological performance in sliding contacts, Surface and Coatings Technology 201 (2006) 4253–4259. [4] P. Harlin, U. Bexell, M. Olsson, Influence of surface topography of arcdeposited TiN and sputter-deposited WC/C coatings on the initial material transfer tendency and friction characteristics under dry sliding contact conditions, Surface and Coatings Technology 203 (2009) 1748–1755. [5] J. Eriksson, M. Olsson, Tribological testing of commercial CrN, (Ti,Al)N and CrC/C PVD coatings—Evaluation of galling and wear characteristics against different high strength steels, Surface and Coatings Technology 205 (2011) 4045–4051. [6] M. Olsson, U. Bexell, Friction characteristics and material transfer tendency in metal powder compaction, Wear 271 (2011) 1903–1908. [7] P.L. Menezes, Kishore, S.V. Kailas, M.S. Bobji, Influence of tilt angle of plate on friction and transfer layer—a study of aluminium pin sliding against steel plate, Tribology International 43 (2010) 897–905.
[8] P.L. Menezes, Kishore, S.V. Kailas, M.R. Lovell, The role of surface texture on friction and transfer layer formation during repeated sliding of Al–4Mg against steel, Wear 271 (2011) 1785–1793. [9] P.L. Menezes, Kishore, S.V. Kailas, Influence of surface texture and roughness parameters on friction and transfer layer formation during sliding of aluminium pin on steel plate, Wear 267 (2009) 1534–1549. [10] J. Heinrichs, M. Olsson, S. Jacobson, New understanding of the initiation of material transfer and transfer layer build-up in metal forming—in situ studies in the SEM, Wear 292–293 (2012) 61–73. [11] J. Heinrichs, M. Olsson, S. Jacobson, Mechanisms of material transfer studied in situ in the SEM: Explanations to the success of DLC coated tools in aluminium forming, Wear 292–293 (2012) 49–60. [12] J. Heinrichs, M. Olsson, S. Jacobson, Initiation of galling in forming of ductile metals—differences between aluminium and austenitic stainless steel studied in situ in the SEM, in: Proceedings of the Nordtrib 2012, Trondheim, Norway, June 12–15, 2012. [13] G. Mro´wka-Nowotnik, J. Sieniawski, Influence of heat treatment on the microstructure and mechanical properties of 6005 and 6082 aluminium alloys, Journal of Materials Processing Technology 162–163 (2005) 367–372. ¨ Influence of tool steel microstructure on galling resistance against [14] I. Heikkila, stainless steel, Tribology Series 43 (2003) 641–649.
Contents lists available at SciVerse ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Influence of tool steel microstructure on initial material transfer in metal forming—In situ studies in the SEM J. Heinrichs a,n, M. Olsson a,b, S. Jacobson a a b
˚ Angstr¨ om Tribomaterials group, Uppsala University, Sweden Materials Science, Dalarna University, Sweden
a r t i c l e i n f o
abstract
Article history: Received 25 January 2013 Received in revised form 30 January 2013 Accepted 31 January 2013 Available online 8 February 2013
Metal forming constitutes a group of industrially important processes to form metallic components to net shape. When forming aluminium and other materials that tend to stick to the tools, problems associated with material transfer, e.g. galling, may occur. In a previous study by the present authors, in situ observations of aluminium transfer during sliding contact in the SEM revealed that the surface topography and chemical composition of the tool steel counter surface have a strong impact on the initial material transfer tendency. Even if carefully polished to a very smooth surface (Rao 50 nm), transfer of aluminium was found to immediately take place on a very fine scale and preferentially to the surface irregularities presented by the slightly protruding M(C,N) particles (height 15 nm) in the tool steel. In contrast, the less protruding M6C carbides, as well as the martensitic steel matrix exhibited very little initial transfer. The mechanism behind the preferential pick-up tendency displayed by the M(C,N) particles was not fully understood and it was not possible to determine if the decisive mechanism operates on the microstructural scale, the nanoroughness scale or the chemical bonding scale. In the present study, these mechanisms have been further investigated and analysed by comparing the very initial stages of material transfer onto different types of tool steels in sliding contact with aluminium in the SEM. The tool steels investigated cover conventional ingot cast and powder metallurgy steel grades, selected to possess a range of different types, amounts and sizes of hard phase particles, including MC, M(C,N), M7C3 and M6C. The transfer mechanisms are investigated using high resolution SEM, and the differences between the different microstructures and carbide types are carefully analysed. The implications for real metal forming are discussed. & 2013 Elsevier B.V. All rights reserved.
Keywords: Surface topography Microstructure Material transfer Scanning electron microscopy In situ studies
1. Introduction High friction and transfer of work material to tool surfaces constitute important problems in many industrial forming operations, such as in cold forming of aluminium. The high friction results in high forming forces that limit the forming possibilities; for example the cavities of dies will not be completely filled when the shapes become too complex or their radii too small. The transferred work material roughens the surfaces and results in increased friction forces when forming the following piece and also impairs its surface finish. Thick and uneven transferred material will result in scratches and other types of imperfections on the surface formed. This process is commonly called galling. The hardened aluminium can be removed from the tools by wet chemical etching. However, this causes production stops that are
n
Corresponding author. Tel.: þ46 184 717 236; fax: þ 46 18 471 35 72. E-mail addresses: [email protected] (J. Heinrichs), [email protected] (M. Olsson), [email protected] (S. Jacobson). 0043-1648/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2013.01.114
highly undesired and further the etching agent might etch the tool, although to a lesser degree. The area has been researched for many years, and the forming processes (tool materials, coatings, surface preparation processes, lubricants, etc.) have gradually been developed so that today more advanced shapes and more units can be formed before the tool must be exchanged or reconditioned [1–6]. The transfer of aluminium to harder (tool) surfaces and its influence on friction has been investigated using several approaches. Despite the extensive research, there is still a lack of understanding of the physical and chemical mechanisms involved. Such understanding would be highly useful when developing tool materials, tool coatings, tool surface preparation techniques or lubricant systems. One outstanding problem is to describe the decisive mechanisms behind the initiation of transfer. Usually, the very initial stages of transfer have not been studied at all. Rather, the surfaces have been investigated first when a lot of material has been transferred, and therefore covered any signs of where it originated. Furthermore, the evaluation techniques or the instrument
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settings selected have typically not allowed detection of thin transfer layers [7–9]. Recently, the present authors have developed and employed a technique to evaluate the very first sliding contact between tool material surfaces and aluminium and stainless steel [10–12]. The technique involves a sharp tip of work material put into contact with a tool material under a controlled load, followed by a relative movement of the two surfaces. The test rig is adapted in a Scanning Electron Microscope (SEM) to allow direct observation of the dynamics of the tribological contact. With this well controlled and limited contact, the observed movement and passages of surface features can be directly related to load variations, friction responses and material transfer. The passage of the tip involves a sliding distance determined by the diameter of its flattened end (typically 100–200 mm). This allows observations of transfer and other changes to the tool material flat during very limited sliding distances, not easily attainable by other experimental techniques. The contact surfaces are further studied in the SEM, using low acceleration voltage to be able to also detect such thin transfer layers that would be transparent when using the typical, higher acceleration voltages. In Ref. [10], the tested power metallurgical tool steel (Vancron 40) immediately showed transfer of aluminium, even if carefully polished. The transfer took place at different scales: on the scale of surface damage (filling up scratches, building up in front of ridges and protrusions around scratches) and on the scale of microstructure of the steel (on top of or in front of slightly protruding hard phase particles). The effect was very strong despite of the very limited roughness on the tool surface. In Ref. [11], the tool steel was compared to one of the most successful DLC coatings. The difference in transfer onto the polished surfaces was dramatic. The DLC showed almost no transfer even after 10 passages of the aluminium tip, while the tool steel picked up aluminium already at the first passage, most notably on top of the slightly protruding carbonitride particles. On subsequent passages more aluminium was transferred, now mostly building up in front of the initial transfer. The friction force increased correspondingly for every passage, while the surface became
rougher and rougher. It could therefore be concluded that it is this surface, roughened by transferred lumps, rather than the initial surface that determines the initial ‘‘steady state’’ friction level. Contrastingly, for the DLC, almost no aluminium was transferred and the friction stayed at the initial very low level. It was concluded that the great advantage of the DLC coating over the tool steel primarily seems to be its more advantageous topography after polishing, probably in combination with very weak adhesion tendency towards the aluminium oxide. The objective of the present study was to further investigate the generality of these findings, by including more types of tool steels, with different amounts of hard phase, different hard phase sizes and different hard phase compositions. One main challenge is to be able to separate the influence from local surface roughness and local surface chemistry, in determining the transfer tendency. After fine polishing, these differences also result in different roughness on a nanometre scale. The hard phases protrude the surface to different heights. The contact between aluminium and four different tool steels is investigated and analysed by comparing the very initial stages of material transfer, and the corresponding change of the friction coefficient. The tool steels investigated cover ‘‘carbide free’’ matrix, conventional ingot cast and powder metallurgy steel grades, selected to possess a range of different types, amounts and sizes of hard phase particles, including MC, M(C,N), M7C3 and M6C.
2. Experimental The initial conditions and material transfer in a forming operation have been studied in a contact situation where the work material (aluminium) has been represented by a relatively sharp tip that has been slid against a polished tool steel surface under such a load that the softer work material has deformed plastically. The contact area becomes very limited, initially determined by the load and the hardness of the tip. In these tests the initial contact area has been around 100–200 mm in diameter. 2.1. Materials
Table 1 Chemical composition (in wt%) and hardness of the aluminium alloy 6082 [13]. Si
Mg
Mn
Fe
Cr
Cu
Zn
Others Al
1.2 0.78 0.50 0.33 0.14 0.08 0.05 0.15
Hardness, HV (kg/mm2)
Bal 80
Fig. 1. Sketch showing the cylindrical aluminium pin (work material) with the final tip shape.
2.1.1. The work material The work material used was an industrially important aluminium alloy (6082), including the main alloying elements magnesium and silicon, see Table 1. The work material cylinders were manufactured from sheet metal by turning in a lathe, followed by a rough shaping of the tip. The final tip shape, see Fig. 1, was formed with SiC grinding paper, first 1000 grit followed by 4000 grit. The cylindrical part was 2.9 mm in diameter and about 15 mm long. The conical end part was 5 mm long with a tip angle of about 351. The tips were made as sharp as possible, which with this simple technique typically left a relatively flat end surface, less than 50 mm in diameter. During the first static loading, the sharper tips become plastically flattened, leaving a flat surface of diameter 50–100 mm. Due to this deformation, the differences from manufacturing are evened
Table 2 Chemical composition (in wt%) and hardness of the tool steels investigated. Steel gradea
C
Si
Mn
Cr
Mo
W
N
V
Fe
Hard phase content (vol%)
Hardness, HV3 (kg/mm2)
Dievar Sverker 21 Vancron 40 Vanadis 10
0.35 1.55 1.10 2.90
0.2 0.3 0.5 0.5
0.5 0.4 0.4 0.5
5.0 11.8 4.5 8.0
2.3 0.8 3.2 1.5
– – 3.7 –
– – 1.80 –
0.6 0.8 8.5 9.8
Bal Bal Bal Bal
MC, 1–2 M7C3, 13 M6C, 5 MCN, 19 MC, 16 M7C3, 7
6407 20 7307 20 7907 30 8107 30
a
Uddeholms AB designation.
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Fig. 2. Examples of surface irregularities and microstructure features for the four tool steels. AFM (a, c, e, g) and SEM (b, d, f, h) were used to image the same area for each steel (about 10 mm 10 mm). (a) Topography map of MATRIX. (b) Microstructure of MATRIX, showing lack of large hard phase particles. (c) Topography map of CAST. (d) Microstructure of CAST, showing a large M7C3 carbide, slightly darker than the matrix. (e) Topography map of PM1. (f) Microstructure of PM1, showing M(C,N) appearing black and M6C appearing white. (g) Topography map of PM2. (h) Microstructure of PM2, showing M7C3 appearing grey and MC appearing dark grey.
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Table 3 Topography data for the four steels. The data were achieved from measurements in the AFM. Typical values
Steel type MATRIX
CAST
– –
2 nm (M7C3)
0.59 nm
0.95 nm
PM1
PM2
Hard phase Protrusion Depression Ra (10 mm 10 mm)
6–8 nm (M(C,N)), 1–2 nm (M6C) 2 nm (MC), o1 nm (M7C3) 1.3 nm
0.53 nm
depressions are given in Table 3, together with the average surface roughness value Ra, measured in the AFM. 2.2. Experiments in the in situ manipulator in the SEM
Fig. 3. Schematic drawing of the in situ manipulator in the SEM.
out. The hardness of the alloy depends on the degree of work hardening, and is estimated to be about 80 HV in the contact area.
2.1.2. The tool materials Four different tool steels: a matrix steel grade, Dievar, a conventional high alloyed ingot cast steel grade, Sverker 21 and two PM steel grades, Vancron 40 and Vanadis 10, were included in the tests, see Table 2. In the following sections these grades are denoted MATRIX, CAST, PM1 and PM2. The microstructures of the tool steel grades in the hardened and tempered condition consist of a martensitic matrix (MATRIX) and a martensitic matrix with a hard phase consisting of relatively large M7C3 carbides (CAST), fine M6C carbides and M(C,N) carbonitrides (PM1) and fine MC and M7C3 carbides (PM2). The sizes of the carbide particles are up to 50–100 mm in the ingot cast steel, around 1–2 mm for the carbides in the PM steels and around 0.7 mm for the carbonitrides in the PM1 steel. Prior to the test all steel samples were grinded and carefully polished using 1 mm diamond in the last step. The polished tool steel surfaces were studied using Atomic Force Microscopy (AFM; PSIA XE150) and Scanning Electron Microscopy (SEM; LEO 1550) after the final polishing step, see Fig. 2. The hard phase particles, visible in the SEM due to compositional contrast, were affected differently by the polishing compared to the matrix steel. This caused some protrusion, alternatively depression, of the flat surface in connection to the hard phase particles; compare Fig. 2a, c, e and g with b, d, f, h, respectively. The height of the protrusions and depth of the
The tests were performed in an in situ manipulator comprising a work material tip in contact with a tool steel flat, as schematically illustrated in Fig. 3. The equipment is installed in an SEM (Leo 440) and operates under vacuum, so that the sliding contact can be directly observed using the electron beam and a secondary electron detector. The working distance is fixed at about 40 mm and to achieve decent resolution at this long distance, the lowest acceleration voltage is limited to 20 kV. The tip is mounted orthogonal to the flat surface. To offer a good viewing angle in the SEM, the tip on flat arrangement is tilted to 601. During testing, the tip is held stationary and loaded using a spring while the flat is moved using electric motors. Also the spring loading and tip positioning mechanisms are driven by electric motors. During the sliding test the normal load and friction force are continuously monitored. All tests comprised sliding of the aluminium tips against the prepared tool steel surfaces. The normal load was set to 3 N in all tests, the sliding distance was 4 mm for each passage and the sliding speed was roughly 3 mm/min. The tests were run unlubricated, to represent the critical moments in actual forming when the lubricant film present becomes too thin to separate the surface or is locally missing. Before testing all samples were ultrasonically cleaned in acetone and ethanol for 3þ 3 min. The aluminium tips were initially relatively sharp, but flattened on the first static contact, as mentioned. The contact area then rapidly grew during the sliding contact, and more during high friction conditions. Both plastic deformation and wear were found to contribute to the contact area growth. Typically, the flat end of the tips exhibited diameters in the 100–200 mm range after testing. The tests comprised 1, 5 and 10 passages along three parallel tracks over the tool surface. After each passage, the tip was lifted out of contact, the flat was moved back and the tip again lowered at the original starting position. The passages were studied in the SEM, which facilitated direct observation of tip deformation and transfer of material to the tool surface. This allows for immediate correlation between the work material transfer and changes in tool steel surface appearance as well as influence on friction. 2.3. Post-test characterisation The direct in situ studies were supplemented by highresolution SEM studies of both flats and tips using a Zeiss Ultra 55 FEG-SEM and a LEO 1550 FEG-SEM. The element composition of the surfaces was analysed by energy dispersive X-ray spectroscopy (EDS) using an Oxford Inca Energy system as well as an Oxford Aztec X-max system. Since the transferred material was typically very thin, a low primary electron energy of 3 keV (resulting from using an acceleration voltage of 3 kV) was preferably used in order to limit the interaction depth. When imaging, this results both in a shallow
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‘‘depth of sight’’, and a higher resolution, due to the limited broadening of the area emitting secondary electrons.
3. Results 3.1. In situ observations During testing, only occasional micrometre sized transfer could be observed on the polished steel surfaces, irrespective of number of passages and type of tool steel grade. While sliding over the polished MATRIX and CAST steel surfaces the aluminium tip shows very limited macroscopic deformation. This is also true while sliding over the two polished PM steel surfaces during the first passage. However, with increasing number of passages against the two PM steel surfaces the aluminium tip surface starts to shear towards the rear edge. After 10 successive passages this has resulted in a significant distorted tip geometry.
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3.2. Friction behaviour Fig. 4 shows the friction development during 10 passages in the same track for the four steel grades investigated. As can be seen, significant differences in the friction behaviour can be revealed. For aluminium sliding against the MATRIX grade the coefficient of friction was very low for all 10 passages, starting at around 0.12 during the first passage and then decreasing for each passage in the same track, ending at about 0.08 during the 10th passage. The CAST grade displays a similar decrease in friction coefficient with increasing number of passages but in this case from around 0.25 during the first passage to 0.20 during the 10th passage. The two PM grades display an opposite behaviour where the coefficient of friction is low, around 0.15, during the first passage and then increases with increasing number of passages reaching values in the range 0.45–0.50 for the PM1 grade and 0.35–0.45 for the PM2 grade. For both PM grades the coefficient of friction reaches its maximum in the very start of the track after which it slightly decreases during the sliding event. This tendency is most pronounced for passages 2–5 and especially for the
Fig. 4. Friction recordings for each of ten passages along the same track by the aluminium tips over the different tool steel surfaces. (a) MATRIX, (b) CAST, (c) PM1, and (d) PM2.
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PM2 grade. It should be noted that the obtained friction curves are in good agreement with the in situ observations of the aluminium tip as discussed above, i.e. that the pronounced shearing and deformation of the aluminium tip observed during sliding against the PM surfaces is due to high friction coefficients (high tangential forces).
smoother than both the carbonitrides and the matrix. As a result, the final surface topography of the different steel grades may differ significantly on a nanometre scale.
4. Discussion 3.3. Transfer of work material During testing, only occasional micrometre sized transfer could be observed on the polished surfaces. However, the combination of higher magnifications and lower acceleration voltages in the FEG– SEM makes it possible to resolve thinner transfer layers and finer scratches and the initial transfer mechanisms can be studied in detail, see Figs. 5 and 6. For the MATRIX steel, being almost free from carbides, transfer is very rare and when observed restricted to fine scratches in the sliding track. For the CAST steel containing relatively few but large carbides transfer is initiated and more or less restricted to regions with small pores in connection to the larger carbides or fine scratches caused by carbide fragments being detached during the polishing process. For the two PM grades transfer is initiated in connection to the hard phase particles. However, the hard phase particles show different tendencies to pick up material. For the PM1 grade MCN carbonitrides show a higher material pick-up tendency as compared with the M6C carbides while for the PM2 grade the MC carbides show a higher material pick-up tendency as compared with the M7C3 carbides. For the CAST, PM1 and PM2 grades the constituent phases differ with respect to composition, mechanical properties (hardness and stiffness) and topography. The difference in topography is caused by the polishing process which affects the phases differently. The finescale topography caused by the polishing is best imaged with atomic force microscopy (AFM), as illustrated in Fig. 2. For example, for the PM1 grade, the carbonitrides protrude about 6–8 nm from the matrix after the fine polishing, while the M6C carbides protrude only about 1–2 nm. Similar results have been reported by Heikkil¨a [14]. It can further be noted that the polishing here made the carbides much
Although finely polished to a very smooth surface, i.e. Rao10 nm, the different tool steel grades display different friction characteristics in sliding contact with the aluminium tip. Post-test high resolution SEM of the steel samples revealed that the friction characteristics correlate well with the tendency to transfer aluminium to the tool steel surfaces. For the carbide containing tool steels, i.e. CAST, PM1 and PM2, the first transfer occurs on a very fine scale and is localised to individual surface irregularities in the surface. For the PM steels these surface irregularities were found to be individual hard phase particles while for the CAST steel the surface irregularities were represented by small pores in connection to the larger carbides or fine scratches caused by carbide fragments being detached during the polishing process. In contrast, the MATRIX steel containing a very small amount of carbides displays a surprisingly low friction in combination with a low tendency to aluminium pick-up. The reason for the obtained results is not fully understood and it is still difficult to separate the influence from local surface roughness and local surface chemistry in determining the transfer tendency. In a previous study by the present authors it was concluded that the most probable reason for the initial pick-up of aluminium on polished tool steel (PM1) was due to the combined action of protruding ‘‘rough’’ carbonitride particles that damage the native aluminium oxide film, expose the highly reactive aluminium to the particle surface resulting in welding and tear of aluminium. However, in the present study the polished PM2 surface shows a similar initial pick-up tendency without showing any protruding hard phase particles which makes the overall picture even more complex. Common for the two PM steel grades is a very high density of hard particles at a slightly (nm scale) different level as the surrounding steel matrix and that this surface morphology may promote initial material transfer.
Fig. 5. Initial aluminium transfer to the polished tool steel surfaces after 1 passage. (a) MATRIX, (b) CAST, (c) PM1 and (d) PM2. Sliding direction from left to right.
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Fig. 6. General appearance of the fine polished tool steel surfaces after 10 passages of the aluminium tip. (a) MATRIX, (b) CAST, (c) PM1 and (d) PM2. Sliding direction from left to right. SEM tilted view.
The reason for the very low friction coefficient displayed by the aluminium–MATRIX steel sliding contact is not clear. However, it should be noted that the tested surfaces, although ultrasonically clean in acetone and ethanol and handled with care, are far from atomically clean. The presence of contaminant films resulting from exposure to the atmosphere in addition to the oxide films may, in the case of smooth surfaces and under mild sliding contact conditions, prevent direct metal–metal contact and material transfer. Analysis of the aluminium tip shows that the contact area after sliding against the fine polished MATRIX surface is smooth and also shows an enrichment of carbon and oxygen, indicating the pick-up of surface contaminants from the tool steel surface. Despite a nominal hardness significantly higher than the aluminium pin the MATRIX surface shows shallow scratches, typically some 100 nm in width, in the sliding path. The origin of these is not fully understood since no protruding particles were found on the aluminium tip surface. However, since the Al-alloy used in the experiments contain small inclusions of Si as well as AlFeMnSi of sub-mm size these may have been responsible for the mild plastic deformation associated with these scratches. Also, the presence of small alumina particles must be considered. The fact that the aluminium tip surface is not subjected to any wear (due to the smooth counter surface and very small transfer tendency) makes the surface as well as the action of inclusion particles stable. No signs of small chips, due to abrasion, could be detected on the tribo surfaces. It should be noted that the CAST surface also shows some shallow scratches in the sliding path although to a lower number. In contrast, scratches in the PM surfaces are uncommon. The relative ranking of the tool steels with respect to their material pick-up tendency correlates well with their topographies on a nanometre scale as well as their microstructures. Obviously, the presence of a large number of protrusions/depression as associated with the fine hard phase particles in the PM tool steels will increase the tribological interaction with the counter surface and the tendency to initial material transfer and consequently a matrix steel should be
selected if material transfer must be avoided. However, the latter steel type may exhibit a higher wear which with increasing time may result in significant roughening of the surface and as a result an increased material transfer tendency.
5. Conclusions Based on the observations from the present study of an aluminium alloy pin sliding over tool steel surfaces of different microstructures it can be concluded that:
The nanoscale surface topography of the fine polished tool
steel surfaces is dependent on the distribution, size and composition of hard phase particles. Of the tool steel grades investigated, the matrix steel shows very little initial transfer while the two PM steels show a high initial transfer tendency. For both PM steels initial transfer occurs on a very fine scale and is located to surface irregularities presented by the hard phase particles. The friction coefficient is strongly dependent on the transfer of aluminium to the tool steel surface. For both PM steels the accumulation of transferred aluminium during each passage results in a gradually increasing friction coefficient for each passage until it reaches a saturation level. In contrast, the matrix steel and the cast steel show a slight decrease in friction coefficient with increasing number of passages.
Acknowledgements The authors would like to acknowledge the Swedish Foundation for Strategic Research for financial support via the programme Technical advancement through controlled tribofilms.
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