1. INTRODUCTION

Over the last decade, significant progress has been made on developing new and advanced techniques for additive manufacturing (AM). Both practitioners and academicians in the field have been striving in developing ways in which sustainable and economi-cal products can be additively manufactured. Selec-tive Laser Melting (SLM) is an additive manufactur-ing process which has excellent prospects in the manufacturing of complex lightweight metal com-ponents and cellular lattice structures.

Support structures are essential for AM metallic parts to build the overhanging and undercut surfaces in complex geometries and to control part curling and deformation during layer-by-layer fabrication. These supports affect the material use, build-time, surface finish, energy consumption and post-processing time of the manufactured parts. Minimis-ing the amount of support structures could reduce the time needed to remove the part from the platform and therefore help to reduce manufacturing costs (Frank and Fadel, 1995). A suitable part deposition orientation can improve part accuracy and surface finish as well as reduce the production time and sup-port structure required for building a part (Pham and Demov, 2001). Therefore, researchers have devel-oped methods to minimize the support volume by changing the orientation of the supported part (Frank and Fadel, 1995; Allen and Dutta, 1995; Pham, et al. 1999).

It is worth mentioning, that the majority of the previous studies on support structures were specifi-cally developed for non-metallic processes such as

Stereolithography (SLA) and Fused Deposition Modelling (FDM). The current solutions for support generation are either automatic or manual. The later depends on the experience of an operator to manual-ly design and customize the support structure de-pending on the requirement for a particular job. Whether automatic or manual, the efficiency and re-liability of the generated support is always under question, as it is difficult to determine if the selected support is excessive or inadequate for the part.

In SLM, support requirement is even more com-plicated. The thermal stresses cause distortions and cracks in the part if inadequately supported. There-fore, metallic support structures should be able to support and take heat away from the part. Addition-ally, the removal of metallic supports from the part and base plate is a tedious job, sometimes requiring machining. Particularly, large amount supports for delicate parts would increase the difficulties of sup-port removal and even destroy fine details on the downward facing surfaces.

In order to utilize the full potential of AM tech-nology, it is important to investigate the opportuni-ties in developing new and improved designs for metallic support structures. This study aims to ex-plore the potential of using cellular structures to support metallic parts manufactured by SLM. A key advantage offered by cellular structures is their low volume, which offers opportunities to reduce the support material volume and increase the ease with which raw powders trapped inside these sacrificial supports can be removed.

Preliminary investigation on cellular support structures using SLM process

Ahmed Hussein, Chunze Yan, Richard Everson and Liang Hao

College of Engineering, Mathematics and Physical Science, University of Exeter, Exeter EX4 4QF, Devon,

United Kingdom

ABSTRACT: Selective Laser Melting (SLM) is an additive manufacturing (AM) technique in which 3D metallic parts are manufactured in a layer-by-layer fashion, typically in small series. To build complex geom-etries with overhanging and undercut surfaces, SLM has to solidify expensive materials into sacrificial sup-port structures in order to control curling and shrinkage of the part. The majority of previous studies on sup-port structures concentrate on non-metallic processes. This study aims to explore the potential of using cellular structures to support metallic parts manufactured by SLM. A key advantage offered by the cellular structures is their low volume fraction which provides opportunities to greatly reduce the volume of support materials and build time as well as allow for easy removal of raw powders which are trapped inside these sac-rificial supports. The preliminary experimental results reveal that these cellular support structures have the potential to be used for supporting metallic parts and prevent part deformation. KEYWORDS: Additive manufacturing; Selective laser melting (SLM), Cellular structure, Support structure

2. DESIGN OF CELLULAR SUPPORT STRUCTURES

Figure 1 shows the unit cell types used for the gen-eration of the support structures. These unit cells are generated through the software provided by Sim-pleware Ltd. These are periodic lattice structures which could assist in reducing the time and material volume needed for building support structures. The volume fraction defines the relative solid volume that is inside the generated support. The lower the volume fraction is, the more open the support struc-ture will be. If the value of volume fraction is very low, it may result in loss of connectivity between ad-jacent cells in the support; very high values may re-sult in closed support volume.

Figure 1. Unit cell types used for support structure generation

Figure 2. Design of support structures with the part

To investigate the deformation of the parts supported by the cellular structures, specimens with dimen-sions of 30 ×20 × 0.3 mm

3 were built and the deflec-

tions in z-direction were measured. As is shown in Figure 2, the support structure designs were used with unit cell sizes of 6 mm and 2.5 mm and with a volume fraction of 12% and 15% respectively. The

height of the support was 5 mm and built on the steel base plate bolted into the machine platform.

3. MATERIALS AND EQUIPMENT

Stainless steel (316L) powders with average particle size of 45 ± 10 µm were used to build the test sam-ples. A SLM MCP-Realizer machine with 100 w CW Ytterbium fibre laser, operating within 1068-1095 nm was used. TalySurf from Taylor Hobson Ltd was used to measure the deflection of test parts. This is the 3D surface topography profiling instru-ment that moves the workpiece under a stationary gauge head.

4. RESULTS AND DISCUSSION

As shown in Figure 3, the specimens were fabricated with the Schoen gyroid and Schwartz diamond type cellular support structures. When a cell size of 6 mm was used, the support structure did not connect well to the edges of the specimen to restrict deformation, as is shown in Figures 3 (a) and (b). The supports connected to the specimens were able to hold the ar-eas above and prevent the deformation of the speci-mens. In addition, supports act as a heat sink and take the heat away from newly melted regions, so without support the temperature gradients cause high thermal stresses which lead deflection to the speci-mens.

Figure 3. SLM manufactured parts with support structures

Schoen gyroid Schwartz diamond

(a) Schoen gyroid

Volume fraction= 12%

Cell size = 6mm

(b) Schwartz diamond

Volume fraction= 12%

Cell size = 6mm

(c) Schoen gyroid

Volume fraction= 15%

Cell size = 2.5mm

(d) Schwartz diamond

Volume fraction= 15%

Cell size = 2.5mm

a) Schoen gyroid

Volume fraction= 12%

Cell size = 6mm

b) Schwartz diamond

Volume fraction= 12%

Cell size = 6mm

c) Schoen gyroid

Volume fraction= 15%

Cell size = 2.5 mm

d) Schwartz diamond

Volume fraction= 15%

Cell size = 2.5 mm

Figure 4. Measured deflection profiles in z-axis (a) Gyroid, cell size= 6 mm, the volume fraction= 12% (b) Diamond, cell size=

6 mm, volume fraction= 12% (c) Gyroid, cell size= 2.5 mm, volume fraction= 15% (d) Diamond, cell size= 2.5 mm, volume

fraction=15%

Figures 4 (a) and (b) show the obtained deflec-tions of the two types of support structure in the z-axis. The measured profiles show higher deflections on the edges for both support types with a 6 mm cell size.

Support structures with the smaller cell size of 2.5 mm were used to remove the edge deformation. However, with a 12% volume fraction and the smaller cell sizes, structures were weakly connected and thus a volume fraction of 15% was preferred. As in Figures 4 (c) and (d), significant reduction in part deflection can be observed. As the number of con-tact points on the downward facing surfaces of the part is increased, more heat was taken away and edge deformation was constrained. Even with the cell size used, there were no issues with removing the raw powder trapped inside the support structures.

There are no obvious differences between the two types of cellular support structures, the gyroid and diamond cells, in terms of the resultant part deflec-tion. Both can be built in SLM if the cell size and volume fraction are chosen properly. Concerning the powder removal, the gyroid cell type is relatively easier due to the structure formation as it is more open compared to the diamond type. Comparisons based on other aspects such as ease of removal of the support from the part and base-plate are the sub-jects for future investigation.

The presented results demonstrate that smaller cell sizes can restrain deflection. However, more contact points on the downward facing surfaces might result in a rougher surface. Lowering the vol-ume fraction can minimise the material used for the support, but very low values may result in weaker supports and failure. A well designed support struc-ture has to take into consideration the manufactura-bility, support removal, support volume, build time and surface quality of the supported regions.

5. CONCLUSION AND FUTURE WORK

The majority of previous studies on support structure concentrated more on non-metallic processes. Since the support requirements of metallic laser processes like SLM are more complicated, due to high thermal gradients involved in the phase transformation pro-cess resulting in thermal stresses and distortion of the part, proper study on support structure design and manufacturing is required. This work explores the potential of using cellular structures to support metallic parts manufactured by SLM. Two types of cellular structures were tested for their suitability for support structures. The results reveal that these cel-lular support structures can be used for supporting metallic parts. As the cell size of the structure was

(d)

Def

lecti

on

in

z-

axis

X-axis (mm)

µm

-400

-300

-200

-100

0

100

200

300

400

500

0 2 4 6 8 10 12 14 16 18 20 22 24 26 mm

Length = 27.6 mm Pt = 416 µm Scale = 1000 µm

(c)

Def

lecti

on

in

z-

axis

X-axis (mm)

µm

-400

-300

-200

-100

0

100

200

300

400

500

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 mm

Length = 28.8 mm Pt = 360 µm Scale = 1000 µm

(b)

Def

lecti

on

in

z-

axis

X-axis (mm)

µm

-300

-200

-100

0

100

200

300

400

500

600

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 mm

Length = 28.7 mm Pt = 674 µm Scale = 1000 µm

(a) µm

-300

-200

-100

0

100

200

300

400

500

600

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 mm

Length = 28.9 mm Pt = 657 µm Scale = 1000 µm

Def

lecti

on

in

z-

axis

X-axis (mm)

made smaller, less deformation of the part occurred. The loose powder trapped inside these support struc-tures can be removed easily due to the structures’ porous nature.

Smaller cell size on the interface of support and the part should be used to restrain distortion without compromising the down facing surface quality. To minimise the support material, lower volume frac-tions should be used without compromising the abil-ity of the support to withstand loads.

Future work will investigate the effects of using

different volume fractions on the building time, the quality of down-facing surface and removal of these cellular support structures.

6. ACKNOWLEDGEMENT

This work has been supported by the UK Technolo-gy Strategy Board (TSB) Research Project (BA036D). The TSB funded project is entitled ‘SAVING - Sustainable product development via design optimisation and AdditiVe manufacturING’ and is a collaboration between the Simpleware Ltd, Delcam PLC, University of Exeter, 3T RPD, Cruci-ble Industrial Design Ltd, EOS Electro Optical Sys-tems Ltd and Plunkett Associates Ltd.

REFERENCES

Frank D., Fadel G. 1995. Expert system-based selection

of the preferred direction of build for rapid prototyp-

ing processes. J. Intll. Manufg 6: 339-345.

Pham D.T, Demov S.S. 2001. Rapid manufacturing: the

technologies and applications of RP and RT. Spring-

er-verlag, London limited.

Allen S., Dutta D. 1995. Determination and evaluation of

support structure in layered manufacturing. J. Design

manufacturing 5: 153-162

Pham D.T, Demov S.S, Gault R.S. 1999. Part orientation

in SLA. Int J. of adv. Manufg. 15: 677-682