Application of Abrasive Flow Machining in Removing Surface Defects in

Metal Parts of 3D Printing

Haiquan Wang1, a, Haibo Wei1,b, Can Peng1,c, Xuanping Wang1,2,d,*, and Hang Gao1,2,e

1MOE Key Laboratory for Precision and Non-traditional Machining Technology, Dalian University of Technology, China

2School of Mechanical Engineering, Dalian University of Technology, China

awanghaiquan311@163.com, b985747545@qq.com, c517350148@qq.com, d xpwang@dlut.edu.cn, e gaohang@dlut.edu.cn

Keywords: Additive manufacturing; Abrasive flow machining; Balling effect; Powder adhesion

Abstract. As the quality of as-built surfaces of additive manufacturing(AM) metal parts is far inferior to service requirements, it is emergent to improve the complex as-built surfaces deteriorated by inherent characteristics of powder adhesion and balling effect for metal AM technology, e.g. selective laser melting (SLM). Abrasive flow machining (AFM) is a finishing technique offering high efficiency for parts with difficult-to-machine structures, and the application of AFM to finishing metal parts of AM process is taken in this paper. The effects of abrasive grits size and pressure on removal of surface defects of AM metal parts are discussed, with pretty good finishing effect achieved for non-trial surface defects .

Introduction

Additive manufacturing (AM) provides vast fabrication freedom to make parts with complex geometries that cannot be fabricated in conventional subtractive manufacturing, based-on the layer-wise building pattern of AM[1]. The AM techniques building metal parts include selective laser sintering, selective laser melting (SLM), electron beam melting, and DMLS, etc. In SLM, metal powders are subsequently melted in a powder bed with laser as heat source, to form part geometry in layers[2, 3].

The advantages of AM in geometrical freedom fabrication are pretty attractive to many industries in enhancing parts functionality, reducing parts weight and shortening fabrication period.

However, a major challenge in fabricating metal parts with AM is the poor as-built surface roughness. The poor surface quality of AM parts is inherent to the characteristics of balling effect and powder adhesion for metal AM technologies. The latter is most evident in SLM process where powders are adhered to part due to residual heat, particularly at steep angles where step edges are close together.

Fig.1 illustrates the effect of partially melted powder on as-built surfaces of AM parts. The surface roughness Ra is no less than 10 μm and variable depending on different AM processes, the processing parameters and the part geometry. The functional properties of metal AM part are influenced by its surface roughness, including fatigue resistance, frictional properties. For example, pressure drops and heat transfer efficiency in narrow channels are immediately affected by the roughness of the channel walls.

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Fig.1 As-built surface of Titanium AM part

Consequently, the as-built surface quality of AM parts is unqualified for many applications. If there are no significant improvements in the resolution of AM technology, post-processing is still an inevitable step to improve surface quality of parts for AM process. Because of the geometric complexities, it is more challengeable for those of conventional polishing processes for polishing AM parts[4].

Conventional polishing technologies like manual polishing and sand blasting could not achieve the required polishing effect of complex AM parts. However, Abrasive flow machining(AFM) are advantageous in polishing difficult-to-access area[5, 6].

In this paper, AFM process is applied to polish additively manufactured metal grille parts. The internal and external surface of the grille part are polished at the same time. The effect of AFM process on removal of balling effect and powder adhesion is studied.

Experimental Setup

As shown in Fig.2, the titanium grille is built up by Ti-6Al-4V powder through Selective Laser Melting(SLM), where inclined square holes with 3mm long each side are distributed in equal distance. As shown in Fig.2(b) and Fig.2(c) , there are obviously titanium alloy balls from the SLM process and alloy powders adhered on the grille surface, which results in the poor surface quality. Initial surface profile of the titanium grille is given in Fig. 3, with value of roughness Ra as 3.6μm.

(a) (b) (c)

Fig. 2 As-built outer and inner surface of titanium AM grille

Fig. 3 Profile of as-built surface of AM grille

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In the polishing process of AFM, abrasive media are composed of polymer, abrasive grits, et al, acting as the continuously deforming cutting tool. The active abrasive grits in abrasive media scratch the surface and micro-chips are produced, as abrasive media are extruded across the workpiece surface, and the material removal occur during this process. The AFM machine used in the present work is shown in Fig. 4, composed of hydraulic system, electric system, and mechanical system.

Fig.4 AFM experimental system

Mesh size and mass fraction of the abrasive media, and the process parameters in the AFM polishing are listed in Table 1. Two types of SiC grits with mixed size of 120#- 50# and 80#- 50# are used to polish the AM grille part.

Table 1 Processing parameters in AFM polishing

Media Pressure

/MPa Cycles Time /min

SiC120-50 2.5 30 30 SiC120-50 3 51 30 SiC120-50 3 137 60 SiC80-50 3.1 50 30

Discussion

With abrasive media of mixed size 120#- 50#, experimental results show that variation of pressure and cycles nearly have no influence on the final polished inner surface, as the size of SiC grits with 120# is too small to generate enough material removal for polishing inner surface.

After 50 cycles of AFM process by abrasive media with mixed size of 80#- 50#, the outer surface and inner surface of the AM grille are provided in Fig.5. As shown in Fig.5, the adhered melten balls and alloy powders are all removed with comparison to as-built surface textures in Fig. 2. The surface profile of the polished titanium grille is given in Fig. 6, and the surface roughness is reduced from the initial Ra 3.6μm to Ra 1.6μm.

With comparison of surface polishing by abrasive media with 120#- 50# and 80#- 50# respectively, it is observed that proper grit size is critical for implementation of effective inner surface polishing. It could be concluded from the experimental results that AM parts with non-trivial internal structures can be polished effectively, including outer surface and inner surface.

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Fig. 5 The outer and inner surface of titanium AM grille after AFM Polishing

Fig. 6 Profile of surface of titanium AM grille after AFM polishing

Summary

In this paper abrasive flow machining(AFM) technique is taken to polish as-built surface of additively manufactured titanium grille with inclined square holes equally distributed. Abrasive media with hybrid grit sizes of 50# and 80# are adopted to polish the inner and outer surface of the grille in the same time. After AFM polishing process, the poor surface quality of as-built surface due to the inherent "balling effect", "powder adhesion" in additive manufacturing is significantly improved.

Acknowledgement

The authors gratefully acknowledge the financial support from Science Fund for Creative Research Groups of NSFC (51621064), the National Natural Science Foundation of China (Grant No. 11302043, 51475074) , the Joint Fund of MOE and GAD (6141A02022106), and the support from “the Fundamental Research Funds for the Central Universities”.

References

[1] W E. Frazier, Metal additive manufacturing: A review, Journal of Materials Engineering and Performance. 23(2014) 1917-1928. [2] A. Lamikiz, J.A. Sanchez, L.N.L. de Lacalle, et al, Laser polishing of parts built up by selective laser sintering, International Journal of Machine Tools & Manufacture. 47(2007)2040-2050. [3] B. Rosa, P. Mognol, J.Y. Hascoet, Laser polishing of additive laser manufacturing surfaces, Journal of Laser Applications, 27(2015) S29102-1. [4] G. Strano, L. Hao, R.M. Everson, et al, Surface roughness analysis in selective laser melting. Innovative Developments on Virtual and Physical Prototyping, 3(2012)561-565. [5] Y.Z. Fu, H. Gao, X.P. Wang, et al, Machining integral impeller & blisk of aero-engine: a review of surface finishing and strengthening technologies, Chinese journal of mechanical engineering, 30(2017) 528-543. [6] Y.Z. Fu, X.P. Wang, H. Gao, et al, Blade surface uniformity of blisk finished by abrasive flow machining, International Journal of Advanced Manufacturing Technology, 84(2016)1725–1735.

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