Microstructure and mechanical properties of selective laser melted

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  • Applied Surface Science 257 (2011) 74477454

    Contents lists available at ScienceDirect

    Applied Surface Science

    journa l homepage: www.e lsev ier .com

    Micros ele

    C.C. Nga,a Department o gb Department o

    a r t i c l

    Article history:Received 12 NReceived in reAccepted 1 MaAvailable onlin

    Keywords:MagnesiumSelective laserMicrostructurMechanical prHardness

    tersigateent oty inccrea

    respeng elamisinn bon

    1. Introdu

    Duringimplant materials for use in the orthopedic eld has increasedsignicantly [13]. Metal materials are of considerable impor-tance among diverse material types for load-bearing orthopedicapplications [4,5]. Metallic biomaterials such as titanium, stainlesssteel, chromiumcobalt and its alloy are widely used due to theirsuperior meand chemicexpandingof mechanibones, espeloosening frelease of to

    One menew kind oate mechanan excellenmagnesiummaterial w[9,10] and fabone comp[11]. Therefuse of magsitu also im

    CorresponE-mail add

    duegnessubs

    of 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 have

    0169-4332/$ doi:10.1016/j.chanical properties, biocompatibility, biodegradabilityal stability [6]. However, the major disadvantages intheir use in the orthopedic sectors are the mismatchcal properties between these biomaterials and naturalcially in the elastic modulus, which results in implantsollowing stress shielding of the bone [7] and also thexic species which may cause allergy and cancer [8].thod to alleviate these problems is to investigate af metal-based biocompatible material with appropri-ical properties close to that of human bone and witht biocompatibility as well as low toxicity. Recently,has been suggested as a very promising metallic bio-

    hich offers both inherent biocompatible performancevorablemechanical properties in relation to thenaturalared with any other widely used metallic biomaterialsore, stress shielding effects can be minimized by thenesium implant. In addition, its ability to degrade inplies that the need for secondary surgery whole elim-

    ding author. Tel.: +852 3400 3190; fax: +852 2362 5267.ress: mmfsmm@inet.polyu.edu.hk (M.M. Savalani).

    a direct inuence 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 solidication processing could beobtained by varying the process parameters and is related to therened microstructures in the molten zone. Zheng et al. [16] illus-trated that laser deposited material experiences a signicant rapidquenching effect andaveryhigh cooling rate canbeattained,wherethemicrostructural 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 ametal powder injected into amolten pool createdby a focused, high-powered laser beam. High cooling rates wereproduced 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

    see front matter 2011 Elsevier B.V. All rights reserved.apsusc.2011.03.004tructure and mechanical properties of s

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

    f Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hong Konf Mechanical Engineering, The Hong Kong Polytechnic University, Hong Kong

    e i n f o

    ovember 2010vised form 27 February 2011rch 2011e 8 March 2011

    meltingeoperties

    a b s t r a c t

    The effects of laser processing paramelaser-melted magnesium were investthe laser-melted samples are dependzone coarsen as the laser energy densizone decreases signicantly with an inrelatively high laser energy density irfrom0.59 to 0.95GPa and correspondilaser-meltedmagnesiumparts are proare more closely matched with huma

    ction

    the last several decades, development of medical

    inatedthe marently,/ locate /apsusc

    ctive laser melted magnesium

    on the microstructure and mechanical properties of selectived. The results show that the microstructure characteristics ofn the grain size of SLM magnesium. The grains in the moltenreases. In addition, the average hardness values of themoltense of the laser energy densities and then decreased slowly at active of mode of irradiation. The hardness value was obtainedsticmodulus ranging from27 to 33GPa. The present selectiveg for biomedical applications since themechanical propertiese than other metallic biomaterials.

    2011 Elsevier B.V. All rights reserved.

    to gradually dissolvable, resorption and excretion ofium implant in human body environment [12]. Cur-tantial efforts have been put to investigate the potential

  • 7448 C.C. Ng et al. / Applied Surface Science 257 (2011) 74477454

    extremely high cooling rates (the order of 108 K/s) were obtainedin the laser deposition of Al +Al2O3 onmagnesiumalloys processedusing a pulsed Nd:YAG laser. In the presence of a high coolingrate, grain structures do not grow signicantly during solidication[19]. Effectsteristics ofstudied by Kgrain renein surface htures was foYu et al. [21of the weldand showeing heat inpwelding ofand presenequixed inrate and thmicrohardn

    The nanbiomedicalof human bals [23,24].progressivewhen the itest. The hamaterial cacharacteristtion methoon an extrenique has bof the laservious studymagnesiumparametersthe -Mg) iical propert

    2. Method

    2.1. Process

    SLMofpics Markerlong wereparametersproperties.been descri

    2.2. Micros

    To reveamagnesiumwere grouning paper fto obtain gowas perpenpolished byfree surface(6 g picric aStandard E4were perforing techniqacceleratingmanually a

    size of each sample. 30 interceptswere for eachmeasurementweretaken by linear line intercept method, based on ASTM StandardE112-96 [28].

    denta

    oindon, Ielimietersloadloadloadximluesongiata ptopmodispla

    ults

    icros

    gitudre maretionain sd. Thelted thhightionus sttructed pid so

    The twaveer ctrac

    ely sed inion otrac

    roces,30].e retionallisfterzonrainike thund

    Mg ppulstionhe ins torevais thuof process parameters on the microstructural charac-laser surface melted AISI 410 stainless steel have beenrishna and Bandyopadhyay [20] and it was found thatment after surface laser melting lead to an increaseardness. The fusion zone with the rened microstruc-und to be down to a maximum depth of 13510m.] demonstrated the correlation between yield strengthjoint and heat input of ber laser welding of Mg alloyd that yield strength increases with decreasing weld-ut due to ner microstructure obtained. Electron beamMgAl based alloy has been conducted by Su et al. [22]ted that the grains inside the fusion zone were nearlysharp with about 10m in size, due to rapid coolinge renement of grains responsible for the increase iness in the fusion zone.oindentation technique has recently been used in theengineering sector to measure mechanical propertiesones as well as metallic implants and other biomateri-In a nanoindentation instrument, hardness is recordedly by measuring the loads and displacements prolendenter penetrates the surface of the material underrdness values and elastic modulus information of then be determined by analyzing the loaddisplacementics [25]. One of the main advantages of nanoindenta-d is the capability of measuring a materials propertiesmely small scale. At present, the nanoindentation tech-een introduced to measure the mechanical properties-melted magnesium tracks. To our knowledge, no pre-is concerned with the mechanical properties of SLM. Therefore, in this paper, the effects of laser processon the microstructure (grain size and morphology ofn the molten zone of SLMmagnesium and the mechan-ies (hardness and elastic modulus) were investigated.

    s

    ing

    uremagnesiumpowderwasconductedonaGSI Lumon-SPe Nd:YAG (=1.06m) laser. Single tracks of 20mmfabricated to investigate the effects of laser processon the microstructure characteristics and mechanicalThe detailed experimental setup and procedures havebed in the authors previous study [26].

    tructural analysis

    l the microstrucutral characteristic of the laser-melted, the longitudinal cross-sectional surface of the samplesded and polished under water on Silicon carbide grind-rom lower grade 60 up to higher grade 2400. In orderod grinding surface, the abrasion direction of specimendicular to the pre-existing scratches. The specimenwasgamma diamond paste up to grade 1 um until scratch-was obtained and etched with picric acid solution

    cid +100ml ethanol +1ml H3PO4) according to ASTM07-07 [27]. Metallurgical observation of each samplemed by means of a secondary scanning electron imag-ue (JEOL scanning electron microscope), operated at anvoltage of 20kV. The grain boundaries were marked

    nd computed statistically to obtain the average grain

    2.3. In

    Nan(Hysitrthe prparamtationand unimumthe maThe vaof the lof 60 dsurfaceelasticload-d

    3. Res

    3.1. M

    Loncast punesiumconvenage grmetholaser-minferrewith aconvenpreviomicrossolidithe rap

    3.1.1.pulsed

    Undme