Advanced lightweight 316L stainless steel cellular lattice structures fabricated via lective lar
melting
Chunze Yan a ,b ,Liang Hao a ,⇑,Ahmed Husin a ,Philippe Young a ,David Raymont c
a
College of Engineering,Mathematics and Physical Sciences,University of Exeter,Exeter EX44QF,Devon,United Kingdom
梦到自杀b
State Key Laboratory of Materials Processing and Die &Mould Technology,Huazhong University of Science and Technology,Wuhan 430074,Hubei,China c
Simpleware Ltd.,Exeter EX43PL,Devon,United Kingdom
a r t i c l e i n f o Article history:
Received 22May 2013Accepted 10October 2013
Available online 22October 2013Keywords:
Additive manufacture Selective lar melting Cellular lattice structures
a b s t r a c t
This paper investigates the manufacturability and performance of advanced and lightweight stainless
steel cellular lattice structures fabricated via lective lar melting (SLM).A unique cell type called gyroid is designed to construct periodic lattice structures and utili its curved cell surface as a lf-sup-ported feature which avoids the building of support structures and reduces material waste and produc-tion time.The gyroid cellular lattice structures with a wide range of volume fraction were made at different orientations,showing it can reduce the constraints in design for the SLM and provide flexibility in lecting optimal manufacturing parameters.The lattice structures with different volume fraction were well manufactured by the SLM process to exhibit a good geometric agreement with the original CAD models.The strut of the SLM-manufactured lattice structures reprents a rough surface and its size is slightly higher than the designed value.When the lattice structure was positioned with half of its struts at an angle of 0°with respect to the building plane,which is considered as the worst building orientation for SLM,it was manufactured with well-defined struts and no defects or broken cells.The compression strength and modulus of the lattice structures increa with the increa in the volume fraction,and two equations bad on Gibson–Ashby model have been established to predict their compression properties.
Ó2013Elvier Ltd.All rights rerved.
1.Introduction
晚上吃啥
Metal cellular structures are a unique classification of materials,which can exhibits a combination of high performance features such as high strength accompanied by a relatively low mass,good energy absorption characteristics and good thermal and acoustic insulation properties [1,2].Metal cellular structures are classified into two common types:stochastic porous structures and periodic cellular lattice structures.Metal stochastic porous structures typi-cally have a random distribution of open or clod voids,whereas metal periodic cellular lattice structures have uniform structures that are generated by repeating a unit cell.Periodic lattice struc-tures exhibit property profiles greatly superior to tho demon-strated by the stochastic analogues at the same volume fraction (or weight)[2,3].Therefore,metal periodic cellular lattice struc-tures can be ud to develop light-weight structures that can pro-vide advanced or multifunctional performance for high value engineering products such as automobile,aerospace and medical products [4].The periodic lattice structures,however,currently face a higher manufacturing complexity and costs than the stochastic structures [2].It can be time and cost consuming to u conventional methods (i.e.investment casting,deformation forming,metal wire approaches,brazing,etc.)to make periodic cellular lattice structures.The structures made by conventional methods posss relatively simple geometries and limited design freedoms,and conquently lack advanced functionality to meet more advanced requirements and applications.
Selective lar melting (SLM)is an additive manufacturing (AM)process,which can directly make complex three-dimensional me-tal parts according to a CAD data by lectively melting successive layers of metal powders [5,6].Manufacture of fully den metal parts (even over 99.9%)without the need of post-process such as infiltration,sintering or HIPing,and a high individuality and de-gree of geometric freedom are considered to be its major advanta-ges [7].SLM has the capability of producing structures of complex freeform geometry.It has been demonstrated to manufacture cel-lular lattice structures with fine features,showing a great potential to make advanced lightweight structures and products which are highly desired by engineering ctors such as aerospace,automo-tive and medical industries [8,9].In addition,the u of low volume lattice structures can prevent the limitations of the SLM process being an expensive manufacturing method due to the expensive powdered metal materials requiring fine particle sizes and proper
0261-3069/$-e front matter Ó2013Elvier Ltd.All rights rerved.dx.doi/10.1016/j.matdes.2013.10.027
⇑Corresponding author.Tel.:+44(0)1392723665.
E-mail address:c.yan@exeter.ac.uk (C.Yan),l.hao@exeter.ac.uk (L.Hao).
morphology,and the immen consumption of building time since only considerably small quantities of material can be procesd per time[10].
However,SLM requires support structure to build overhang c-tion if its angel from the horizontal is less than certain degree.This introduces design and manufacturing complications for the SLM of lightweight cellular structures and engineering components.A few previous works have investigated the fabrication of cellular lattice structures using the SLM process.Brooks et al.[11]examined the SLM production of the regular316L stainless steel lattice structures containing any combination of three element types including pil-lar,diagonal and octahedral elements.They tested the minimum angels of elements from the horizontal that could be manufac-tured,and found that the elements with angles lower than30°were problematic to fabricate.Santorinaios et al.[12]studied the manufacturability of open cellular lattice structures with the cell sizes of1.25,2.5and5mm.The structures only consists of vertical and diagonal cross struts,as currently,the SLM process cannot build horizontal struts.McKown et al.[13]made a range of metal-lic lattice structures bad on two kinds of unit cells posssing octahedral and pillar-octahedral topologies by the SLM process, and studied the compression and blast loading behavior of the lat-tice structures.Mullen et al.[8]fabricated periodic cellular tita-nium structures by SLM,which were generated by repeating a singl
e octahedral unit cell.Van Bael et al.[14]investigated a mi-cro-CT-bad protocol for increasing the controllability of porous structures produced by SLM.The porous structures were created using Magics software bad on the same unit cell consisting of the struts with the angles of90°or45°with respect to the horizontal.
However,the cellular lattice structures investigated in the pre-vious works do not exhibit good manufacturability in SLM.The cel-lular lattice structures with large unit cell size or low strut angles from the horizontal(usually lower than30°)could not be built using the SLM process becau overhanging struts led to the occur-rence of rious deformation.Conquently,most of the previous cellular lattice structures were designed to only include the struts with angles of90°or45°relative to the build plane and manufac-tured via the SLM process at only one orientation[8,11–14].This adds considerable constraints on manufacturing versatile and com-plex cellular structures to meet requirements of
and applications,sacrificing the design freedom of
tures and geometrical capability of AM
manufacturability of cellular lattice structures,we
月亮姑娘做衣裳
design and manufacturing of periodic cellular
using a novel unit cell type called‘‘Schoen Gyroid’’,
gyroid unit cell,to enhance the geometrical
process.Unlike the majority of previous rearch
ployed a unit cell with straight beam-like struts and
core,the gyroid unit cell has circular and smooth
spherical core.The inclination angle of the circular
百战不殆
struts of the gyroid unit cell continuously varies along
伤感的歌core,which makes layers grow up gradually with
overhanging area and position between two
the previously manufactured layer can,therefore,
next layer in the SLM process.This lf-supported
up new capability for the SLM process to manufacture
tile and advanced cellular lattice structures with
unit cell size and volume fraction at different
the need of support structures.Fabricating such
structures with less design and manufacturing
enable SLM to make lightweight products as well
迷恋expensive functional materials,build time and微信网页版登录手机版
tion.In our previous works,we reported the
od for generating gyroid cellular lattice structures
the manufacturability of SLM for the fabrication of lattice structures with a wide unit cell size range fr
om2to8mm and investigated the effect of unit cell size on the strut density and compression properties of gyroid lattice structures[16].This study investigated the influences of the volume fraction and orien-tation on the manufacturability and compression properties of the gyroid cellular lattice structures fabricated by SLM using a316L stainless steel powder.
2.Experimental details
2.1.Materials
Cellular lattice structures were made from a316L stainless steel powder with average particle size of45±10l m,which was gas atomized and produced by Sandvir Osprey Ltd.,UK.The SEM micrograph of the stainless steel powder is shown in Fig.1.It is en that the powder has a narrow particle size distribution and
a nearly spherical shape.
2.2.Design of advanced cellular lattice structures
The CAD models of gyroid unit cell and periodic cellular lattice structures with the volume fraction of6%,8%,10%,12%and15% andfixed unit cell size of5mm were generated through the Scan-CAD sof
tware provided by Simpleware Ltd.,UK.Volume fraction is defined as the volume percentage of the solid material in the cellu-lar lattice structure.The CAD model of gyroid unit cell is shown in Fig.2(a).The cellular lattice structures that are directly generated by the ScanCAD software always have a strut angle of45°with re-spect to the horizontal,and are referred to as normal orientation cellular structures in this study.A normal orientation cellular lat-tice structure with15%volume fraction and5mm unit cell size is shown in Fig.2(b).The normal orientation cellular lattice struc-ture was rotated along the Y axis by45°,and thus the worst orien-tation cellular lattice structure as shown in Fig.2(c)was obtained. From Fig.2(c),it is obrved that the half of the struts of the worst orientation cellular lattice structures is orientated at an angle of0°with respect to the horizontal building platform and completely overhanging during the SLM process.Hence,it is considered as the most difficult condition for the SLM to build the lattice struc-Fig.1.SEM micrograph of the stainless steel powder.
534 C.Yan et al./Materials and Design55(2014)533–541
is0.1mm.All processing occurs in an Argon atmosphere with less than1.0%O2.The processing parameters ud in this study were as follows:the lar power was95W;the scanning time per point was250l s and the point distance was40l m;the scan spacing was75l m;the layer thickness was75l m.9Gyroid cellular lattice structures with the dimensions of25Â25Â15mm3were built by the SLM proc
ess on a ba plate,and then cut off from the ba plate using Electrical Discharge Machining(EDM)wire cutting for various tests.
2.4.Measurements
A micro-CT scanner(Benchtop CT160Xi,X-Tek)at27l m reso-lution using120kV voltage and182l A current was ud to scan the lattice structures,and2D slice image data were collected. VGstudio MAX2.1software was ud to reconstruct the3D models of the fabricated cellular lattice structures using the2D slice images data obtained from micro-CT scans.By analyzing the recon-struction3D models,the features of the SLM-manufactured lattice structures such as internal defects and volume of solid struts were determined.The316L stainless steel powder and struts of the SLM-manufactured gyroid cellular lattice structure underwent micro-morphological characterization using HITACHI S-3200N Scanning Electron Microscope.An optical microscope(Dino-lite Digital Microscope)was ud to investigate the morphologies of the SLM-fabricated lattice structures and analyze the strut size.As the strut diameter of the gyroid unit cell is not uniform as shown in Fig.2(a),we took the diameter of the middle andfinest part of the struts as strut size.For every optical microscope image,10 dimensional values of the strut size were measured and average value was calculated.Uni-axial compression tests were carried out to asss the compression properties of the lattice structures by
using EZ20Universal Material Testing Machine,Lloyd Instru-ments Ltd.,UK equipped with a20kN load cell.The speed of load-ing was t a constant of0.4mm/min for all of the tests.The stress–strain curves,yield strengths and Young’s moduli of the SLM-manufactured lattice structures were obtained through the compression tests.
3.Results and discussion
3.1.SEM morphological analysis of lattice strut surface
Fig.3shows the SEM images of the SLM-manufactured cellular lattice structures with the volume fractions of6%,8%,10%and12%. All the structures have the same unit cell size of5mm.It can be en from the SEM images that the SLM-manufactured cellular lat-tice structures show circular struts and spherical cores,which is in agreement with the CAD model of the gyroid unit cell shown in Fig.2(a),and no interlayer delamination indicating metallurgical bonding between the layers during the manufacturing process.It is also obrved that the lattice structures exhibit very rough sur-faces with curvatures and corrugations.A higher magnification SEM micrograph of the strut in Fig.4(a)demonstrates a stair-ca-shaped profile and many partially melted metal particles bonded on the surfaces of the lattice structures.The rough strut surfaces of the SLM-manufactured lattice str
uctures can mainly be attributed to the four reasons:(1)Stair stepping effect.As shown in Fig.4(b),CAD model of the part is decompod into many right-angular polyhedron layers which are then built one by one and combined together to form3D physical part in the SLM man-ufacturing process of the circular strut.For any curved surfaces or inclined plane,the effect of laminar build is noticed as stair step, which is referred to as stair stepping effect,leading to the stair-ca-shaped profile of the circular strut as shown in Fig.4(a).The stair stepping effect has a great influence on the surface quality of SLM parts,and can be diminished by decreasing the layer thick-ness,but this increas the time required to complete the fabrica-tion[17].(2)Circular struts are partially built on the loo powder. To ensurefirm combination of adjacent layers,lar melting depth, which is the depth of lar melting and permeation into the pow-der,is slightly higher than the layer thickness to form overlaps be-tween layers as shown in Fig.4(b).However,the circular struts with varying inclined angles are partially built on the loo pow-der,and thus some metal particles below each layer will be totally or partially melted and then bonded on the bottom of the layer.(3) Thermal diffusion.Thermal diffusion occurs between loo powder and solid material due to big temperature difference,leading to powder particles sticking to the strut surface[14].(4)Partially melted raw metal particles on the boundary of each layer.A new layer of metal particles is scanned by the contour lar track,fol-lowed by the hatching lar track.Some stainless steel particles on the boundary are partially melted by the contour lar track, and thus bonded to the boundary of each new formed layer[16].
3.2.Optical microscope analysis of the manufacturability
Fig.5shows the optical microscope images of the SLM-manu-factured cellular lattice structures with thefixed unit cell size of 5mm and different designed volume fractions of6%,8%,10%and 12%.It is en that the struts of the lattice structures are well man-ufactured by the SLM process,and the struts are solid,connected and continuous although their surfaces are rough.
The strut sizes of the SLM-manufactured cellular lattice struc-tures were measured from the optical microscope images as shown in Fig.5(d),referred to as experimental strut sizes.The designed strut sizes were measured from the CAD models of the cellular lat-tice structures.The experimental and designed strut sizes in func-tion of the volume fraction were plotted and compared in Fig.6.It is found that the experimental strut sizes are higher than the de-signed values.The experimental strut sizes were found to be 0.50,0.70,0.86and1.01mm against the designed strut sizes
of CAD models of(a)gyroid unit cell,(b)normally orientation cellular lattice structure and(c)worst orientation cellular lattice
0.42,0.61,0.79and0.92mm for the lattice structures with the de-signed volume fractions of6%,8%,10%and12%,respectively.The increa in the strut size compared with the designed values can be attributed to the following:(1)The partially melted metal par-ticles bonded on the strut surfaces as shown in the SEM micro-graphs in Fig.4(a).(2)The melt pool size of the scan vectors that describe boundaries of a strut is much higher than the lar spot size although the scan vectors are usually shifted half of the lar spot size inwards for compensation[14].
Similar obrvations were made when porous metal structures were manufactured via the AM technologies.Van Bael et al.[14] evaluated the SLM-manufactured Ti6Al4V porous structures though micro-CT image analysis and noticed the increa in strut size with112l m compared to the designed value,and in accor-dance the structure volume and surface area incread signifi-cantly.Parthasarathy et al.[18]reported a140l m increa in the strut size of the EBM-produced porous Ti6Al4V structures com-pared with the designed value,and thus decread pore size by 210l m.
From Fig.6,it is also found that the strut size increas with increasing the volume fraction at afixed unit cell size.If the unit cell size is kept unchanged,the unit cell number and total strut length of the lattice structure do not vary.Therefore,when increas-ing the volume fraction,the strut will become thicker and thus the strut size increas.
The worst orientation cellular lattice structure with15%volume fraction and5mm unit cell size,the half of who struts is at an angle of0°with respect to the building platform of the SLM ma-chine,was manufactured with no obvious deformation by the SLM process and the obtained structure is shown in Fig.7(a).The optical microscope images of the SLM-manufactured worst orien-tation cellular lattice structure in top,bottom and lateral view are exhibited in Fig.7(b)–(d),respectively.Int images in Fig.7(b)–(d)prent the CAD model of the worst orientation lattice structure in top,bottom and lateral view respectively for compar-ison.It can be en from the optical micrographs that there are no defects or broken cells in the structures.By comparing the opti-cal microscope images of the lattice structure in the different view with the corresponding CAD models,the SLM-manufactured worst orientation cellular lattice structure is in good agreement with its CAD model except for the rough surfaces.Manufacturability of gyroid lattice structures with half of its struts orientated at an
images of the SLM-manufactured cellular lattice structures with different volume fractions and thefixe
d unit cell magnification SEM micrograph of the strut and(b)schematic illustration of the SLM manufacturing process of the
云南教师招聘angle of0°with respect to the building platform by SLM can be attributed to the lf-supported feature of the gyroid unit cell. The inclination angle of the circular and smooth struts of the gyro-id unit cell continuously varies along the spherical core,which makes layers grow up gradually with slight changes in overhang-ing area and position between two adjacent layers during the SLM process.This curve surface of gyroid overhang structure pro-vides a small length overhanging ction between the layers.Con-quently,the previous manufactured layer can almost support next layer indicating a lf-supported feature,which thus enables the SLM process to manufacture the lattice structures with0°strut angle and relative cell unit size.For the cellular lattice structures with straight beam-like struts and a polyhedral core propod and investigated in majority of the previous rearch works,if their strut angles from the horizontal plane are
gree and its overhanging ction is over a
tion will occur during the SLM or other direct
becau the struts are mostly built on loo
fects or broken cells in produced lattice structures,
failure of the manufacturing process.Mullen
unit cell geometry with45°strands bad
suitability for SLM manufacture,and believed
structure such as the dodecahedron do not
they contain many horizontal and low angle
cult to build becau they are unsupported
Santorinaios et al.[12]addresd that this
with horizontal struts cannot be built through
rect metal AM process.Cansizoglu et al.
tices who struts were oriented at an
with respect to the build plane had little
the successive melted layers resulting in
Hence,the gyroid lattice structures reprent
facturability in comparison with previous
straight beam-like struts or
unit cell).They can be built any orientation,
large cell unit size.This could mitigate the
and manufacturing process for the SLM fabrication.It therefore al-lows the u of optimal design and processing parameters such as large unit size and optimal building orientation to reduce material and energy consumption and the production time in the SLM of lightweight products.
3.3.Micro-CT analysis of volume fractions
The CT reconstruction models of the SLM-manufactured cellular lattice structures with thefixed unit c
ell size of5mm and the dif-ferent volume fractions of6%,8%,10%,12%,15%(normal orienta-tion)and15%(worst orientation)are shown in Fig.8(a)–(f) respectively.Micro-CT analysis shows well defined struts and no defects or broken cells throughout the lattice structures,indicating
microscope images of the SLM-manufactured cellular lattice structures with different volume fractions and thefixed unit
Fig.6.Strut sizes measured from optical microscope images(experimental strut
size)and CAD models(designed strut size)in function of the volume fraction at a
constant unit cell size of5mm.
