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Applied Surface Science 257 (2011) 7447–7454

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Applied Surface Science

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icrostructure and mechanical properties of selective laser melted magnesium

.C. Nga, M.M. Savalania,∗, M.L. Laub, H.C. Mana

Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hong KongDepartment of Mechanical Engineering, The Hong Kong Polytechnic University, Hong Kong

r t i c l e i n f o

rticle history:eceived 12 November 2010eceived in revised form 27 February 2011ccepted 1 March 2011

a b s t r a c t

The effects of laser processing parameters on the microstructure and mechanical properties of selectivelaser-melted magnesium were investigated. The results show that the microstructure characteristics ofthe laser-melted samples are dependent on the grain size of SLM magnesium. The grains in the moltenzone coarsen as the laser energy density increases. In addition, the average hardness values of the molten

vailable online 8 March 2011

eywords:agnesium

elective laser meltingicrostructure

zone decreases significantly with an increase of the laser energy densities and then decreased slowly at arelatively high laser energy density irrespective of mode of irradiation. The hardness value was obtainedfrom 0.59 to 0.95 GPa and corresponding elastic modulus ranging from 27 to 33 GPa. The present selectivelaser-melted magnesium parts are promising for biomedical applications since the mechanical propertiesare more closely matched with human bone than other metallic biomaterials.

echanical propertiesardness

. Introduction

During the last several decades, development of medicalmplant materials for use in the orthopedic field has increasedignificantly [1–3]. Metal materials are of considerable impor-ance among diverse material types for load-bearing orthopedicpplications [4,5]. Metallic biomaterials such as titanium, stainlessteel, chromium–cobalt and its alloy are widely used due to theiruperior mechanical properties, biocompatibility, biodegradabilitynd chemical stability [6]. However, the major disadvantages inxpanding their use in the orthopedic sectors are the mismatchf mechanical properties between these biomaterials and naturalones, especially in the elastic modulus, which results in implants

oosening following stress shielding of the bone [7] and also theelease of toxic species which may cause allergy and cancer [8].

One method to alleviate these problems is to investigate aew kind of metal-based biocompatible material with appropri-te mechanical properties close to that of human bone and withn excellent biocompatibility as well as low toxicity. Recently,agnesium has been suggested as a very promising metallic bio-aterial which offers both inherent biocompatible performance

9,10] and favorable mechanical properties in relation to the natural

one compared with any other widely used metallic biomaterials11]. Therefore, “stress shielding” effects can be minimized by these of magnesium implant. In addition, its ability to degrade initu also implies that the need for secondary surgery whole elim-

∗ Corresponding author. Tel.: +852 3400 3190; fax: +852 2362 5267.E-mail address: mmfsmm@inet.polyu.edu.hk (M.M. Savalani).

169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2011.03.004

© 2011 Elsevier B.V. All rights reserved.

inated due to gradually dissolvable, resorption and excretion ofthe magnesium implant in human body environment [12]. Cur-rently, substantial efforts have been put to investigate the potentialof magnesium to replace presently used metallic biomaterials fororthopedic applications [13].

Selective laser melting (SLM) is a new approach of manufactur-ing parts directly from metal powders [14]. Among those processparameters, laser power and laser scan speed were found to havea direct influence on the microstructures and mechanical proper-ties of the laser-melted parts [15]. The relationship between theparameters can be combined into a term called linear energy den-sity. The linear energy density governs the amount of laser powerincident to the powder bed per unit area per unit scan speed. Manyresearchers showed that the improvement of the surface mechan-ical properties by laser rapid solidification processing could beobtained by varying the process parameters and is related to therefined microstructures in the molten zone. Zheng et al. [16] illus-trated that laser deposited material experiences a significant rapidquenching effect and a very high cooling rate can be attained, wherethe microstructural evolution is related to process parameters dur-ing the laser engineered net shaping (LENS®) process, which isa laser-powder additive manufacturing technology for fabricat-ing metal parts directly from a computer-aided design (CAD) solidmodel by using a metal powder injected into a molten pool createdby a focused, high-powered laser beam. High cooling rates were

produced by high power density welding, resulting in microstruc-tures that are far from equilibrium and the cooling rates at thesolid-liquid interface were found to be from 104 to 105 K/s [17].Laser surface alloying under pulsed-wave irradiation resulted ineven higher cooling rates. Samant and Dahotre [18] illustrated that

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xtremely high cooling rates (the order of 108 K/s) were obtainedn the laser deposition of Al + Al2O3 on magnesium alloys processedsing a pulsed Nd:YAG laser. In the presence of a high coolingate, grain structures do not grow significantly during solidification19]. Effects of process parameters on the microstructural charac-eristics of laser surface melted AISI 410 stainless steel have beentudied by Krishna and Bandyopadhyay [20] and it was found thatrain refinement after surface laser melting lead to an increasen surface hardness. The fusion zone with the refined microstruc-ures was found to be down to a maximum depth of 135 ± 10 �m.u et al. [21] demonstrated the correlation between yield strengthf the weld joint and heat input of fiber laser welding of Mg alloynd showed that yield strength increases with decreasing weld-ng heat input due to finer microstructure obtained. Electron beam

elding of Mg–Al based alloy has been conducted by Su et al. [22]nd presented that the grains inside the fusion zone were nearlyquixed in sharp with about 10 �m in size, due to rapid coolingate and the refinement of grains responsible for the increase inicrohardness in the fusion zone.The nanoindentation technique has recently been used in the

iomedical engineering sector to measure mechanical propertiesf human bones as well as metallic implants and other biomateri-ls [23,24]. In a nanoindentation instrument, hardness is recordedrogressively by measuring the loads and displacements profilehen the indenter penetrates the surface of the material under

est. The hardness values and elastic modulus information of theaterial can be determined by analyzing the load–displacement

haracteristics [25]. One of the main advantages of nanoindenta-ion method is the capability of measuring a material’s propertiesn an extremely small scale. At present, the nanoindentation tech-ique has been introduced to measure the mechanical propertiesf the laser-melted magnesium tracks. To our knowledge, no pre-ious study is concerned with the mechanical properties of SLMagnesium. Therefore, in this paper, the effects of laser process

arameters on the microstructure (grain size and morphology ofhe ˛-Mg) in the molten zone of SLM magnesium and the mechan-cal properties (hardness and elastic modulus) were investigated.

. Methods

.1. Processing

SLM of pure magnesium powder was conducted on a GSI Lumon-cs Marker SPe Nd:YAG (� = 1.06 �m) laser. Single tracks of 20 mmong were fabricated to investigate the effects of laser processarameters on the microstructure characteristics and mechanicalroperties. The detailed experimental setup and procedures haveeen described in the author’s previous study [26].

.2. Microstructural analysis

To reveal the microstrucutral characteristic of the laser-meltedagnesium, the longitudinal cross-sectional surface of the samplesere grounded and polished under water on Silicon carbide grind-

ng paper from lower grade 60 up to higher grade 2400. In ordero obtain good grinding surface, the abrasion direction of specimenas perpendicular to the pre-existing scratches. The specimen wasolished by gamma diamond paste up to grade 1 um until scratch-ree surface was obtained and etched with picric acid solution6 g picric acid + 100 ml ethanol + 1 ml H3PO4) according to ASTM

tandard E407-07 [27]. Metallurgical observation of each sampleere performed by means of a secondary scanning electron imag-

ng technique (JEOL scanning electron microscope), operated at anccelerating voltage of 20 kV. The grain boundaries were markedanually and computed statistically to obtain the average grain

nce 257 (2011) 7447–7454

size of each sample. 30 intercepts were for each measurement weretaken by linear line intercept method, based on ASTM StandardE112-96 [28].

2.3. Indentation testing

Nanoindentation tests were carried out using a Triboindenter(Hysitron, Inc.) equipped with a diamond Berkovich probe. Afterthe preliminary trails to obtain the best loading and unloadingparameters, the measurement sequence is as follows. The inden-tation load was fixed at 8 mN at room temperature and the loadand unload rates were kept at a constant of 0.8 mN. After the max-imum load was held for 5 s, the samples were unloaded to 10% ofthe maximum load and kept for 15 s for thermal drift correction.The values of the nanomechanical properties of the molten zoneof the longitudinal cross-section were made from an average valueof 60 data points. The optical microscope was used to examine thesurface topographies prior to and after indentation. The reducedelastic modulus (Er) and hardness (nHV) were computed from theload-displacement curve [23].

3. Results and discussion

3.1. Microstructure

Longitudinal cross-section microstructures of the conventionalcast pure magnesium ingot and the selective laser-melted mag-nesium are presented in Fig. 1. A typical optical micrograph of theconventional casting has an equiaxed grain structure with the aver-age grain size of about 7 �m determined by linear line interceptmethod. The grain size is relatively large as compared with thelaser-melted specimen due to the slower cooling rates. It can beinferred that the microstructure of the laser-melted magnesiumwith a higher solidification rate is much finer (2–5 �m) than theconventional cast magnesium (Fig. 1b). The result is similar to theprevious study by Teng et al. which showed that a highly refinedmicrostructure of magnesium alloy was obtained under rapidlysolidified process compared to the cast magnesium alloy due tothe rapid solidification during processing [29].

3.1.1. The typical microstructure of the continuous wave andpulsed wave laser melted magnesium

Under continuous wave irradiation, the center of the laser-melted track had a fine, uniform and single phase structure withrelatively small grains as shown in Fig. 2a. The ˛-Mg single phasesolidified in form of equiaxed crystals. The laser melting leads to theformation of fully recrystallised grains in melted zones of the laser-melted track. This microstructure has also been observed by otherlaser processing techniques of magnesium alloy such as laser weld-ing [17,30]. Since the peak temperature of the melted zone is higherthan the recrystallisation temperature and a longer time of laserirradiation in CW mode as compared with pulse mode, completerecrystallisation and complete grain growth occurred in the meltedzone after laser melting [31]. This is a possible reason why themelted zone consists of fine homogeneous and full recrystallised˛-Mg grains.

Unlike the continuous wave irradiation, laser-melted tracks fab-ricated under pulsed irradiation consist of incomplete growth ofthe ˛-Mg phase (Fig. 2b). Since the solidification rate achievedunder pulsed laser irradiation is higher than continuous wave

irradiation [32], this inhibits the full growth of ˛-Mg grains andsince the interaction time is short, it has insufficient time for thecrystals to arrange themselves such that thermodynamic equilib-rium prevails at the solid/liquid interface, the average size of thegrains is thus smaller than those grains obtained in the continuous

C.C. Ng et al. / Applied Surface Science 257 (2011) 7447–7454 7449

Fig. 1. Optical micrographs of (a) cast pure Mg ingot and (b) laser-melted Mg at longitinudual cross-section.

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ig. 2. Typical microstrucutral images of laser-melted magnesium under, (a) contin

ode laser melted tracks, as determined by the linear interceptethod.

.1.2. Effects of the laser processing parameters on theicrostructures of CW samples

Under continuous wave irradiation, in order to discern grainize of the laser melted tracks processed under different lasernergy densities, scanning electron microscopy (SEM) was pre-ormed for further observation. Acceptable melting of magnesiumakes place between the energy densities values of 1.27 × 109 J/m2

nd 7.84 × 109 J/m2. When the energy density values are below.27 × 109 J/m2, the single line specimens could not be meltedr were too fragile to handle [25]. This may possibly be becausehe melting temperature of magnesium (650 ◦C) has not beeneached. Higher energy values were not attainable due to the lim-tation of the laser. Thus, SEM images of the melted zones of the.27 × 109 J/m2, 2.11 × 109 J/m2, 3.92 × 109 J/m2, 6.33 × 109 J/m2

nd 7.84 × 109 J/m2 samples are shown in Fig. 3. The laser-meltedagnesium is free of pores or cracks. However, it can be seen

hat the laser-melted magnesium has some white phases alonghe grain boundaries of ˛-Mg grains. The grain boundary precip-tates referred to magnesium oxide according to EDX analysis, andt is associated with oxygen pick-up during SLM process. The rapidolidification rates achieved after laser melting inhibit the parti-ioning of the oxide elements between the ˛-Mg grains due tooo little time for the MgO atoms to arrange themselves such thathermodynamic equilibrium prevails at the grain boundary [33]. As

xpected, various laser parameters resulted in different solidifica-ion rates of the molten pool and thermal cycles, leading to variationn microstructures of the melted zone. The SEM images confirm thatll the melted zones consist of only equiaxed ˛-Mg single phaseith a high density of intergrainular boundaries. The average grain

wave irradiation at 1.27 × 109 J/m2, (b) pulsed wave irradiation at 1.13 × 1012 J/m2.

sizes are about 2.30 �m, 2.82 �m, 3.65 �m 4.10 �m and 4.87 �m,respectively. Evidently, the average grain size of the ˛-Mg grainsis larger for higher laser energy density values, the increase in theaverage grain size with increasing laser energy density is expected,since the temperature during laser melting generally increases withthe increasing laser energy density. Therefore, the melted zoneshows coarser grain at a lower cooling rate. The lower the coolingrate during solidification, the longer the time available for graincoarsening [34]. Thus, increasing laser energy density would resultin grain coarsening. This trend can explain the relationship betweengrain size and laser energy density in the melted zone of the presentsamples, as illustrated in Fig. 4a. Similarly, as shown in Figs. 4b and5, the increase of the laser power or decrease of the laser scan speedlead to the coarsening of the grains in the melted zone because thehigher the laser power or slower the laser scan speed providedmore driving force for grain boundary movement which then pro-moted the growth of grains. Previous studies on laser welding ofmagnesium alloys also indicated that grain coarsening occurred byincreasing the heat input and improved heat conduction [35].

3.1.3. Effects of the laser processing parameters on themicrostructures of pulsed samples

Under pulsed wave irradiation, the energy density providedby pulsed wave irradiation for melting track fabrication is muchhigher, ranging between 1.13 × 1012 J/m2 and 9.80 × 1012 J/m2 dueto the higher peak power under pulse wave irradiation. Despiteenergy densities as high as 1 × 1013 J/m2 insufficient pulse dura-

tion time (20 ns) may have not brought the molten material tothe evaporation temperature (1050 ◦C) since no vaporization wasobserved during processing. Energy densities below 1 × 1012 J/m2

have resulted in the lack of melting of powders. SEM micrographs(Fig. 7) of the equiaxed structures of the melted zone show a very

7450 C.C. Ng et al. / Applied Surface Science 257 (2011) 7447–7454

Fig. 3. Microstructural images of the continuous wave laser-melted zone with different laser energy densities, (a) 1.27 × 109 J/m2, (b) 2.11 × 109 J/m2, (c) 3.92 × 109 J/m2, (d)6.33 × 109 J/m2 and (e) 7.84 × 109 J/m2.

Table 1The grain size, hardness and elastic modulus with various energy densities under continuous wave irradiation.

Energy density (J/m2) 1.27 × 109 2.11 × 109 3.92 × 109 6.33 × 109 7.84 × 109

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Grain size (�m) 2.30 ± 0.3 2.82 ± 0.Hardness (GPa) 0.87 ± 0.13 0.75 ± 0.Young modulus (GPa) 32.76 ± 1.07 27.99 ± 4.

ne grain size (2.3–5 �m). Like the continuous wave irradiation,hite precipitates are present along the grain boundaries of the ˛-g grains. The grains of the melted zone in all different energy

ensities are uniformly distributed as shown in Fig. 6(a–e), andhe effect of laser energy density on the average grain size of the

elted zone is given in Fig. 7a. It is shown that the average grain sizencreases from approximately 2.3 �m to 5.0 �m with the increasen laser energy density from 1.13 × 1012 J/m2 to 9.80 × 1012 J/m2.he results above indicated that different degrees of grain refine-ent can be obtained under different laser energy densities. Thisas attributed to the difference in solidification and cooling rates

s observed in the continuous wave irradiation. Finer grains cane obtained by fast cooling whereas slow cooling leads to the for-ation of coarse grains. Likewise, the results for the quantitative

nalysis of the effect of the laser power and the laser scan speedn the average grain size of ˛-Mg are shown in Figs. 7b and 8,espectively. When the laser power was low or the laser scan speedas high, the average gain size became small since decreasing the

aser power in effect acts in the same way as reducing the preheatemperature of the powder when the next pulse hits the base. Theelatively high cooling rate at low laser power or high laser scanpeed restrained the growth of ˛-Mg grain during solidification. Sut al. showed that high peak power density would lead in increasing

3.65 ± 0.5 4.10 ± 0.3 4.87 ± 0.70.67 ± 0.05 0.62 ± 0.09 0.59 ± 0.05

26.22 ± 3.67 24.09 ± 2.46 20.84 ± 4.06

the absorption of energy in the material and a reduction in coolingrate at the center of weld pool during laser welding [22]. On thecontrary, Mg grains in the melted zone coarsened with either anincrease in laser power or a reduction in scan speed.

3.2. Hardness

3.2.1. Effects of the laser processing parameters on themechanical properties of continuous wave samples

Nanoindentation was performed on the longitudinal cross-section of the laser-melted tracks. Fig. 9 shows the hardness profilesagainst different laser energy densities ranged from 0.59 GPa to0.87 GPa. As the laser energy density increased, the melted zonehardness value decreased. Table 1 summarizes the results of thegrain size, hardness and elastic modulus with samples fabricated atvarious energy densities under continuous wave irradiation. Hard-ness values of the melted zone were 0.87 GPa, 0.75 GPa, 0.67 GPa,

0.63 GPa, 0.62 GPa and 0.59 GPa for laser energy density valuesof 1.27 × 109 J/m2, 2.11 × 109 J/m2, 3.92 × 109 J/m2, 6.33 × 109 J/m2

and 7.84 × 109 J/m2 samples, respectively. The higher hardness val-ues fabricated with low laser energy densities are attributed to laserrapid solidification and subsequent grain refinement [36].

C.C. Ng et al. / Applied Surface Science 257 (2011) 7447–7454 7451

Table 2The grain size, hardness and elastic modulus with various energy densities under pulsed wave irradiation.

Energy Density (J/m2) 1.13 × 1012 1.63 × 1012 2.64 × 1012 3.27 × 1012 9.80 × 1012

Grain size (�m) 2.33 ± 0.09 2.50 ± 0.47 3.3 ± 0.35 3.82 ± 0.20 4.86 ± 0.31Hardness (GPa) 0.95 ± 0.08 0.85 ± 0.02 0.80 ± 0.02 0.69 ± 0.02 0.67 ± 0.05Young modulus (GPa) 38.18 ± 4.04 38.10 ± 1.66 37.20 ± 0.31 36.25 ± 2.28 27.41 ± 1.6

Table 3Summary of the mechanical properties of various metallic implant materials in comparison to natural bone edited from Ref. [10].

Properties Natural bone SLM magnesium Ti alloy Stainless steel

Hardness (GPa) 0.39–0.77 0.59–0.95 1.42–2.67 1.37–1.77Elastic modulus (GPa) 3–20 27–33 110–117 189–205

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ig. 4. (a) The effect of laser energy density on the average grain size of ˛-Mg underontinuous wave irradiation. (b) The effect of laser power on the average grain sizef ˛-Mg under continuous wave irradiation.

.2.2. Effects of the laser processing parameters on theechanical properties of pulsed wave samples

The effect of laser energy density on the hardness of the meltedone is shown in Fig. 10. It can be seen that the lower the lasernergy density is, the higher is the hardness of the melted zone.he increase of the hardness values can be attributed to the grainefinement effect [37], as observed in continuous wave mode. Dur-ng laser melting process, grain coarsening occurred in melted zone

ith an increase of laser energy density. Hence, a higher hardnessalue increases with decreasing in laser energy density. Moreover,t can be seen that with a further increase of laser energy density up

o 2.11 × 109 J/m2, the hardness value changed slightly, the effect ofrain coarsening on the decrease of the hardness is partially offset-ed. Table 2 summarized the results of the grain size, hardness andlastic modulus with samples fabricated at various energy densitiesnder pulsed wave irradiation.

Scan Speed (mm/s)

Fig. 5. The effect of laser scan speed on the average grain size of ˛-Mg under con-tinuous wave irradiation.

3.2.3. Effects of the grain size on the hardness of both continuousand pulsed wave samples

The melted zones had homogeneous equiaxed grain structures,and the hardness values increased as the laser energy densitydecreased. The higher cooling rate induced by low laser energydensity resulted in smaller grain sizes, so that the hardness val-ues were mainly related to the grain size in the melted zone. Fig. 11shows the relationship between the hardness (GPa) and the recip-rocal of the square root of grain size (d) for the melted zone. Thehardness value (GPa) is found to be proportional to the recipro-cal of the square root of grain size (d). According to the Hall–Petchequation for the melted zone of the present sample is expressed as[38]

H = H0 + kd−1/2 (2)

where H is the hardness of material, H0 and k are appropriate con-stants associated with the hardness measurements. The regressionequations for continuous-wave irradiation and pulsed-wave irra-diation are found to be y (GPa) = 0.069x (�m−1/2) + 0.493 (GPa) andy (GPa) = 0.068x (�m−1/2) + 0.644 (GPa) respectively (as shown inFig. 11). The smaller the grain size is, the higher is the hardnessvalue. The results confirm that the Hall–Petch relationship per-sists down to at least the finest grain size examined experimentally(2 �m). Hence, the change of the hardness value is inversely pro-portional to the square root of the grain size. This illustrated thatgrain coarsening in the melted zone due to the effect of the thermalcycling resulted in the reduction of the hardness of the laser-melted magnesium, which is consistent with the microstrucutres

displayed above. It can be concluded that smaller ˛-Mg size crystalsresulted in higher hardness values. In the Hall–Petch relationshiponly high angle grain boundaries are considered as obstacles to thedislocation movement. The finer grain size can facilitate the grainboundary sliding process [25].

7452 C.C. Ng et al. / Applied Surface Science 257 (2011) 7447–7454

Fig. 6. SEM images of the microstructure of the pulsed wave laser-melted zone with different laser energy densities, (a) 1.13 × 1012 J/m2, (b) 1.63 × 1012 J/m2, (c)2.64 × 1012 J/m2, (d) 3.27 × 1012 J/m2 and (e) 9.80 × 1012 J/m2.

0 2x1012

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Fig. 7. (a) The effect of laser energy density on the average grain size of ˛-Mg underpulsed wave irradiation. (b) The effect of laser power on the average grain size of˛-Mg under pulsed wave irradiation.

7060504030201000

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Fig. 8. The effect of scan speed on the average grain size of ˛-Mg under pulsed waveirradiation.

0 2x109 4x109 6x109 8x109 1x1010

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ness (

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Fig. 9. The effect of laser energy density on the hardness of laser-melted magnesiumunder continuous wave irradiation.

C.C. Ng et al. / Applied Surface Scie

0 2x1012 4x1012 6x1012 8x1012 1x1013

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ard

ness (

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Linear Enery Density (J/m3)

Fig. 10. The effect of laser energy density on the hardness of laser-melted magne-sium under pulsed wave irradiation.

0.70.60.50.4

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R2 = 0.928

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ig. 11. The relationship between hardness and grain size under (a) continuous-ave irradiation and (b) pulsed-wave irradiation.

Table 3 depicts the range of elastic modulus and hardness valueseasured on the laser-melted magnesium using the nanoindenta-

ion technique, and compared with the mechanical properties ofome commonly used metallic biomaterials [39]. It can be seen thathe obtainable hardness values of laser-melted magnesium rangedetween 0.59 and 0.95 GPa and the elastic modulus varied between7 and 33 GPa, which are within the range of conventional cast

agnesium (∼28 GPa). It is worth noting that the higher the value

f elastic modulus of the laser-melted magnesium, the stronger theismatch between the implant and living bone tissue, because of

he small value of the elastic modulus of the bone tissues (3–20 GPa)

[

nce 257 (2011) 7447–7454 7453

[10]. Compared with the conventional metallic biomaterials such astitanium alloy and stainless steel, the mechanical properties of thelaser-melted magnesium show the closest to the natural bone. Theremarkable reduction in hardness and elastic modulus of the laser-melted magnesium has a potential contribution towards using as abone fixation implant (Table 3).

4. Conclusions

Microstructural characteristics and mechanical properties ofSLM magnesium were investigated. The microstructure of moltenzone consists of fine equiaxed grains of the ˛-Mg phase after SLMprocess. The microstructure evolution was explained in terms ofhigh cooling rate and rapid solidification during SLM. An increaseof laser energy density resulted in a decrease of cooling rate andthe grain coarsening of ˛-Mg in the melted zone. The hardnessvalue in the melted zone increased with decreasing grain size,which is in agreement with the Hall–Petch equation. An increase oflaser energy density, the hardness value of melted zone decreasedsharply at first and then decreased slightly. The hardness values andelastic modulus of the laser melted magnesium were in the range of0.59 and 0.95 GPa and 27 and 33 GPa, respectively, which is compa-rable with the cast magnesium ingot and are closely matched withthe mechanical properties of human bone than other conventionalmetallic biomaterials.

Acknowledgement

The authors would like to express their gratitude for financialsupport from the Hong Kong Polytechnic University, under theproject code A-PK57 and the Department of Industrial SystemsEngineering, The Hong Kong Polytechnic University.

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