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Crystal structure analysis of M2 high speed steel parts produced by selective laser melting
Crystal Structure Analysis of M2 High Speed Steel Parts Produced by Selective Laser Melting Z.H. Liu, D.Q. Zhang, K.F. Leong, C.K. Chua PII: DOI: Reference:
S1044-5803(13)00211-8 doi: 10.1016/j.matchar.2013.07.010 MTL 7382
To appear in:
Materials Characterization
Received date: Revised date: Accepted date:
22 February 2013 10 July 2013 12 July 2013
Please cite this article as: Liu ZH, Zhang DQ, Leong KF, Chua CK, Crystal Structure Analysis of M2 High Speed Steel Parts Produced by Selective Laser Melting, Materials Characterization (2013), doi: 10.1016/j.matchar.2013.07.010
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ACCEPTED MANUSCRIPT Title: Crystal Structure Analysis of M2 High Speed Steel Parts Produced by Selective Laser Melting
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Affiliation: aNanyang Technological University, Singapore
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Authors: 1,aZ. H. LIU, 2,aD. Q. ZHANG, 3,aK. F. LEONG & 4,aC. K. CHUA
Corresponding author: Chee Kai CHUA (C. K. CHUA)
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Contact number: +65 6790 5486, Fax number: +65 6791-1859
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Email: [email protected], [email protected], [email protected] & [email protected]
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Abstract: M2 high speed steel (HSS) samples were produced by an additive manufacturing (AM)
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process, selective laser melting (SLM). The observed microstructure from SEM and FIB was
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characterised by a continuous and homogeneous network of dendrites within two different phases. These phases were characterised to be bcc single crystals and fcc/bcc polycrystals from
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TEM-SADP. EBSD results also indicated that the bcc grains were randomly orientated. In addition, the lattice constants from two different SADPs of bcc crystal were calculated to be 2.892 Å and 2.905 Å, larger than high purity iron bcc structure of lattice constant 2.867 Å. The elements V, Cr, Mo, W and Si were detected in the iron matrix with TEM-EDS. These elements were responsible for the enlargement of the lattice constant. TEM-EDS results also indicated that the network of dendrites were carbides. These results gave insights on the rapid solidification phenomenon in SLM. Keywords: Selective Laser Melting, Transmission Electron Microscopy, High Speed Steel, Microstructure Characterisation, Crystal Structure Study.
ACCEPTED MANUSCRIPT 1. Introduction Selective laser melting (SLM) is an additive manufacturing (AM) technique that fabricates net
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shape parts from the melting and fusion of powder material in a layer by layer manner [1, 2]. In the SLM process, a laser is focused onto a thin powder layer creating a high energy density spot.
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This high density energy melts the powder material which then fuses with the dense underlying substrate. The relatively thin molten material is brought into good thermal contact with this
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substrate that effectively performs as a powerful heat sink [3]. Hence, SLM is characterised by
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rapid solidification that results in several key structural features, namely, grain size refinement, extended solid solubility, chemical homogeneity and reduction in number and sizes of phase
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segregation. These features contribute to excellent mechanical properties of SLM parts [4]. However, SLM is a very complex process that involves complicated heat and mass transfer that
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are influenced by several factors [5] such as laser power, scan speed and powder layer thickness
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[6-9]. The basic principle of the SLM process in terms of the heat transfer phenomenon, energy
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absorption, the material laser interaction during rapid melting and solidification in the SLM process is not yet fully understood [5]. Furthermore, unexpected phase transformation may occur in the underlying material in each layer as it is thermally affected by the rapidly melted and solidified subsequent layers of powder above. This paper aims to characterise the microstructure evolution of M2 high speed steel (HSS) SLM parts. In addition, the crystal structure and grain orientation of M2 HSS parts produced by the SLM process will be studied and analysed. These findings provide an insight to the SLM process and contribute to the fundamental understanding of the SLM forming mechanism.
ACCEPTED MANUSCRIPT 2. Experimental Details In this work, a 100 W Nd:YAG laser in continuous wave (cw) mode was used to produce M2
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HSS parts. The laser beam profile follows the Gaussian’s distribution and has a wavelength of 1.064 nm. The beam diameter, laser scan speeds and powder layer thickness were set at 180 µm,
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700 mm/s and 30 µm respectively. In addition, the substrate was preheated to a temperature of 180 oC to reduce the thermal gradients and reduce thermal stresses experienced by the parts
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during the SLM process.
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The microstructures of polished M2 HSS samples were first observed with focused ion beam (FIB) imaging and x-ray diffraction (XRD) followed by electron backscatter diffraction (EBSD).
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These samples were then etched with Vilella’s reagent to reveal the ferrite-carbide structures. The etched and un-etched samples were further observed and characterised using scanning
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electron microscopy (SEM) in two different detection modes, the standard secondary electron
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imaging (SEI) and low angle backscatter electron (LABe) imaging for detail compositional
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contrast observation. Samples were also carefully prepared with FEI Helios 600i FIB equipment. The crystal structures were also studied using TEM (FEI Tecnai F20ST TEM equipment with a calibration tolerance within 3 to 5 %.) and selected area diffraction pattern (SADP) techniques. In addition, energy dispersion x-ray spectroscopy (EDS) was used to identify the type and amount of elements present where it has an accuracy of ± 4 to 5 % deviation for concentration above 5 weight percent in accordance to standard EDS analysis guideline. 3. Results 3.1 Focused Ion Beam Imaging
ACCEPTED MANUSCRIPT Ferritic, austenitic and martensitic phases were observed to be present in the M2 HSS SLM samples. Three different zones, A, B and C identified from the FIB image (taken along the melt
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pool boundary to indicate the different phases present) are shown in Fig. 1. Zone A is
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characterised by ferrite phase where grain boundaries could be clearly seen whereas zone B is characterised by a mixture of disoriented austenitic and ferritic phases and lastly, zone C, on the
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other hand, is characterised by a disoriented mixture of ferritic, austenitic and martensitic phases.
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Zone C displayed acicular martensite features that are characteristic of tempered M2 HSS. Since the SLM process is a layer by layer additive manufacturing process, the material in each layer
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was influenced by the laser during the SLM process and experienced a thermal condition where
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partial tempering took place resulting in formation of martensites. 3.2 X-Ray Diffraction
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The XRD patterns of three different scan speeds, 250, 550 and 700 mm/s, were compared. The
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XRD results showed fcc and bcc phases in all samples. Comparing the results in Fig. 2, the fcc
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peaks in Fig. 2a were sharper and more distinct, indicating the presence of more austenite in samples produced by scan speed 700 mm/s. This result was consistent with the observations from previous studies [10, 11] where more austenitic phases were observed in samples produced from scan speed of 700 mm/s compared to samples produced from scan speed of 250 mm/s. 3.3 Electron Backscatter Diffraction From the EBSD results, only the ferrite (zone A from Fig. 1) could be indexed and other zones could not be indexed. This was because high solidification rates experienced in zones B and C resulted in mixture of primary martensite/austenite/ferrite phases where the grain growth was incomplete. Therefore, zone B could not be compared to the standard Kikuchi Patterns and hence
ACCEPTED MANUSCRIPT could not indexed. Fig. 3 shows the overall microstructure image taken from light optical microscopy (LOM) compared with the EBSD image. The bright areas (corresponding to ferrite -
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zone A) were indexed with high confidence index (CI) of values ranging from 0.1 to 0.6. The
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dark areas (corresponding to other phases - zones B and C) however, were indexed with low CI values. Generally, the CI value indicates the reliability of the result of the determined orientation.
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An uncompromising approach is to consider only those measurements with CI values greater
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than 0.1 [12-14]. Hence, further analyses on grain shape and grain orientations of the ferrite phase (indexed with high CI values) were carried out with the optical imaging microscopy
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(OIMTM) analysis software. With reference to Fig. 3 where the different phases are shown, a representative orientation map of the indexed area displayed with grain boundaries with a critical
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disorientation angle between 5o to 15o is shown in Fig. 4. The colour-coded inverse pole figure,
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where points on the sample with a <111> axis parallel to the surface normal are blue, <101> green, <001> red (see legend in Fig. 4b) and intermediate orientations have intermediate colours.
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It clearly shows that the grains had random orientations and were not associated with any
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preferred orientations.
3.4 Scanning Electron Microscopy Two distinct zones, A and B (corresponding to zones marked in Fig. 1) were observed from unetched samples under LABe mode shown in Fig. 5a. Upon close up viewing, network shaped dendrites could be observed clearly (Fig. 5b). These network shaped dendrites in zone A were clearly revealed upon etching under SEI mode (Fig. 5c). Under LABe mode, the same etched sample revealed a homogeneous network shaped dendrites in both zones, A and B. These network shaped dendrites were also observed to be continuous along the phase boundaries as shown in Fig 5d.
ACCEPTED MANUSCRIPT 3.5 Crystal Structure Analysis
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3.5.1 Transmission Electron Microscopy
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The area containing zones A and B in the sample was specially selected and was first prepared carefully by polishing and thinned down (around 100 nm thickness) by FIB milling. Zones A and
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B are shown to be separated and were consistent with the SEM and FIB images.
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Fig. 6 shows two SADPs of selected grains taken from zone A. The length ratio and angle ratio from SADP (i) fit the standard diffraction pattern for bcc crystals in the [001] beam direction
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[15]. As seen from the very clear SADP obtained, the electron beam is almost parallel to the zone axis and the grain is oriented in such a way where the measurements of lattice spacing is
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very accurate. Based on this diffraction pattern, the inter-planar spacings were measured to be
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d200 = 0.1447 nm and d110 = 0.2062 nm. The lattice constant was calculated based on these interplanar spacings and the average lattice constant of this bcc crystal was determined to be 2.905 Å,
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larger than that of high purity iron. The respective inter-planar spacings for high purity iron are
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d200 = 0.1433 nm and d110 = 0.2027 nm, resulting in a lattice constant of 2.867 Å. Similar to SADP (i), the length ratio and angle ratio from SADP (ii) fit the standard diffraction pattern for bcc crystals but in the [-111] beam direction [15]. Based on this diffraction pattern, the inter-planar spacings were measured to be d110 = 0.2020 nm and d101 = 0.2070 nm. The average lattice constant of this crystal was determined to be 2.892 Å, again larger than that of high purity iron. The respective inter-planar spacing for high purity iron is d110 = 0.2027 nm, resulting in a lattice constant of 2.867 Å. These results indicated that zone A consisted of bcc single crystals of lattice constant that is larger than that of high purity iron.
ACCEPTED MANUSCRIPT Fig. 7 and Fig. 8 show two SADPs of selected locations taken from zones B and C. The diffraction patterns clearly indicated that zones B and C consisted of polycrystals [15]. From
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SADP (i) shown in Fig. 7, the radius ratio of the rings observed was 1: 1.118 (fcc): 1.386 (bcc):
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1.724 (bcc): 1.991 (bcc/fcc): 2.211 (bcc) [15]. From SADP (ii) shown in Fig. 8, the radius ratio of the rings observed was 1: 1.123 (fcc): 1.293 (inconclusive); 1.412 (bcc): 1.736 (bcc): 1.923
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(fcc): 2.024 (fcc/bcc): 2.254 (bcc) [15]. These results strongly suggested that zones B and C were
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3.5.2 Energy Dispersive X-ray Spectroscopy
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a mixture of bcc and fcc polycrystals.
TEM-EDS analysis was carried out on three areas where network shaped dendrites could be
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observed under scanning TEM (STEM). Fig. 9 shows the STEM image of the locations, (i), (ii) and (iii), from zone A and zone B where EDS was carried out to identify the elemental
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composition in the network shaped dendrites correspond to those shown in Fig. 5. In each EDS
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location, the x-ray spectrum acquisition live time for each point was 10 seconds and a total of
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three points were measured, two on the network shaped dendrites and one on the background as reference. The purpose of the reference was to compare the carbon content of the background and the network shaped dendrites from the x-ray spectrum. Since carbon is typically associated with surface contamination, the total carbon content quantified from the background was assumed to be the contamination. Hence, x-ray counts of acquired x-ray spectrum, from 0 keV to 0.36 keV, were subtracted from x-ray counts of the same energy range of point 3 obtained in all EDS locations, (i), (ii) and (iii). The quantitative results are summarised in Table 1. The resultant carbon in atomic % in EDS (i) point 1, EDS (ii) point 2 and EDS (iii) point 1 were at 3.46%, 3.61% and 4.99% respectively.
ACCEPTED MANUSCRIPT This suggested that the homogeneous and continuous network shaped dendrites were carbides although they could not be positively identified. On the other hand, the resultant carbon content
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could not be quantified for EDS (i) point 2, EDS (ii) point 1 and EDS (iii) point 2 after
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subtraction. Hence, quantitative results on these points were left out. As shown in Table 1, V, Cr, Mo, W and a small amount of Si were detected to be present within the bcc phase shown as point
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3 in EDS (i) and bcc/fcc mixture phase in EDS (ii) and (iii).
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4. Discussion
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In conventional casting of M2 HSS based on the M2 HSS phase diagram [16], δ-ferrite (a more stable ferrite form at higher temperatures) first forms from the melt at 1435o (L δ + L)
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followed by γ-iron and δ-ferrite at 1330o (δ + L δ + γ + L), followed by δ-ferrite, γ-iron and MxC (δ + γ + L δ + γ + MxC) at 1260o and finally forms α-ferrite and MxC (δ + γ + MxC α
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+ MxC) at room temperature upon cooling in air [16].
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In SLM of M2 HSS, the phase transformation process did not follow the conventional phase
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diagram. An etched microstructure of a single SLM track shown in Fig. 10 indicated that the M2 HSS experienced non-equilibrium solidification during the SLM process. The ferritic phases present from the melt (dark regions in Fig. 10 correspond to zone A in Fig. 1) in contact with the substrate experienced a lower solidification rate because there was enough time to form single bcc crystals. On the other hand, a mixture of bcc/fcc polycrystals (zone B) were observed from the centre of the melt. In addition, the FIB image showed clear grain boundaries in zone A and disorganised morphology in zone B. This was a clear indication of higher solidification rates experienced in zone B.
ACCEPTED MANUSCRIPT Upon heating to its austenitisation temperature in M2 HSS [17], austenite is formed and can only be retained under quenching conditions where cooling rates are higher than 103 K/s [18]. Under
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lower cooling rates, austenite will evolve and transform to ferrite [16]. As such, the observation
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of austenite presence in the centre of the melt and ferrite presence in the melt surrounding the austenite indicated that the centre of the melt experienced a higher cooling rate (of more than 103
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K/s) compared to the rest of the melt.
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The resultant solidification and cooling rates of a melt in the SLM process are largely dependent
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on the thermal gradients experienced in the build direction as well as the laser scan direction [1921]. This determines the resultant thermal gradient vector experienced by the melt during the
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SLM process. While the ferrite crystals were observed to grow towards the centre of the melt beginning from the melt boundary, the influence of the huge thermal gradient along the laser
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scan direction was larger than the thermal gradient along the build direction. The presence of
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austenite at the centre of the melt suggested that the huge thermal gradient in the laser scan direction had a larger influence on the melt compared to the thermal gradient in the build
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direction caused by the substrate heat sink. This resulted in a higher cooling and solidification rate at the centre of the melt as observed from the microstructure. Nevertheless, both zones experienced rapid solidification sufficient to suppress the precipitation of bulk shape carbides resulting in a continuous and homogeneous network of carbide dendrites within zones A and B as observed in Fig. 5d. In addition, uniformity of the carbide dendrite network are highly desired in M2 HSS because such a morphology resists dimensional and hardness changes.
ACCEPTED MANUSCRIPT Since, the solubility of carbon is low in bcc crystals (maximum of 0.02% at 723oC), most of the carbon was rejected into the remaining melt during the crystallisation of bcc crystals. Upon
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further solidification of the remaining melt, the carbon was trapped in the fcc crystals during
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crystallisation. Some of these fcc crystals in zone B experienced a thermal situation (rapid heating and cooling of the succeeding material above) where the martensite start temperature (Ms)
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was reached. They became mechanically unstable and transformed to bct crystals (martensite)
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during the cooling phase. Therefore, partial martensitic transformation took place resulting in a mixture of bcc/fcc/bct crystals as observed in zone C in Fig. 1. As the SLM is a layer by layer
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AM process [2], each layer experienced similar thermal circumstances resulting in a heterogeneous microstructure following a repetitive/alternating morphology of bcc and a mixture
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of bcc/fcc/bct crystals (see Fig. 3). As a result, the hardness was inconsistent and was measured
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to be 682 ± 26 HV in zone A and 846 ± 35 HV in zone B from previous studies [10]. Nevertheless, microstructural homogeneity and high hardness could be obtained with appropriate
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hardening and tempering heat treatments shown in another study [22].
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EBSD results indicated that the resultant M2 HSS material was isotropic since the grains were random and not associated with any preferred directions. The bcc lattice constants obtained from the SADPs in zone A were calculated to be larger than that of high purity iron bcc. According to the TEM-EDS results, the elements V, Cr, Mo, W and Si detected in the iron matrix could be responsible for the enlargement of the lattice constant. From the crystal structure study of the M2 HSS part produced by SLM, the SLM process was characterised by rapid solidification where mechanical properties were enhanced. The growth morphology of the iron matrix and the distribution of the dendritic carbide network were significantly influenced by the applied laser radiation. The combination of microstructure and
ACCEPTED MANUSCRIPT mechanical properties could be achieved through this rapid solidification in SLM. The rapid change of thermal energy associated with this process permits large deviations from equilibrium,
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as evidenced by the SLM process characteristics and several key structural features established
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in this work. Firstly, the solid solubility is directly related to the temperature [23]. During the SLM process, the alloying elements do not have time to diffuse due to rapid solidification. Under
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such a condition, the solid solubility is extended which would increase the solid solution
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strengthening of SLM parts [4]. Secondly, the reduction in size and number of phase segregation was observed from the continuous and homogeneous network of carbide dendrites of nanometric
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scale. Size refinement was also seen from the small grains of less than 0.5 µm (see Fig. 11)
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which would improve fracture and impact toughness of SLM parts [4]. 5. Conclusions
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1. The resultant microstructure of M2 HSS parts produced from SLM was characterised by
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three different zones consisting of bcc, bcc/fcc and bcc/fcc/bct phases as seen from FIB
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images. A continuous and homogeneous network of carbide dendrites was observed under SEM within these zones. 2. The phases were revealed to be fcc and bcc from the XRD results. The bcc grains had random orientations and were not associated with any preferred orientations. 3. The bcc phase was single crystals with enlarged lattice constant from TEM-SADP results. Further analysis from TEM-EDS suggested that the detected elements V, Cr, Mo, W and Si within the iron matrix were responsible for the enlargement of the bcc lattice constant. 4. The rapid solidification phenomenon in the SLM process resulted in extension in solid solubility and grain size refinement.
ACCEPTED MANUSCRIPT Acknowledgement The authors would like to thank Professor Jean-Pierre Kruth and his colleagues from Katholieke
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Universiteit Leuven, Belgium for providing the M2 HSS SLM samples for this research.
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ACCEPTED MANUSCRIPT 19. Thijs L, Kempen K, Kruth JP, Van Humbeeck J. Fine-structured aluminium products with controllable texture by selective laser melting of pre-alloyed AlSi10Mg powder.
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Figure Captions
Fig. 1 – FIB image of the microstructure of M2 HSS SLM sample.
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Fig. 2 – XRD of M2 HSS SLM samples produced from scan speeds: (a) 250 mm/s, (b) 550 mm/s, and (c) 700 mm/s.
Fig. 3 – LOM and EBSD images of the microstructure of M2 HSS SLM samples. Fig. 4 – (a) Indexed area and (b) EBSD orientation map of indexed area. Fig. 5 – (a) Image of unetched sample under LABe mode showing the distinct separated phases where zones A and B correspond to zones marked in Fig. 1, (b) close up image showing network shaped dendrites, (c) etched sample under SEI mode revealing network shaped dendrites in zone
ACCEPTED MANUSCRIPT A, and (d) etched sample under LABe mode revealing a homogeneous and continuous network of dendrites in zones A and B.
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Fig. 6 – SADP (i) and (ii) obtained from zone A. Fig. 7 – SADP (i) obtained from zone B.
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Fig. 9 – EDS locations identified from STEM.
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Fig. 8 – SADP (ii) obtained from zone B.
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Fig. 10 – Microstructural morphology of the cross section in a single SLM track.
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Fig. 11 – FIB image showing small grains in zone A.
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ACCEPTED MANUSCRIPT Table 1 – Summary of the EDS quantitative results from EDX (i), (ii) and (iii).
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EDS (i) EDS (ii) EDS (iii) Element Point Weight % Atomic % Weight % Atomic % Weight % Atomic % 1 0.67 3.46 0.88 4.99 C(K) 2 0.61 3.61 0.08 0.48 3 Carbon content is excluded from quantitative analysis 1 11.71 14.07 4.97 6.60 V(K) 2 14.17 19.55 5.01 6.69 3 1.02 1.20 1.12 1.29 2.83 3.24 1 8.75 10.30 10.12 13.16 Cr(K) 2 10.72 14.49 11.18 14.63 3 3.45 3.96 4.16 4.67 3.72 4.17 1 53.29 58.43 45.98 55.67 Fe(K) 2 25.60 32.22 46.84 57.05 3 81.57 87.26 85.06 88.90 84.40 88.03 1 17.05 10.88 16.52 11.64 Mo(K) 2 24.24 17.75 22.09 15.66 3 5.78 3.60 0.80 0.48 1 8.50 2.83 21.49 7.90 W(L) 2 23.26 8.89 14.77 5.46 3 7.43 2.41 8.46 2.68 7.40 2.34 1 Si(K) 2 1.38 3.45 3 0.72 1.54 1.17 2.43 0.82 1.72
ACCEPTED MANUSCRIPT Highlights:
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Microstructure of SLM m2 high speed steel was investigated through TEM A continuous and homogeneous network of carbide dendrites were observed Two distinct phases of bcc single crystals and fcc/bcc polycrystals were observed The iron bcc lattice constant calculated from TEM-SADP was larger Alloying elements V, Cr, Mo, W and Si were detected in the iron matrix by TEM-EDS
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S1044-5803(13)00211-8 doi: 10.1016/j.matchar.2013.07.010 MTL 7382
To appear in:
Materials Characterization
Received date: Revised date: Accepted date:
22 February 2013 10 July 2013 12 July 2013
Please cite this article as: Liu ZH, Zhang DQ, Leong KF, Chua CK, Crystal Structure Analysis of M2 High Speed Steel Parts Produced by Selective Laser Melting, Materials Characterization (2013), doi: 10.1016/j.matchar.2013.07.010
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Title: Crystal Structure Analysis of M2 High Speed Steel Parts Produced by Selective Laser Melting
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Affiliation: aNanyang Technological University, Singapore
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Authors: 1,aZ. H. LIU, 2,aD. Q. ZHANG, 3,aK. F. LEONG & 4,aC. K. CHUA
Corresponding author: Chee Kai CHUA (C. K. CHUA)
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Contact number: +65 6790 5486, Fax number: +65 6791-1859
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Email: [email protected], [email protected], [email protected] & [email protected]
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Abstract: M2 high speed steel (HSS) samples were produced by an additive manufacturing (AM)
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process, selective laser melting (SLM). The observed microstructure from SEM and FIB was
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characterised by a continuous and homogeneous network of dendrites within two different phases. These phases were characterised to be bcc single crystals and fcc/bcc polycrystals from
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TEM-SADP. EBSD results also indicated that the bcc grains were randomly orientated. In addition, the lattice constants from two different SADPs of bcc crystal were calculated to be 2.892 Å and 2.905 Å, larger than high purity iron bcc structure of lattice constant 2.867 Å. The elements V, Cr, Mo, W and Si were detected in the iron matrix with TEM-EDS. These elements were responsible for the enlargement of the lattice constant. TEM-EDS results also indicated that the network of dendrites were carbides. These results gave insights on the rapid solidification phenomenon in SLM. Keywords: Selective Laser Melting, Transmission Electron Microscopy, High Speed Steel, Microstructure Characterisation, Crystal Structure Study.
ACCEPTED MANUSCRIPT 1. Introduction Selective laser melting (SLM) is an additive manufacturing (AM) technique that fabricates net
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shape parts from the melting and fusion of powder material in a layer by layer manner [1, 2]. In the SLM process, a laser is focused onto a thin powder layer creating a high energy density spot.
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This high density energy melts the powder material which then fuses with the dense underlying substrate. The relatively thin molten material is brought into good thermal contact with this
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substrate that effectively performs as a powerful heat sink [3]. Hence, SLM is characterised by
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rapid solidification that results in several key structural features, namely, grain size refinement, extended solid solubility, chemical homogeneity and reduction in number and sizes of phase
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segregation. These features contribute to excellent mechanical properties of SLM parts [4]. However, SLM is a very complex process that involves complicated heat and mass transfer that
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are influenced by several factors [5] such as laser power, scan speed and powder layer thickness
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[6-9]. The basic principle of the SLM process in terms of the heat transfer phenomenon, energy
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absorption, the material laser interaction during rapid melting and solidification in the SLM process is not yet fully understood [5]. Furthermore, unexpected phase transformation may occur in the underlying material in each layer as it is thermally affected by the rapidly melted and solidified subsequent layers of powder above. This paper aims to characterise the microstructure evolution of M2 high speed steel (HSS) SLM parts. In addition, the crystal structure and grain orientation of M2 HSS parts produced by the SLM process will be studied and analysed. These findings provide an insight to the SLM process and contribute to the fundamental understanding of the SLM forming mechanism.
ACCEPTED MANUSCRIPT 2. Experimental Details In this work, a 100 W Nd:YAG laser in continuous wave (cw) mode was used to produce M2
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HSS parts. The laser beam profile follows the Gaussian’s distribution and has a wavelength of 1.064 nm. The beam diameter, laser scan speeds and powder layer thickness were set at 180 µm,
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700 mm/s and 30 µm respectively. In addition, the substrate was preheated to a temperature of 180 oC to reduce the thermal gradients and reduce thermal stresses experienced by the parts
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during the SLM process.
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The microstructures of polished M2 HSS samples were first observed with focused ion beam (FIB) imaging and x-ray diffraction (XRD) followed by electron backscatter diffraction (EBSD).
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These samples were then etched with Vilella’s reagent to reveal the ferrite-carbide structures. The etched and un-etched samples were further observed and characterised using scanning
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electron microscopy (SEM) in two different detection modes, the standard secondary electron
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imaging (SEI) and low angle backscatter electron (LABe) imaging for detail compositional
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contrast observation. Samples were also carefully prepared with FEI Helios 600i FIB equipment. The crystal structures were also studied using TEM (FEI Tecnai F20ST TEM equipment with a calibration tolerance within 3 to 5 %.) and selected area diffraction pattern (SADP) techniques. In addition, energy dispersion x-ray spectroscopy (EDS) was used to identify the type and amount of elements present where it has an accuracy of ± 4 to 5 % deviation for concentration above 5 weight percent in accordance to standard EDS analysis guideline. 3. Results 3.1 Focused Ion Beam Imaging
ACCEPTED MANUSCRIPT Ferritic, austenitic and martensitic phases were observed to be present in the M2 HSS SLM samples. Three different zones, A, B and C identified from the FIB image (taken along the melt
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pool boundary to indicate the different phases present) are shown in Fig. 1. Zone A is
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characterised by ferrite phase where grain boundaries could be clearly seen whereas zone B is characterised by a mixture of disoriented austenitic and ferritic phases and lastly, zone C, on the
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other hand, is characterised by a disoriented mixture of ferritic, austenitic and martensitic phases.
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Zone C displayed acicular martensite features that are characteristic of tempered M2 HSS. Since the SLM process is a layer by layer additive manufacturing process, the material in each layer
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was influenced by the laser during the SLM process and experienced a thermal condition where
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partial tempering took place resulting in formation of martensites. 3.2 X-Ray Diffraction
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The XRD patterns of three different scan speeds, 250, 550 and 700 mm/s, were compared. The
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XRD results showed fcc and bcc phases in all samples. Comparing the results in Fig. 2, the fcc
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peaks in Fig. 2a were sharper and more distinct, indicating the presence of more austenite in samples produced by scan speed 700 mm/s. This result was consistent with the observations from previous studies [10, 11] where more austenitic phases were observed in samples produced from scan speed of 700 mm/s compared to samples produced from scan speed of 250 mm/s. 3.3 Electron Backscatter Diffraction From the EBSD results, only the ferrite (zone A from Fig. 1) could be indexed and other zones could not be indexed. This was because high solidification rates experienced in zones B and C resulted in mixture of primary martensite/austenite/ferrite phases where the grain growth was incomplete. Therefore, zone B could not be compared to the standard Kikuchi Patterns and hence
ACCEPTED MANUSCRIPT could not indexed. Fig. 3 shows the overall microstructure image taken from light optical microscopy (LOM) compared with the EBSD image. The bright areas (corresponding to ferrite -
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zone A) were indexed with high confidence index (CI) of values ranging from 0.1 to 0.6. The
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dark areas (corresponding to other phases - zones B and C) however, were indexed with low CI values. Generally, the CI value indicates the reliability of the result of the determined orientation.
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An uncompromising approach is to consider only those measurements with CI values greater
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than 0.1 [12-14]. Hence, further analyses on grain shape and grain orientations of the ferrite phase (indexed with high CI values) were carried out with the optical imaging microscopy
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(OIMTM) analysis software. With reference to Fig. 3 where the different phases are shown, a representative orientation map of the indexed area displayed with grain boundaries with a critical
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disorientation angle between 5o to 15o is shown in Fig. 4. The colour-coded inverse pole figure,
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where points on the sample with a <111> axis parallel to the surface normal are blue, <101> green, <001> red (see legend in Fig. 4b) and intermediate orientations have intermediate colours.
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It clearly shows that the grains had random orientations and were not associated with any
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preferred orientations.
3.4 Scanning Electron Microscopy Two distinct zones, A and B (corresponding to zones marked in Fig. 1) were observed from unetched samples under LABe mode shown in Fig. 5a. Upon close up viewing, network shaped dendrites could be observed clearly (Fig. 5b). These network shaped dendrites in zone A were clearly revealed upon etching under SEI mode (Fig. 5c). Under LABe mode, the same etched sample revealed a homogeneous network shaped dendrites in both zones, A and B. These network shaped dendrites were also observed to be continuous along the phase boundaries as shown in Fig 5d.
ACCEPTED MANUSCRIPT 3.5 Crystal Structure Analysis
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3.5.1 Transmission Electron Microscopy
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The area containing zones A and B in the sample was specially selected and was first prepared carefully by polishing and thinned down (around 100 nm thickness) by FIB milling. Zones A and
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B are shown to be separated and were consistent with the SEM and FIB images.
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Fig. 6 shows two SADPs of selected grains taken from zone A. The length ratio and angle ratio from SADP (i) fit the standard diffraction pattern for bcc crystals in the [001] beam direction
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[15]. As seen from the very clear SADP obtained, the electron beam is almost parallel to the zone axis and the grain is oriented in such a way where the measurements of lattice spacing is
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very accurate. Based on this diffraction pattern, the inter-planar spacings were measured to be
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d200 = 0.1447 nm and d110 = 0.2062 nm. The lattice constant was calculated based on these interplanar spacings and the average lattice constant of this bcc crystal was determined to be 2.905 Å,
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larger than that of high purity iron. The respective inter-planar spacings for high purity iron are
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d200 = 0.1433 nm and d110 = 0.2027 nm, resulting in a lattice constant of 2.867 Å. Similar to SADP (i), the length ratio and angle ratio from SADP (ii) fit the standard diffraction pattern for bcc crystals but in the [-111] beam direction [15]. Based on this diffraction pattern, the inter-planar spacings were measured to be d110 = 0.2020 nm and d101 = 0.2070 nm. The average lattice constant of this crystal was determined to be 2.892 Å, again larger than that of high purity iron. The respective inter-planar spacing for high purity iron is d110 = 0.2027 nm, resulting in a lattice constant of 2.867 Å. These results indicated that zone A consisted of bcc single crystals of lattice constant that is larger than that of high purity iron.
ACCEPTED MANUSCRIPT Fig. 7 and Fig. 8 show two SADPs of selected locations taken from zones B and C. The diffraction patterns clearly indicated that zones B and C consisted of polycrystals [15]. From
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SADP (i) shown in Fig. 7, the radius ratio of the rings observed was 1: 1.118 (fcc): 1.386 (bcc):
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1.724 (bcc): 1.991 (bcc/fcc): 2.211 (bcc) [15]. From SADP (ii) shown in Fig. 8, the radius ratio of the rings observed was 1: 1.123 (fcc): 1.293 (inconclusive); 1.412 (bcc): 1.736 (bcc): 1.923
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(fcc): 2.024 (fcc/bcc): 2.254 (bcc) [15]. These results strongly suggested that zones B and C were
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3.5.2 Energy Dispersive X-ray Spectroscopy
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a mixture of bcc and fcc polycrystals.
TEM-EDS analysis was carried out on three areas where network shaped dendrites could be
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observed under scanning TEM (STEM). Fig. 9 shows the STEM image of the locations, (i), (ii) and (iii), from zone A and zone B where EDS was carried out to identify the elemental
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composition in the network shaped dendrites correspond to those shown in Fig. 5. In each EDS
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location, the x-ray spectrum acquisition live time for each point was 10 seconds and a total of
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three points were measured, two on the network shaped dendrites and one on the background as reference. The purpose of the reference was to compare the carbon content of the background and the network shaped dendrites from the x-ray spectrum. Since carbon is typically associated with surface contamination, the total carbon content quantified from the background was assumed to be the contamination. Hence, x-ray counts of acquired x-ray spectrum, from 0 keV to 0.36 keV, were subtracted from x-ray counts of the same energy range of point 3 obtained in all EDS locations, (i), (ii) and (iii). The quantitative results are summarised in Table 1. The resultant carbon in atomic % in EDS (i) point 1, EDS (ii) point 2 and EDS (iii) point 1 were at 3.46%, 3.61% and 4.99% respectively.
ACCEPTED MANUSCRIPT This suggested that the homogeneous and continuous network shaped dendrites were carbides although they could not be positively identified. On the other hand, the resultant carbon content
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could not be quantified for EDS (i) point 2, EDS (ii) point 1 and EDS (iii) point 2 after
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subtraction. Hence, quantitative results on these points were left out. As shown in Table 1, V, Cr, Mo, W and a small amount of Si were detected to be present within the bcc phase shown as point
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3 in EDS (i) and bcc/fcc mixture phase in EDS (ii) and (iii).
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4. Discussion
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In conventional casting of M2 HSS based on the M2 HSS phase diagram [16], δ-ferrite (a more stable ferrite form at higher temperatures) first forms from the melt at 1435o (L δ + L)
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followed by γ-iron and δ-ferrite at 1330o (δ + L δ + γ + L), followed by δ-ferrite, γ-iron and MxC (δ + γ + L δ + γ + MxC) at 1260o and finally forms α-ferrite and MxC (δ + γ + MxC α
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+ MxC) at room temperature upon cooling in air [16].
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In SLM of M2 HSS, the phase transformation process did not follow the conventional phase
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diagram. An etched microstructure of a single SLM track shown in Fig. 10 indicated that the M2 HSS experienced non-equilibrium solidification during the SLM process. The ferritic phases present from the melt (dark regions in Fig. 10 correspond to zone A in Fig. 1) in contact with the substrate experienced a lower solidification rate because there was enough time to form single bcc crystals. On the other hand, a mixture of bcc/fcc polycrystals (zone B) were observed from the centre of the melt. In addition, the FIB image showed clear grain boundaries in zone A and disorganised morphology in zone B. This was a clear indication of higher solidification rates experienced in zone B.
ACCEPTED MANUSCRIPT Upon heating to its austenitisation temperature in M2 HSS [17], austenite is formed and can only be retained under quenching conditions where cooling rates are higher than 103 K/s [18]. Under
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lower cooling rates, austenite will evolve and transform to ferrite [16]. As such, the observation
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of austenite presence in the centre of the melt and ferrite presence in the melt surrounding the austenite indicated that the centre of the melt experienced a higher cooling rate (of more than 103
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K/s) compared to the rest of the melt.
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The resultant solidification and cooling rates of a melt in the SLM process are largely dependent
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on the thermal gradients experienced in the build direction as well as the laser scan direction [1921]. This determines the resultant thermal gradient vector experienced by the melt during the
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SLM process. While the ferrite crystals were observed to grow towards the centre of the melt beginning from the melt boundary, the influence of the huge thermal gradient along the laser
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scan direction was larger than the thermal gradient along the build direction. The presence of
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austenite at the centre of the melt suggested that the huge thermal gradient in the laser scan direction had a larger influence on the melt compared to the thermal gradient in the build
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direction caused by the substrate heat sink. This resulted in a higher cooling and solidification rate at the centre of the melt as observed from the microstructure. Nevertheless, both zones experienced rapid solidification sufficient to suppress the precipitation of bulk shape carbides resulting in a continuous and homogeneous network of carbide dendrites within zones A and B as observed in Fig. 5d. In addition, uniformity of the carbide dendrite network are highly desired in M2 HSS because such a morphology resists dimensional and hardness changes.
ACCEPTED MANUSCRIPT Since, the solubility of carbon is low in bcc crystals (maximum of 0.02% at 723oC), most of the carbon was rejected into the remaining melt during the crystallisation of bcc crystals. Upon
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further solidification of the remaining melt, the carbon was trapped in the fcc crystals during
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crystallisation. Some of these fcc crystals in zone B experienced a thermal situation (rapid heating and cooling of the succeeding material above) where the martensite start temperature (Ms)
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was reached. They became mechanically unstable and transformed to bct crystals (martensite)
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during the cooling phase. Therefore, partial martensitic transformation took place resulting in a mixture of bcc/fcc/bct crystals as observed in zone C in Fig. 1. As the SLM is a layer by layer
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AM process [2], each layer experienced similar thermal circumstances resulting in a heterogeneous microstructure following a repetitive/alternating morphology of bcc and a mixture
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of bcc/fcc/bct crystals (see Fig. 3). As a result, the hardness was inconsistent and was measured
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to be 682 ± 26 HV in zone A and 846 ± 35 HV in zone B from previous studies [10]. Nevertheless, microstructural homogeneity and high hardness could be obtained with appropriate
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hardening and tempering heat treatments shown in another study [22].
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EBSD results indicated that the resultant M2 HSS material was isotropic since the grains were random and not associated with any preferred directions. The bcc lattice constants obtained from the SADPs in zone A were calculated to be larger than that of high purity iron bcc. According to the TEM-EDS results, the elements V, Cr, Mo, W and Si detected in the iron matrix could be responsible for the enlargement of the lattice constant. From the crystal structure study of the M2 HSS part produced by SLM, the SLM process was characterised by rapid solidification where mechanical properties were enhanced. The growth morphology of the iron matrix and the distribution of the dendritic carbide network were significantly influenced by the applied laser radiation. The combination of microstructure and
ACCEPTED MANUSCRIPT mechanical properties could be achieved through this rapid solidification in SLM. The rapid change of thermal energy associated with this process permits large deviations from equilibrium,
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as evidenced by the SLM process characteristics and several key structural features established
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in this work. Firstly, the solid solubility is directly related to the temperature [23]. During the SLM process, the alloying elements do not have time to diffuse due to rapid solidification. Under
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such a condition, the solid solubility is extended which would increase the solid solution
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strengthening of SLM parts [4]. Secondly, the reduction in size and number of phase segregation was observed from the continuous and homogeneous network of carbide dendrites of nanometric
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scale. Size refinement was also seen from the small grains of less than 0.5 µm (see Fig. 11)
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which would improve fracture and impact toughness of SLM parts [4]. 5. Conclusions
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1. The resultant microstructure of M2 HSS parts produced from SLM was characterised by
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three different zones consisting of bcc, bcc/fcc and bcc/fcc/bct phases as seen from FIB
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images. A continuous and homogeneous network of carbide dendrites was observed under SEM within these zones. 2. The phases were revealed to be fcc and bcc from the XRD results. The bcc grains had random orientations and were not associated with any preferred orientations. 3. The bcc phase was single crystals with enlarged lattice constant from TEM-SADP results. Further analysis from TEM-EDS suggested that the detected elements V, Cr, Mo, W and Si within the iron matrix were responsible for the enlargement of the bcc lattice constant. 4. The rapid solidification phenomenon in the SLM process resulted in extension in solid solubility and grain size refinement.
ACCEPTED MANUSCRIPT Acknowledgement The authors would like to thank Professor Jean-Pierre Kruth and his colleagues from Katholieke
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Universiteit Leuven, Belgium for providing the M2 HSS SLM samples for this research.
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ACCEPTED MANUSCRIPT 8. Rombouts M, Kruth JP, Froyen L, Mercelis P. Selective Laser Melting of Iron and Steel powders. Annuals of the CIRP 2006;55:187-92.
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ACCEPTED MANUSCRIPT 19. Thijs L, Kempen K, Kruth JP, Van Humbeeck J. Fine-structured aluminium products with controllable texture by selective laser melting of pre-alloyed AlSi10Mg powder.
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Figure Captions
Fig. 1 – FIB image of the microstructure of M2 HSS SLM sample.
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Fig. 2 – XRD of M2 HSS SLM samples produced from scan speeds: (a) 250 mm/s, (b) 550 mm/s, and (c) 700 mm/s.
Fig. 3 – LOM and EBSD images of the microstructure of M2 HSS SLM samples. Fig. 4 – (a) Indexed area and (b) EBSD orientation map of indexed area. Fig. 5 – (a) Image of unetched sample under LABe mode showing the distinct separated phases where zones A and B correspond to zones marked in Fig. 1, (b) close up image showing network shaped dendrites, (c) etched sample under SEI mode revealing network shaped dendrites in zone
ACCEPTED MANUSCRIPT A, and (d) etched sample under LABe mode revealing a homogeneous and continuous network of dendrites in zones A and B.
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Fig. 6 – SADP (i) and (ii) obtained from zone A. Fig. 7 – SADP (i) obtained from zone B.
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Fig. 9 – EDS locations identified from STEM.
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Fig. 8 – SADP (ii) obtained from zone B.
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Fig. 10 – Microstructural morphology of the cross section in a single SLM track.
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Fig. 11 – FIB image showing small grains in zone A.
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ACCEPTED MANUSCRIPT Table 1 – Summary of the EDS quantitative results from EDX (i), (ii) and (iii).
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EDS (i) EDS (ii) EDS (iii) Element Point Weight % Atomic % Weight % Atomic % Weight % Atomic % 1 0.67 3.46 0.88 4.99 C(K) 2 0.61 3.61 0.08 0.48 3 Carbon content is excluded from quantitative analysis 1 11.71 14.07 4.97 6.60 V(K) 2 14.17 19.55 5.01 6.69 3 1.02 1.20 1.12 1.29 2.83 3.24 1 8.75 10.30 10.12 13.16 Cr(K) 2 10.72 14.49 11.18 14.63 3 3.45 3.96 4.16 4.67 3.72 4.17 1 53.29 58.43 45.98 55.67 Fe(K) 2 25.60 32.22 46.84 57.05 3 81.57 87.26 85.06 88.90 84.40 88.03 1 17.05 10.88 16.52 11.64 Mo(K) 2 24.24 17.75 22.09 15.66 3 5.78 3.60 0.80 0.48 1 8.50 2.83 21.49 7.90 W(L) 2 23.26 8.89 14.77 5.46 3 7.43 2.41 8.46 2.68 7.40 2.34 1 Si(K) 2 1.38 3.45 3 0.72 1.54 1.17 2.43 0.82 1.72
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Microstructure of SLM m2 high speed steel was investigated through TEM A continuous and homogeneous network of carbide dendrites were observed Two distinct phases of bcc single crystals and fcc/bcc polycrystals were observed The iron bcc lattice constant calculated from TEM-SADP was larger Alloying elements V, Cr, Mo, W and Si were detected in the iron matrix by TEM-EDS
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