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Journal of Materials Processing Technology xxx (2015) xxx–xxx
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Journal of Materials Processing Technology
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nfluence of microstructural evolution of Al-Si coated UHSS on itsribological behaviour against tool steel at elevated temperatures
eonardo Pelcastre ∗, Jens Hardell, Anthony Rolland, Braham Prakashivision of Machine Elements, Luleå University of Technology, Luleå SE-971 87, Sweden
r t i c l e i n f o
rticle history:eceived 10 September 2014eceived in revised form 5 March 2015ccepted 7 March 2015vailable online xxx
a b s t r a c t
The usage of the hot stamping process is of great importance due to the high demands for production ofultra-high strength steels (UHSS). An Al-Si coating is normally applied to the steel to prevent decarburi-sation and scaling during heating and to improve the corrosion resistance of the final component. Duringheating, the Al and the Si from the coating combine with the Fe from the steel substrate to form hardintermetallic phases. Little is known about the influence of the heating conditions on the tribological
eywords:lSi coatingot stampingallinglevated temperature
behaviour of the Al-Si coating during interaction with tool steels. The present work investigated differ-ent heat treatment parameters and the influence they had on the microstructure of the coating and thegalling behaviour. With low alloying temperatures (700 ◦C), severe galling occurred and increasing thealloying temperature to 900 ◦C resulted in almost negligible material transfer. The reduction in gallingwas associated to the development of Fe2Al5 and FeAl2 at the surface.
. Introduction
In the hot stamping process an Al-Si coating is commonlypplied on the ultra-high strength steel (UHSS) sheets. This coat-ng gives good corrosion protection and has good paintability and
eldability as has been reported by Suehiro et al. (2003) in theirtudies.
The Fe-Al-Si system is known for its complexity as it can formn extensive amount of phases that are stable at various ranges ofemperatures and composition as highlighted in different studies.rendelsberger et al. (2007) studied this system and the chal-
enge involved for the analysis and identification of different phasess temperature increases through power diffraction, nonetheless,hey were able to confirm the existence of different ternary phaseseported in the open literature as well as the invariant tempera-ures of the reactions. Gupta (2003) carried out an analysis of thel-Fe-Si system with the aim of confirming which phases can bencountered at different temperatures. Gupta was able to confirmhe existence of nine intermetallic phases despite the small differ-nces in chemical composition. In a later study, Maitra and Gupta2003) found that the binary phases (Fe-Al) form solid layers. They
Please cite this article in press as: Pelcastre, L., et al., Influence of mbehaviour against tool steel at elevated temperatures. J. Mater. Process.
lso stated that ternary phases such as �1 and �5 can crystalliseirectly from liquid during cooling or by the occurrence of invarianteactions.
In the case of the Al-Si coated UHSS in the hot stamping process,when exposed to high temperatures, the Al-Si coating undergoesmicrostructural changes due to the formation of intermetallics ashighlighted by Fan and De Cooman (2012) in their review. Theseintermetallic phases have also been observed and studied by otherauthors; Hardell et al. (2010) pointed out the formation of suchintermetallic layers and the morphological changes that it causeson the surface of the coating. This was also confirmed by Borsettoet al. (2009) as they observed that increasing the holding timeaffects the chemical distribution of the constituents through thecoating as well as the morphology of the surface. These changes inthe chemistry of the coating are related to the formation of inter-metallic phases, these intermetallics are formed by inter-diffusionof Al, Si and the Fe from the steel substrate. When exposed tohigh temperature (over 577 ◦C), the Al-Si coating melts. This allowsthe Fe from the substrate to diffuse quickly through the coatingtowards the surface. The increased concentration of Fe in the coat-ing causes the formation of intermetallic phases and the coatingsolidifies. As the intermetallics are formed, hardness of the coatingis increased and the coating becomes brittle. A multi layered struc-ture after exposure to elevated temperatures has been observed byGrigorieva et al. (2011). They reported up to five layers (i.e. phases)that can be found within the Al-Si coating; however, this number isreduced with increased temperature and time. Furthermore, in the
icrostructural evolution of Al-Si coated UHSS on its tribological Tech. (2015), http://dx.doi.org/10.1016/j.jmatprotec.2015.03.009
study by Suehiro et al. (2003), it was stated that if enough time andtemperature is given, the coating homogenises into a single layer.
Veit et al. (2011) showed that even with the use of fast heatingprocess, the multi-layered structure of the coating is still formed.
Table 1Test conditions to analyse influence of alloying time.
Temperature (◦C) Soaking time (min)
ARTICLEROTEC-14326; No. of Pages 8
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As mentioned before, the changes of the coating at high temper-tures are not limited to the phase transformations; morphologicalhanges of the surface have also been reported in the studies doney Hardell et al. (2010) in the form of nodules at the surface afterxposure to high temperature and also an increase in the aver-ge surface roughness. Borsetto et al. (2009) also observed thencreased roughness of heated specimens and also showed that
ith long holding times, the roughness continues to increase. Veitt al. (2011) suggested that the formation of such nodules is causedy the melting of the unalloyed Al situated at the outermost layerf the coating.
Several authors have addressed the tribological behaviour ofhe Al-Si coated UHSS and reported occurrence of material trans-er when it interacts with different tool steels with and withoutoatings. The wear behaviour of tool steels with and withoutoatings sliding against Al-Si coated UHSS has been investigated byondratiuk and Kuhn (2011) and found that severe adhesive wearan occur when coatings such as AlCrN or TiAlN are used. Azushimat al. (2012) observed that under lubricated conditions frictionan be controlled to a steady level and suggests that wear can beeduced. Pelcastre et al. (2012) studied the occurrence of materialransfer or galling on the tool steel surface and concluded that wearragments coming from the Al-Si coated UHSS accumulate withinurface defects or valleys of the surface of the tool and generateaterial built up. The occurrence of adhesion was also observed by
elcastre et al. in untreated tool steels (2011); however, this adhe-ive tendency was magnified when certain PVD coatings were usedPelcastre et al., 2013), however, the use of oxidised plasma nitridedool steel reduced considerably the occurrence of galling. Hou et al.2010) observed that die corners are sites where severe materialransfer occurs and suggested that this was due to the increasedontact pressure and temperature at these regions.
It is clear that the heating conditions have a significant effect onhe resulting microstructure and morphology of the coating, butqually important is the fact that the existing phases also affect theribological response. The effect of the changes of surface rough-ess of the Al-Si coating has not yet been thoroughly studied and
ts impact on galling or friction is not well understood. It has beenbserved that the temperature, to which the coating is exposed to,as an effect on material transfer and friction. For instance, in atudy by Kondratiuk and Kuhn (2011), they suggested that at tem-eratures around 920 ◦C material transfer was reduced comparedo lower temperatures when Al-Si was sliding against tool steel.n the study done by Borsetto et al. (2009) the authors observed
reduced coefficient of friction between the Al-Si coating and aardened hot work tool steel at 700 ◦C in a pin-on-disc configura-ion. However, it is not yet clear the reason for the above mentionedases.
In the actual forming process, the coating undergoes heatinguring the austenitisation of the steel (∼930 ◦C). Upon exiting theurnace, the work-piece cools down to around 750 ◦C before it getsnto contact with the forming tools. Even though it is acknowledgedhat the heating history of the coating has an influence on the tri-ological response, the correlation between the microstructure ofhe coating and the tribological response still lacks understanding.
In this work, two temperatures (700 ◦C and 900 ◦C) were cho-en for the investigations with a given soaking time for alloyingf the coating. From a fundamental point of view, the selectedemperatures allow the formation and evolution of typical phasesncountered in the forming process and others that are not ofteneen in the process but that are also typical in the Al-Si-Fe system.ven though the microstructural evolution of the Al-Si-Fe system
Please cite this article in press as: Pelcastre, L., et al., Influence of mbehaviour against tool steel at elevated temperatures. J. Mater. Process.
as been studied, as described in the review done by Fan and Deooman (2012), there is still lack of understanding concerning theribological behaviour of most of the phases that form in this sys-em, and in the particular case, in the Al-Si coated UHSS.
700 0 4 20900 0 4 20
In the forming industry, there is always a drive to modify theheating conditions to increase productivity and reduce costs. How-ever, little information is available concerning possible effects thatmay occur as a result of the microstructural evolution of the Al-Sicoating.
This work aims to increase the understanding of the tribologicalbehaviour of Al-Si coated UHSS under different heating conditions.Particular focus is given to the friction behaviour and galling mech-anisms. Furthermore, the effect of the surface roughness of the Al-Sicoating on the tribological behaviour is also studied.
2. Materials and experimental methodology
2.1. Test materials and specimens
The specimens used for the tribological tests were an upper pinof Ø2 mm made from a hot work, quenched and tempered tool steel.A similar composition is normally used for tools in the actual hotstamping process. The tool steel pin surface was polished to anSa value of ∼50 nm. The lower disc (Ø16 mm and 1.7 mm height)was made from Al-Si-coated UHSS which was welded onto a steelbacking plate of Ø24 mm and 6.3 mm height. The Al-Si coated steelis widely used as workpiece in the hot stamping process. Accordingto the review done by Fan and De Cooman (2012), the boron steel isusually aluminised by hot dipping into a molten Al alloy bath witha composition of approximately 88% Al, 9% Si and 3% Fe in weightpercentage.
2.2. Heat treatment of the Al-Si coated boron steel
Initially, the effect of temperature on the evolution of the Al-Sicoating microstructure was considered.
For this study, three different soaking times were used for theanalysis of the microstructural evolution. Table 1 shows the timesused for each temperature. A soaking time of 0 min means thatthe samples were heated up to the desired temperature and thencooled down immediately. The coating undergoes melting whenexposed to temperatures higher than 570 ◦C followed by solidifica-tion caused by diffusion and interaction between the constituentsof the steel substrate and the Al-Si coating. The 0 min soaking timewas selected to determine whether the presence of melted coat-ing material could be observed and its effect on the tribologicalresponse. A soaking time of 4 min was selected as it is close to theduration normally used in the hot stamping process. In the studydone by Suehiro et al. (2003), it was proposed that given enoughtime and temperature, the coating can develop a structure with asingle layer. The soaking time corresponding to 20 min was selectedto evaluate changes in the microstructure of the Al-Si coating (shiftsin the formed layers) and the effects on the tribological behaviour.
A schematic of the heat treatment history is shown in Fig. 1.The heat treatment was performed in the tribometer itself to havethe same heating conditions that are normally obtained during thetribological tests.
2.3. Tribological tests
icrostructural evolution of Al-Si coated UHSS on its tribological Tech. (2015), http://dx.doi.org/10.1016/j.jmatprotec.2015.03.009
Tribological tests were carried out using an Optimol SRV high-temperature reciprocating friction and wear tester. Before the tests,all specimens were cleaned with ethanol and dried. The upper pin
Fig. 1. Heat treatment history for treatments at 700 ◦C and 900 ◦C.
Table 2Test parameters.
Test parameters Value
Normal load 31 NNominal contact pressure 10 MPaTemperature 700 and 900 ◦CStroke length 4 mm
(crahWtsRto
pc
2
ti#dl9imdg
watiTiai(
times (Fig. 4(c)); the main difference was that the Fe Al Si layer
Frequency 12.5 HzDuration 30 s
tool steel) specimen was kept separated from the lower disc (Al-Sioated UHSS) during heating. Once the desired temperature waseached, the lower specimen was retained at that temperature for
given time while still separated from the upper specimen. Theolding times used for the tribological tests were 0, 4 and 20 min.hen the holding time had elapsed, the pin was loaded against
he disc and the test was started. On completion of the test, bothpecimens were left to cool in air and then removed and analysed.epeat tests for all conditions were carried out with new specimens,ool steel pin and work-piece disc, in order to assess repeatabilityf the galling mechanisms.
The conditions used for the tests carried out in this work areresented in Table 2. These parameters were selected based ononditions in the forming operation.
.4. Preparation of the Al-Si coating surface
To account for the influence of the surface of the Al-Si coating,he specimens were heated at 900 ◦C for 4 min and then quenchedn water at 0 ◦C. The specimens were subsequently ground using600 grit SiC abrasive paper and grinding was performed in ran-om directions to avoid creating a defined orientation of the surface
ay. After the surface preparation, the specimens were reheated to00 ◦C and the tribological experiments were carried out. It was
nitially intended to grind the specimens before any heat treat-ent. However, the thickness of the coating was severely reduced
ue to the low hardness of the coating. The obtained surfaces afterrinding are shown in Fig. 2.
A reduction of the surface roughness Sa from 3.2 �m to 1.2 �mas observed (the surface roughness was measured by means of
Wyko 1100NT 3D optical interferometer). It is important to notehat the coating has a high waviness after the hot dip aluminis-ng process which is one of the reasons for its high roughness.his means that the thickness of the coating is not uniform andt changes from one point to another. This type of structure persists
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fter heating and this could be observed when grinding the spec-men as patches of flat (ground) and rough (not ground) surfaceFig. 2(c)).
PRESSssing Technology xxx (2015) xxx–xxx 3
To achieve a totally flat surface, it needed to be ground for aconsiderable amount of time which caused the removal of a greatpart of the coating at some locations. Altering the thickness of thecoating has an effect on the diffusion kinetics of the constituentsand hence on the phase transformations during heating, meaningthat large variations would be obtained when comparing with otheranalysis found in the literature. To avoid this, all samples were set tobe ground for the same duration (30 s), so as to reduce the surfaceroughness without reducing too much the thickness of the coat-ing. It is clear that the optimal conditions (totally smooth and flatsurface) were not obtained, however, it does give a good insight tocharacterise the effect of the surface topography on the tribologicalbehaviour.
3. Results and discussion
3.1. Microstructural evolution of the Al-Si coating
For the identification of the different phases, backscatterelectron microscopy coupled with EDS analysis was used. Theobservations were then compared with what has been reportedin the open literature, for instance the review done by Fan and DeCooman (2012) or the studies by Grigorieva et al. (2011) and Veitet al. (2011). In these references, the analysis has been done mainlyby means of EDS and then the composition has been compared withthe phase diagrams of the Fe-Al-Si system. A cross section of thecoating in the as-delivered state can be seen in Fig. 3(a). The coatingis composed of two layers, an intermetallic situated at the interfacecoating/steel substrate which different authors have identified asFe2Al7–8Si (Fan and De Cooman, 2012; Veit et al., 2011) and a mix-ture of unalloyed Al and Si. The coating is characterised by a verylow hardness of the Al + Si outer layer (∼55 HV25g), whilst the layerat the interface coating/steel has higher hardness (∼475 HV25g).
When the coating is heated up, new phases are formed as the Fefrom the steel substrate diffuses outwards and the Al of the coatingdiffuses inwards. The formation of these phases depends on the dif-fusion of the coating and steel constituents, thus the existing phasesdepend on factors such as temperature, time and thickness of thecoating. The most common phases observed during heating of theAl-Si coated UHSS are shown in Fig. 3(b). As mentioned before, amulti-layered structure is obtained after heating, from bottom totop the layers are as follows: a diffusion layer between the substrateand the coating, a layer containing FeAl2 and/or Fe2Al5, an interme-diate layer of Fe2Al2Si and a layer of Fe2Al7–8Si. In Fig. 3(b) thesephases are shown. The Fe2Al7–8Si is seldom observed as it is notstable at the process temperatures (it has a melting point of 855 ◦Caccording to the studies done by Gupta (2003), Krendelsberger et al.(2007) and Veit et al. (2011)).
Fig. 4 shows the coating after each of the soaking times used inthis work (0, 4 and 20 min). The temperatures used were 700 and900 ◦C. At 700 ◦C, an interlayer with the appearance of a continu-ous thin line is formed (Fig. 4(a)). This phase is Fe2Al2Si and it ischaracterised by a high hardness (∼600 HV25g). Surrounding thisinterlayer, FeAl2 and/or Fe2Al5 are formed which also have a highhardness (∼830 HV25g) and the outer layer was unalloyed Al andSi. Longer soaking time at 700 ◦C allowed more diffusion of Fe andwhen 4 min had elapsed, the unalloyed Al and Si layer had alreadydisappeared and the whole coating had transformed into inter-metallic phases. Fe2Al7–8Si was formed at the outermost layer andthe Fe2Al2Si had shifted position towards the surface of the coating(Fig. 4(b)). Similar microstructure was observed for longer soaking
icrostructural evolution of Al-Si coated UHSS on its tribological Tech. (2015), http://dx.doi.org/10.1016/j.jmatprotec.2015.03.009
2 7–8was thinner whilst the FeAl2/FeAl5 layer had grown.
At 900 ◦C, the layer with unalloyed Al and Si was not observedfor any soaking time and the Fe2Al2Si phase was already at the
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Fig. 2. Surface morphology of the Al-Si coating: (a) as-delivered, (b) after 900 ◦C treatment and (c) after 900 ◦C treatment and ground with 600 grit size abrasive paper for30 s.
s-deli
m(2wotF
lt
Fig. 3. Phases present on the Al-Si coating: (a) a
iddle of the coating when 0 min soaking time was usedFig. 4(d)–(f)) and it was present for all of the soaking times. When0 min soaking time had elapsed, this layer shifted further out-ards in the coating (Fig. 4(f)). The Fe2Al7–8Si phase was observed
nly when 0 min soaking time was used. For all of the other cases,his phase had dissolved and only layers containing Fe2Al2Si and
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eAl2/Fe2Al5 were observed (Fig. 4(e) and (f)).In addition to the aforementioned phases, at 900 ◦C another
ayer was observed at the interface between the coating andhe steel substrate (Fig. 4(d)–(f)), other authors identify it as a
Fig. 4. Structure of the coating for all the conditions used. (a)–(c) show 0
vered and (b) after exposure to 900 ◦C for 4 min.
diffusion layer (Fan and De Cooman, 2012). The thickness of thislayer increased with the soaking time and its hardness was in therange of 330–380 HV25g.
It is clear that the most stable phases of this system, underthe heating conditions used, are Fe2Al5/FeAl2 and Fe2Al2Si. In theirstudy, Suehiro et al. (2003) stated that if enough activation energy
icrostructural evolution of Al-Si coated UHSS on its tribological Tech. (2015), http://dx.doi.org/10.1016/j.jmatprotec.2015.03.009
is provided, the coating ends up with a single layer. The conditionsselected in the present study did not provide with such activationenergy or time therefore the final state of the coating could not bereached.
, 4 and 20 min at 700 ◦C and (d)–(f) show 0, 4 and 20 min at 900 ◦C.
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iction
bicFftuFFr
3a
btt(sIwtscb
F(
Fig. 5. Evolution of the coefficient of fr
The formation of a homogeneous coating is limited or hinderedy the diffusion of different constituents through the coating and
n the steel substrate. Initially, part of the Al + Si layer melts. Thisauses a rapid diffusion of the Fe from the steel substrate and thee2Al2Si, Fe2Al5 and FeAl2 phases are formed. As time is increasedurther, more Fe diffuses towards the surface of the coating andhe Fe2Al7–8Si is formed whilst at the same time the remainingnalloyed Al + Si phase combines with the Fe and more Fe2Al5 andeAl2 are formed. With further increase of the holding time, thee2Al7–8Si phase dissolves and the constituents combine and as aesult, only Fe2Al2Si, Fe2Al5 and FeAl2 remain.
.2. Influence of heating conditions on the tribological behaviournd galling
Concerning the influence of the heating conditions on the tri-ological behaviour, the soaking time had a significant effect onhe friction behaviour. For the specimens heated up to 700 ◦C,he coefficient of friction had in general an unstable behaviourFig. 5(a)). Using 4 min soaking time resulted in a lower and moretable coefficient of friction compared to the other soaking times.t was observed that at 900 ◦C (Fig. 5(b)), the coefficient of friction
as unstable and large fluctuations were seen with 0 min soaking
Please cite this article in press as: Pelcastre, L., et al., Influence of mbehaviour against tool steel at elevated temperatures. J. Mater. Process.
ime. Once the soaking time was increased from 0 min to 4 min, atable coefficient of friction was observed. Comparing the coeffi-ient of friction at 900 ◦C after 4 and 20 min soaking time, a similarehaviour was observed for the two soaking times.
ig. 6. Tool steel specimen after tribological test at 700 ◦C with soaking time of (a) 0 min,
f) 20 min.
from tests at (a) 700 ◦C and (b) 900 ◦C.
The wear mechanisms present on the tool steel specimen sur-faces were analysed by means of SEM. The analysis focused mainlyon the material transfer mechanisms from the Al-Si coated steelonto the tool steel.
Fig. 6 shows the material transfer observed after the tribologicaltests. A strong dependence between the material transfer (galling)and the heating conditions was observed. Severe galling occurredat 700 ◦C for all of the soaking times. From Fig. 6(a) and (c) it is clearthat thick lumps were developed on the surface of the tool speci-mens with 0 and 20 min soaking time. The test with 4 min soakingtime showed severe material transfer but a more uniform trans-fer layer (Fig. 6(b)). The uniform layer correlates with the observedfriction curve from Fig. 5(a), as 4 min soaking time resulted in amore stable coefficient of friction. Formation of lumps is normallyassociated with unstable and high coefficient of friction.
Tests carried out at 900 ◦C underwent severe galling only when0 min soaking time was used. Increasing the soaking time signifi-cantly reduced the occurrence of material transfer. The amount oftransferred material did not change noticeably upon increasing thesoaking time from 4 min to 20 min. This behaviour also correlateswith the observed friction behaviour (Fig. 5(b)), as 4 and 20 minresulted in a stable coefficient of friction.
icrostructural evolution of Al-Si coated UHSS on its tribological Tech. (2015), http://dx.doi.org/10.1016/j.jmatprotec.2015.03.009
3.2.1. Correlation between the phases of the coating and thetribological response
It is known that the intermetallic formation in the coating startsat around 570 ◦C. In their work, Suehiro et al. (2003) observed that
(b) 4 min, and (c) 20 min, and at 900 ◦C with soaking time of (d) 0 min, (e) 4 min and
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F oaking time, (b) 900 ◦C and 0 min soaking time and (c) 900 ◦C and 4 min soaking time.
msiAtr
cbaIsFtinltwrhrftyt
plon
(oiwt
F
ig. 7. Cross section of the coating after the tribological tests. (a) 700 ◦C and 0 min s
elting of the coating occurred and it solidified as the Fe from theubstrate diffused towards the surface. Initially, the kinetics for thentermetallic formation are high due to the presence of the liquidl + Si phase, however, once it starts to solidify as it combines with
he Fe from the steel substrate, the diffusion rate is then drasticallyeduced and the phase transformations are slowed down.
It was observed that more severe galling occurred at 700 ◦Compared to 900 ◦C. The increased galling and unstable frictionehaviour at 700 ◦C can be attributed to the absence of the hardernd more stable phases at the contacting surface formed at 900 ◦C.t is however not entirely clear whether all the Al-Si coating wasolidified or if part of it was still melted. From the results shown inig. 6, it seems that a liquid phase only exists when 0 min soakingime is used, and as the exposure time increases, the solid phases arendeed formed. 700 ◦C and 4 min soaking time showed a homoge-ous transfer layer on the tool steel surface, which suggests that no
iquid phase was present. Similarly, at 20 min soaking time, evenhough thicker lumps were developed, it is clear that these lumpsere formed as a result of continuous accumulation of wear debris,
ather than severe smearing of a liquid or partially liquid phase. Itas to be noted that the reason for the increased severity of mate-ial transfer, going from 4 min to 20 min soaking time, is not clearrom the conducted experiments. The analysis of the microstruc-ure and the measured properties (hardness) after the tests did notield any conclusive evidence to explain this behaviour. Thereforehis needs to be investigated further.
In a study by Borsetto et al. (2009), they observed low friction in ain-on-disc configuration at 700 ◦C and a holding time of 3 min. The
ow friction they observed may be attributed to the low hardnessf the coating at this temperature and the low shearing stresseseeded to plastically deform it.
In the present work, at 900 ◦C, the phases with high hardnessFeAl2/Fe2Al5) were formed after relatively short soaking time. The
Please cite this article in press as: Pelcastre, L., et al., Influence of mbehaviour against tool steel at elevated temperatures. J. Mater. Process.
bservations suggest that these phases reduce adhesion duringnteraction with the tool steel. Only mild galling was observed
hen these phases (FeAl2/Fe2Al5) had reached the surface con-rary to the severe galling observed when Fe2Al7–8Si phase was
ig. 8. Evolution of the coefficient of friction from tests at 900 ◦C with 4 min soaking time
Fig. 9. Evolution of the coefficient of friction from tests at 900 ◦C with and withoutgrinding.
at the surface. This was linked to the low mechanical propertiesof this phase at the testing temperatures, as it is near its meltingpoint (855 ◦C). The high hardness that the coating develops onceFeAl2/Fe2Al5 are formed all the way to the surface, causes rapidwear of the counter tool steel specimen, mainly through abrasivewear, whilst the surface of the Al-Si coating is protected by an oxidelayer. The occurrence of this phenomenon has been reported by theauthors elsewhere (Pelcastre et al., 2013). The coefficient of fric-tion initially increases but then stabilises once the glaze layers areformed.
If the equilibrium diagrams of the Fe-Al-Si system are consid-ered for the composition of the as-delivered Al-Si coating, for allthe temperatures used in this study, a liquid phase exists togetherwith a solid phase. In the forming operation as well as in the tri-bological tests carried out at 900 ◦C in the present work, diffusion
icrostructural evolution of Al-Si coated UHSS on its tribological Tech. (2015), http://dx.doi.org/10.1016/j.jmatprotec.2015.03.009
of the constituents of the coating and the substrate is activated.Thus, the chemical composition of the system changes locally as theatoms move within the coating. This is the reason for the resultingmulti-layered structure of the coating. With the change in chemical
: (a) without cooling after soaking time and (b) with quenching after soaking time.
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l-Si c
css
beeeipfi
iAsa7sosatwoi
3b
asshTcifiuodpcc
3
hc
Fig. 10. Protective oxide layers developed on the surface of the A
omposition, the position of the alloy in the equilibrium diagramhifts and this allows the existence of different phases and causesolidification of the coating.
It is clear that the friction and galling behaviour are influencedy the phases in the Al-Si coating as they have different prop-rties, both physical and mechanical. Providing the system withnough activation energy to allow diffusion is an important param-ter to consider for reduction of galling. Enough diffusion is neededn order to form the harder phases (FeAl2/Fe2Al5), which haveromoted reduced material transfer and also the most stable coef-cient of friction.
As mentioned before, the increased material transfer observedn some of the tests can be explained by the presence of meltedl-Si coating or extremely soft phases when the tribological teststart. In Fig. 7, cross sections of the wear scar of the Al-Si coatingfter tribological tests are shown. When the test temperature was00 ◦C and 900 ◦C and 0 min soaking time was used, the steel sub-trate and the coating underwent wear. A non-uniform tribolayerf mixed oxidised wear debris was also observed and also a con-iderable amount of the Al-Si coating had been detached (Fig. 7(a)nd (b)). When the exposure time increased, the coating continuedransforming and in consequence no wear on the steel substrateas observed. Furthermore, a protective glaze layer was formed
n top of the Al-Si coating, thus minimising the wear of the coatingtself (Fig. 7(c)).
.3. Influence of Al-Si coating surface roughness on frictionalehaviour and galling
As described in Section 2.4, the specimens were pre-heated tollow the development of the hard intermetallics (900 ◦C and 4 minoaking time) and subsequently quenched and ground with abra-ive paper. Once the surfaces were prepared, the specimens wereeated up once more and the tribological tests were performed.his procedure had no major influence on the frictional behaviourompared to a test where the specimen does not undergo cool-ng before the tribological test. In Fig. 8, the coefficient of frictionor a specimen that had undergone heating, quenching and reheat-ng before the tribological test is compared to one that had notndergone an intermediate quenching. The level of the coefficientf friction as well as the overall behaviour did not show significantifferences. This is in agreement with the observations from therevious section; once only FeAl2/Fe2Al5 and Fe2Al2Si are in theoating, the structure or the existing phases of the coating do nothange unless the soaking time is significantly long.
Please cite this article in press as: Pelcastre, L., et al., Influence of mbehaviour against tool steel at elevated temperatures. J. Mater. Process.
.3.1. Morphological changes of the coating and its surfaceGrinding the Al-Si coated UHSS after a heat treatment did not
ave a considerable effect on the friction behaviour. In Fig. 9, theoefficient of friction is shown for the specimens with and without
oated steel: (a) ground specimen and (b) as-delivered specimen.
grinding. The friction levels as well as the stability of the coefficientof friction were similar in both tests.
The observations from the tests suggest that the surface topog-raphy of the work-piece material is not as significant for thetribological performance when sliding against tool steels. Similarbehaviour was observed by Azushima et al. (2012). However, inthose studies, the tool steel was the one that was tested with dif-ferent surface roughness levels. An explanation for this behaviour isthat the formation of tribo-layers (either adhered material or pro-tective oxide layers), on either of the contacting surfaces, controlthe friction behaviour and not the initial surface topography of thetest specimens. It was observed in previous studies by the authors(Pelcastre et al., 2013), that glaze layers or protective oxide lay-ers are formed on the surface of the Al-Si coated steels. Formationof such layers prevents wear of the Al-Si coating and avoids pro-longed direct contact between the two specimens. Furthermore,these oxide layers also allow the occurrence of a stable, althoughhigh, friction coefficient.
Fig. 10 shows the oxide layers formed on the ground and as-delivered Al-Si coated specimens. Clear differences between thetwo types of layers formed were observed. In the case of the groundspecimen, a more continuous oxide layer was developed on top ofthe plateaus formed during grinding (Fig. 2(c)). On the other hand,the as-delivered specimen showed small patches of agglomeratedoxides randomly distributed over the wear scar.
The observations suggest that the nodules on the as-deliveredsurface prevent the formation of a continuous oxide layer. It is pos-sible that the more irregular surface of the Al-Si coating does notallow the oxidised debris to accumulate and form a large and con-tinuous oxide layer. However, the formation of either type of layerprotects against galling as in both cases direct contact between theAl-Si coating and the tool steel specimen is prevented.
4. Conclusions
The evolution of the microstructure and the surface of the Al-Sicoating under different heating conditions was analysed and the tri-bological performance was correlated to the existing phases in thecoating. From the results, the main conclusions can be summarisedas follows:
- With enough activation energy and holding time, the Al-Si coatingreduces the number of existing phases (layers) in its structure.
- The phases at the surface of the Al-Si coating significantly affectthe tribological response during the interaction with tool steel.Galling is reduced when the Fe5Al2 and FeAl2 phases are present
icrostructural evolution of Al-Si coated UHSS on its tribological Tech. (2015), http://dx.doi.org/10.1016/j.jmatprotec.2015.03.009
at the surface.- The tribological behaviour of the coating does not change upon
cooling and reheating once the stable Fe5Al2 and FeAl2 phases areformed.
Surface roughness of the Al-Si coating has a minor impact on thegalling behaviour when it interacts with uncoated tool steels.
cknowledgements
This work has been carried out with financial support from theentre for High Performance Steel at Luleå University of Technologynd the authors gratefully acknowledge their support. The authorsould also like to thank Gestamp HardTech for providing test spec-
mens for these studies and also thank them for their active interestn this work.
eferences
zushima, A., Uda, K., Yanagida, A., 2012. Friction behaviour of aluminum-coated22MnB5 in hot stamping under dry and lubricated conditions. J. Mater. Process.Technol. 212, 1014–1021.
orsetto, F., Ghiotti, A., Bruschi, S., 2009. Investigations of the high strength steel Al-Sicoating during hot stamping operations. Key Eng. Mater. 410–411, 289–296.
an, D.W., De Cooman, B.C., 2012. State-of-the-knowledge on coating systems forhot stamped parts. Steel Res. Int. 83 (5), 412–433.
Please cite this article in press as: Pelcastre, L., et al., Influence of mbehaviour against tool steel at elevated temperatures. J. Mater. Process.
rigorieva, R., Drillet, P., Mataigne, J.-M., Redjaïmia, A., 2011. Phase transforma-tions in the Al-Si coating during the austenitization step. Solid State Phenom.172–174, 784–790.
upta, S.P., 2003. Intermetallic compound formation in Fe–Al–Si ternary system:Part I. Mater. Character. 46, 269–291.
PRESSssing Technology xxx (2015) xxx–xxx
Hardell, J., Pelcastre, L., Prakash, B., 2010. High-temperature friction and wear char-acteristics of hardened ultra-high-strength boron steel. Proc. IMechE Part J: J.Eng. Tribol. 224, 1139–1151.
Kondratiuk, J., Kuhn, P., 2011. Tribological investigation on friction and wearbehaviour of coatings for hot sheet metal forming. Wear 270, 839–849.
Krendelsberger, N., Weitzer, F., Schuster, J.C., 2007. On the reaction scheme andliquidus surface in the ternary system Al–Fe–Si. Metall. Mater. Trans. A 38 (8),1681–1691.
Maitra, T., Gupta, S.P., 2003. Intermetallic compound formation in Fe–Al–Si ternarysystem: Part II. Mater. Character. 46, 293–311.
Pelcastre, L., Hardell, J., Prakash, B., 2011. Investigations into the occurrence of gallingduring hot forming of Al-Si coated high-strength steel. Proc. IMechE Part J: J. Eng.Tribol. 225, 487–498.
Pelcastre, L., Hardell, J., Prakash, B., 2012. Influence of tool steel surface topographyon galling during hot forming of Al-Si coated ultra-high strength steels. In: Conf.Proc. “Nordic Symposium in Tribology, NordTrib 2012”, Trondheim, Norway,June 2012.
Pelcastre, L., Hardell, J., Prakash, B., 2013. Galling mechanisms during interactionof tool steel and Al-Si coated ultra-high strength steel at elevated temperature.Tribol. Int. 67, 263–271.
Suehiro, M., Kusumi, K., Maki, J., Ohgami, M., Miyakoshi, T., 2003. Properties ofaluminium coated steels for hot forming. Nippon Steel Technical Report 88,
icrostructural evolution of Al-Si coated UHSS on its tribological Tech. (2015), http://dx.doi.org/10.1016/j.jmatprotec.2015.03.009
Veit, R., Hofmann, H., Kolleck, R., Sikora, S., 2011. Investigation of the phase formationof AlSi-coatings for hot stamping of boron alloyed steel. In: AIP Conf. Proc. “Inter-national Conference on Advances in Materials and Processing Technologies”,Paris, France, October 2010, ISBN 978-0-7354-0871-5.
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