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Journal of Materials Processing Tech.

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Laser polishing of additive manufactured CoCr alloy components withcomplex surface geometry

K.C. Yunga, T.Y. Xiaoa,⁎, H.S. Choya,⁎, W.J. Wanga, Z.X. Caib

a Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kongb Shenzhen Sunshine Laser & Electronics Technology Co., Ltd

A R T I C L E I N F O

Keywords:Additive manufacturingLaser polishingComplex surface geometryRoughnessHardness

A B S T R A C T

Laser polishing, capable of polishing selective nonplanar areas, is exploited to improve the surface roughness ofadditive manufactured metal components. It offers a highly repeatable, higher speed polishing process as well aslow labor costs compared with traditional mechanical abrasive polishing. In spite of the fact that many studiescan be found on laser polishing processes, few have reported that focus on metal components, manufacturedadditively by selective laser melting (SLM) technology, with geometrically different complex surfaces. This paperpresents a novel method to reduce the surface roughness of Cobalt Chromium (CoCr) components with complexsurface geometry by using a layered polishing method which can constantly adjust the defocusing distance of thelaser along with the surface shape of the components. The optimized laser polishing parameters used were firstlyobtained from the test results on planar surfaces of CoCr alloy samples and samples with complex surfacegeometry were then polished based on the laser parameters. Characterizations for the laser polished sampleswere conducted using optical profiling and scanning electron microscopy, showing that the surface roughnesswas reduced significantly in comparison with the as-received samples. A reduction of up to 93% in surfaceroughness was achieved. The mechanical hardness was also characterized by testing for Vickers hardness, whichindicated the surface hardness of the laser polished samples was enhanced by 8%. Moreover, a simple andeffective model was developed to illustrate the method of laser polishing on the complex surface geometry ofadditive manufactured CoCr alloy components. The analytical model is helpful in understanding and evaluatingthe underlying mechanisms of laser polishing.

1. Introduction

In recent years, selective laser melting (SLM), which is a category ofadditive manufacturing technology, has been exploited to manufacturemetallic components with complex surface geometries (Bremen et al.,2012). The approach in SLM is to melt minute metallic powder andbuild net-shape components layer by layer. The actual applications forSLM components usually have high requirements on the surfaceroughness. For example, SLM components for medical implants wouldrequire a very smooth surface to prevent bacterial growth and tissuedamage (Gora et al., 2016). However, the surface quality of SLMcomponents is generally lower than that for the other alternativemanufacturing processes. As described (Strano et al., 2013), the surfaceroughness is usually affected by the ‘Stair Step’ effect and the ‘Balling’effect. The ‘Stair Step’ effect refers to the stepped approximation by thelayers of curved and inclined surfaces. During the melting process,many tiny metallic powder particles would adhere to the surface of SLMcomponents, unwantedly, resulting in the ‘Balling’ effect.

Compared with the traditional polishing methods, such as me-chanical abrasive or electrochemical, laser polishing has shown goodpotential for selectively polishing metals and alloys fabricated by SLM.Moreover, as laser polishing is also a type of non-contact polishingtechnique which does not involve load transmission between the po-lished components and equipment, it could overcome some of thedrawbacks and difficulties of traditional surface treatment processesthat affect the structural deformation.

Previous research has shown that the laser polishing process notonly enhances the surface roughness of metallic components, but alsoimproves the surface properties. For example, Mai and Lim (2004) re-duced the roughness of 304 stainless steel from 195 nm to 75 nm, in-creased the surface reflectance to 14% and decreased the diffusive re-flectance to 70% using laser polishing technology. Lamikiz et al. (2007)showed that a laser polished surface presented slightly higher and morehomogeneous hardness, and almost had no heat affected zones orcracks. It is evident that laser polishing could effectively improve thesurface properties of SLM parts. For example, Schreck and Zum Gahr

https://doi.org/10.1016/j.jmatprotec.2018.06.019Received 22 January 2018; Received in revised form 1 June 2018; Accepted 14 June 2018

⁎ Corresponding authors.E-mail addresses: tingyu.xiao@polyu.edu.hk (T.Y. Xiao), choy.henry@polyu.edu.hk (H.S. Choy).

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(2005) found that laser polishing could be beneficial for optimizing thestructure of a component by improving particular tribological proper-ties. Therefore, laser polishing process on SLM parts deserves specialconsideration in melting a certain thickness of the surface and redis-tributing the material, so that a smoother topography than the previoussurface condition could be produced (Ramos-Grez and Bourell, 2004).

At present, studies have been conducted on CoCr alloy samples witha planar surface, which verified that laser polishing can be an effectiveway to smooth the as-received surface of CoCr alloy samples fabricatedby SLM. Different polishing strategies were tested to optimize theparameters, and as a result, the surface roughness could be reducedsignificantly by using identical parameters. However, reports on laserpolishing on complex geometric components made of CoCr alloys arerare, as well as their properties after smoothing by laser polishing. Themorphology of a nonplanar surface is more complex, leading to greatdifficulty in controlling the defocusing distance, so the polishing pro-cess is more complicated compared with that of a planar surface.Moreover, a stable defocusing distance is indeed important during thelaser polishing for a nonplanar surface, which is rarely mentioned in theliterature.

This paper focuses on a study of CoCr alloy samples with complexsurface geometry, fabricated by SLM and processed by using a layeredlaser polishing method. Differing from the polishing process on planarsamples, the interaction between the laser and surface geometry is morecomplicated, and requires optimized parameters to obtain satisfactory

results. Orthogonal testing methodology was applied to design theparameters for the primary experiments on planar samples. Furtherexperiments were based on the optimized parameters and the proper-ties of the polished complex surface geometry were analyzed in detail.A layered laser polishing method was developed, and a geometricmodel was built to help evaluate the process. Based on the geometricmodel, the working distance could remain constant when conductingthe experiments by using a 2D fiber laser, which is important inachieving a final smooth surface morphology. Characteristic observa-tions of the complex surface geometry were conducted using opticalprofiling, scanning electron microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDX), metallographic microscopy, and a Vickershardness tester to analyze and quantify the surface properties of thesamples before and after laser polishing.

2. Experimental procedure

CoCr alloy samples with planar surface and complex surface geo-metry (convex surface, concave surface, slant surface as shown inFig. 1) fabricated independently by means of selective laser meltingwere used in the experiments. Each sample was clearly labelled todistinguish the particular type of surface geometry to be studied. TheCoCr powders used were supplied by SLM SOLUTIONS company with adensity of 4.36 g/cm3 (composition: Cr= 28.23%, Co=64.76%,Mo=5.84%, Si= 0.46%, N=0.06%, Mn=0.50%, Fe=0.04%,

Fig. 1. Different types of samples with complex surface geometry. (a) Convex surface, (b) concave surface and (c) slant surface.

Table 1Basic laser polishing parameters, including power, scanning speed and hatchingspace.

Factors Level 1 Level 2 Level 3 Level 4

Power (W) 30 40 55 70Scanning speed (mm/s) 15 50 100 300Hatching space (mm) 0.02 0.03 0.04 0.05

Table 2Polishing strategies and corresponding energy densities.

Strategy Power(W)

Scanning velocity(mm/s)

Hatching space(mm)

Energy density (J/mm2)

1 30 300 0.04 32 30 100 0.05 63 30 50 0.02 304 30 15 0.03 675 40 300 0.05 36 40 100 0.04 107 40 50 0.03 278 40 15 0.02 1339 55 300 0.02 910 55 100 0.03 1811 55 50 0.04 2812 55 15 0.05 7313 70 300 0.03 814 70 100 0.02 3515 70 50 0.05 2816 70 15 0.04 117

Table 3Experimental parameters designed by using the orthogonal testing metho-dology and the results of the surface roughness.

No. Power (W) Scanningspeed (mm/s)

Hatchingspace (mm)

Surfaceroughness Sa(μm)

Sa reduction(%)

1 30 300 0.04 4.55 −7.62 30 100 0.05 4.93 −16.53 30 50 0.02 6.87 −62.44 30 15 0.03 9.43 −122.85 40 300 0.05 2.11 50.16 40 100 0.04 1.61 62.17 40 50 0.03 4.55 −7.58 40 15 0.02 12.71 −200.39 55 300 0.02 0.92 78.210 55 100 0.03 1.12 73.611 55 50 0.04 1.59 62.512 55 15 0.05 1.91 54.813 70 300 0.03 0.73 82.814 70 100 0.02 1.35 68.215 70 50 0.05 1.59 62.416 70 15 0.04 2.59 38.9K1 25.78 8.32 10.54K2 20.98 9.00 10.33K3 5.54 14.60 15.82K4 6.25 26.63 21.85k1 6.45 2.08 2.64k2 5.24 2.25 2.58k3 1.39 3.65 3.96k4 1.56 6.66 5.46R 5.06 4.58 2.88

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Ni=0.04%, others≈0.07%). The as-received surface morphology ofthe CoCr alloy samples was relatively rough, and could not meet therequirements for practical applications, so post processing is necessaryto improve the surface quality. The laser used for polishing the surfaceof the samples is a fiber laser in a pulse mode operating at a wavelengthof 1064 nm. The maximum operating power of the laser is 70W, withthe beam focused to a spot size of 50 μm using an f-theta lens. Duringthe experiments, the laser was used out of focus to provide a spot with ahigher average energy, and the defocusing distance was 6mm.

2.1. Parameter optimization

To obtain the optimized polishing parameters for further study onCoCr alloy samples with complex surface geometry, experiments werefirstly conducted on the CoCr alloy samples with planar surfaces.During the laser polishing, there are three key parameters: laser power-P (W), scanning speed- v (mm/s), and hatching space- H (mm).Different combinations of the parameter values lead to different pol-ishing results, even if the same energy density is obtained. It is thusnecessary to consider the three key process parameters together whenoptimizing the polishing parameters.

The energy density is determined by a combination of the three

Fig. 2. The surface morphology of planar samples before and after laser polishing by using different polishing strategies. (a) As-received surface, (b) polished surface:P=40W, v=300mm/s, H=0.05mm, (c) polished surface: P= 55W, v= 300mm/s, H=0.02mm and (d) polished surface: P= 70W, v= 300mm/s,H=0.03mm.

Fig. 3. The polished surface morphology of planar sample by using polishingstrategy with a lower laser power of 40W and a higher energy density of 133 J/mm2.

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parameters. The surface roughness of the additively manufactured co-balt chromium alloy can be improved significantly when the laser en-ergy density was set at 30 J/mm2 (Gora et al., 2016). Based on theenergy density and combination with other parameters, a group ofscanning speeds was calculated and selected in a range from low speedto high speed. According to the selected process parameters in thedifferent levels (Table 1), a set of polishing strategies was designed byusing orthogonal testing methodology which could obtain the opti-mized parameters in an effective way. The testing design conditions areshown in Table 2.

The laser energy density for each particular polishing strategy isshown in Table 2. The laser energy density changes from 3 J/mm2 to133 J/mm2 which is a reasonable range for optimizing the laser pol-ishing parameters.

2.2. Further study on samples with complex surface geometry

Differing from planar surfaces, it is difficult to undertake surfacepolishing on a nonplanar surface using a 2D laser polishing system,mainly due to the difficulty in keeping the defocusing distance constant.Therefore, a layered laser polishing method was developed to under-take polishing on the samples with complex surface geometry.

In this layered polishing method, a geometric model was developedto illustrate the process. The whole sample was divided into a numberof layers along the laser processing direction and polished in an orderlyway. The thickness of each layer was consistent with the laser focaldepth, namely each layer thickness could not be larger than the laserfocal depth, at least. The defocusing distance of the laser was adjustedconstantly depending on the height of each layer to be polished duringthe process, thus avoiding the problem of energy instability.Meanwhile, the corresponding area to be polished each time was cal-culated according to the geometric model. The polishing parametersused on the complex surface geometry were optimized from the planarsamples and a fine polishing process was added to smooth the surfacefurther.

3. Results and discussion

3.1. Laser polishing on planar surface

In order to optimize the polishing parameters, 16 groups of pol-ishing strategies were tested on the planar samples. The experimentalparameters and the results of the polished surface roughness are shownin Table 3. The average as-received roughness of the planar surface was4.23 μm, and the roughness reduction for each polishing strategy wasclearly different, ranging from 0.73 μm to 12.71 μm. The combinationof the different laser polishing parameters can smooth the surface atdifferent levels. It is clear that some of the appropriate polishing stra-tegies could effectively reduce the roughness, however, some evenmade the surface quality worse. The laser polishing results and theinfluence of each parameter were analyzed and calculated in terms of K,k, and R values based on orthogonal testing methodology. R representsthe range of each parameter, and it is clearly shown that the ranges ofthe power, scanning speed and hatching space were 5.06, 4.58 and 2.88respectively, revealing the influence of each parameter on the achievedresults. It can be pointed out that the laser power has the most sig-nificant impact during the polishing process, scanning speed is thesecond, and the hatching space is the third.

Since the laser power has the greatest effect on the polishing results,the differences in surface morphology before and after laser polishingwere compared according to different laser powers (Fig. 2). The as-received surface roughness Sa of the planar samples for experimentnumber 5, 9 and 13 was reduced to 2.11 μm, 0.92 μm, 0.73 μm re-spectively. The maximum reduction of surface roughness was 82.8%achieved by a polishing strategy of relatively high laser power andscanning speed, revealing that the appropriate polishing process canevidently improve the surface morphology of the samples.

The surface material structure can be significantly affected by thepolishing temperature during the process (Mohajerani et al., 2017). Theexperiments were conducted under different laser power parametersand energy densities. The results of polished surface by using differentpolishing strategies demonstrate that the energy density has an in-tegrated influence during the laser polishing. The surface structure ofthe specimen polished by using strategy 8, using a lower laser power of40W and a higher energy density of 133 J/mm2, can illustrate thisclearly. The polished surface was seriously damaged and rather rough(Fig. 3).

By comparing the polishing strategies and evaluating the polishedsurface morphology, the polishing parameters for power P=70W,scanning speed v= 300mm/s and hatching space H=0.03mm areconfirmed (Table 4) as optimal.

Table 4Optimized polishing parameters for laser polishing on the planar surface ofCoCr alloys.

Laser polishingparameters

As-receivedroughness (μm)

Polishedroughness (μm)

Sa Reduction(%)

Power (W) 70 4.23 0.73Scanning speed

(mm/s)300

Hatching space(mm)

0.03

Fig. 4. Scanning electron micrographs of as-received convex samples with EDX spectrum. (a) Morphology of the as-received surface and (b) chemical compositionanalysis of the particles.

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Fig. 5. Scanning electron micrographs of as-received concave samples with EDX spectrum. (a) Morphology of the as-received surface and (b) chemical compositionanalysis of the particles.

Fig. 6. Scanning electron micrographs of as-received slant samples with EDX spectrum. (a) Morphology of the as-received surface and (b) chemical compositionanalysis of particles.

Fig. 7. Surface morphology of samples with complex surface geometry before and after laser polishing. (a)–(c) Show the as-received surface roughness of convex,concave and slant surfaces respectively. (d)–(f) Show the laser polished surface roughness of convex, concave and slant surfaces respectively.

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3.2. Laser polishing on complex surface geometry

Before the laser polishing process, the morphology of the as-

received samples, with complex surface geometry, was characterized bySEM. Many spherical powder particles and clusters adhering to theprinted surfaces of the CoCr alloys can be observed clearly in Fig. 4(a),which is an unavoidable problem in the SLM process. Fig. 4(b) showsthe EDX results, and the chemical composition of the spherical powderparticles adhering to the samples surface was determined. Analysis ofthese particles revealed a composition of C=5.90%, Cr= 28.23%,Mn=1.13%, Co= 58.46% and Mo=6.28%.

The initial morphology of the sample with a concave surface isshown in Fig. 5(a). Similar to the convex surface, many un-meltedparticles, with a composition of C= 4.21%, Si= 0.58%, Cr= 28.11%,Mn=0.84%, Co=59.56% and Mo=6.70%, can be observed(Fig. 5(b)). Fig. 6(a) shows the initial morphology of the sample with aslant surface and the EDX result shows that the particles have a com-position of C= 3.48%, Si= 0.58%, Cr= 28.25%, Mn=0.86%,Co=61.01% and Mo=5.82% (Fig. 6(b)).

Based on the results obtained by EDX, it is confirmed that thecomposition of the spherical particles adhering to the surface was si-milar to the original powder, which verified that the particles weremainly from the unmelted powder during the SLM fabrication process.The adhered spherical particles, forming peaks and valleys, led to the

Fig. 8. Roughness reduction of the samples with complex surface geometry(convex, concave and slant surface).

Fig. 9. Scanning electron micrographs of polished convex samples with EDX spectrum. Morphology of the polished surface at (a) 200× and (b) 500× magnification.(c) Chemical composition of as-received and polished surface.

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unsatisfactory surface quality of the as-received samples.Laser polishing conducted on the nonplanar surface was based on

the results obtained on the planar surface. The optimized polishingparameters obtained from the planar samples were exploited to processthe samples with complex surface geometry, as a reference. Moreover, afine polishing strategy was added to achieve a smoother surface.

In order to evaluate the quality of the polished surface, the rough-ness of the complex surface geometry was characterized using opticalprofiling. The optical profiling device used was a Zygo white light in-terferometer system (NEXVIEW). The surface topography repeatabilitymeasured by the system was 0.1 nm. The low-frequency shape factorwas removed by filtering when testing nonplanar surfaces.

Comparison of the complex surface geometry morphology of the as-received and laser polished samples is shown in Fig. 7. It is clear thatthe surface roughness of the polished samples was evidently decreasedcompared with the as-received ones using the layered polishingmethod. The surface roughness Sa of the convex, concave and slantsurface samples was reduced from 4.65 μm, 4.69 μm and 4.51 μm downto 0.28 μm, 0.30 μm and 0.29 μm respectively, and the roughness

reductions were 93.9%, 93.7%, 93.6% respectively (Fig. 8). As ex-pected, surface roughness Sa less than 1.0 μm obtained on planarsamples was successfully achieved on the samples with complex surfacegeometry. Moreover, it should be noted that the samples with differentshapes have almost the same original surface morphology. Both theinitial surface quality and the process parameters are critical inachieving a satisfactory quality of the polished surface. A similar initialroughness of the as-received samples allowed us to achieve a similarfinal roughness after laser polishing. As suggested by Karthik et al.(2015), smaller initial roughness can decrease the final roughness afterlaser peening without coating. The fundamental limit of the surfaceroughness for a rough surface of Ra=2 μm after laser polishing wasreported to be reduced by a factor of 8 to Ra=0.24 μm (Pauli, 2014).

As CoCr alloys are widely applied in the dental field, the surfaceroughness is an important factor, affecting the adhesion of bacteria onthe surface of dentures. The amount of bacteria adhesion on the surfacedecreases with reduction of the surface roughness. In this case, laserpolishing can be an effective method to smooth the surface, therebyinhibiting bacterial adhesion (Ţălu et al., 2015).

Fig. 10. Scanning electron micrographs of polished concave samples with EDX spectrum. Morphology of the polished surface at (a) 200× and (b) 500× magni-fication. (c) Chemical composition of as-received and polished surface.

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Figs. 9–11(a), (b) show the SEM micrographs of the laser tracks afterpolishing with the optimized parameters, confirming the powder par-ticles of different diameters on the surface of the CoCr alloy weremelted during the laser polishing process, which smoothed the as-re-ceived surface. The polished tracks are parallel to the scanning direc-tion. From the SEM observation, it is clearly shown that there are twodistinct areas in the polished surface, the smooth area and the polishedtracks. During the laser polishing process, laser radiation heats up thesurface materials and melts the spherical particles and clusters, and themelted material flows from the peaks to the valleys under surfacetension, smoothing the surface and reducing the surface roughness. Thepolishing tracks are unified without changing along the curvature of thesurface morphology. There are no apparent boundaries between thedifferent layers by using the layered polishing method. The as-receivedsamples and the polished samples exhibited some ripple formation. Itappears to be related to the polarization of the laser beam, since linearpolarization usually creates ripples of light induced periodic surfacestructures (Buividas et al., 2014). By definition, the period, orientation,morphology, and composition of the ripple formation depends stronglyon both the light and material properties. The period depends on thepolarization of the light field E and its projection on to the surface. The

material properties define the energy deposition via absorption and therefractive index as well as thermal properties at the interface. A mor-phology of the ripple pattern occurring can be partially controlled byselecting a suitable laser fabrication wavelength, irradiation conditionsand the exposure dose. Following this principle, the morphology thatoccurred in the as-received samples and polished samples can be ex-plained.

By EDX analysis, the percentage distribution of the elements isshown graphically in Figs. 9–11(c), indicating the composition contrastbefore and after laser polishing. The metallic composition does notdiffer to a great extent for the convex, concave and slant samples,compared with the as-received samples. The Cr content in the smoothedsurface of the polished slant sample is lower than the as-received one by4.1%–9.5%, and the Co content remains almost the same. The EDXanalysis of the laser polished area indicates that the C content experi-ences a decrease of 6.4%–7.2% from the as-received samples, whichwas oxidized during the laser polishing process. However, the O contentis significantly affected by the treatment for all the samples with threedifferent types of surfaces. The O content obviously increases, and isalmost twice as much as the as-received ones. It indicates that the Ocontent accumulates during the laser polishing, probably forming an

Fig. 11. Scanning electron micrographs of polished slant samples with EDX spectrum. Morphology of the polished surface at (a) 200× and (b) 500× magnification.(c) Chemical composition of as-received and polished surface.

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oxide layer with an amount of cobalt, chromium and molybdenumoxide. As a material used in the dental field, the superior surfaceproperties are significant. The surface oxidation can improve the dur-ability of the CoCr alloys (Honda et al., 1988). This is especially the casewhen exposed to the corrosive environment of saliva, where the oxi-dation is beneficial for corrosion protection in CoCr alloys (Díaz et al.,2014).

In comparing the above EDX results in Figs. 9–11, it can be observedthat the amounts of Mo for the convex and slant samples disappearedunexpectedly in the measured areas after laser polishing. It may be dueto the increase in the formation of metal oxide at the selected samplesurface (Adams et al., 2013). The metal oxide layer can thus affect the

EDX detection of Mo underlying the metal surface in those selectedareas. Also, the distribution of elements can differ in different selectedareas during the EDX measurement (Dilawary et al., 2017).

The microstructure of the polished complex surface geometry wasfurther analyzed and characterized cross-section alloy using a me-tallographic microscope. Four distinct zones can be observed in Fig. 12.Moreover, the main chemical compositions were analyzed by using EDX(Fig. 13).

Zone A is the outermost layer, with a thickness of 13 μm, where theoxidation occurs. It is verified in the EDX result that the O content nearthe surface is significantly higher than the interior (Fig. 13). Zone B isthe melted layer where the material was remelted and resolidified. The

Fig. 12. Cross-section metallography of the polished sample with concave surface. (a) Zone A- oxidation layer, (b) Zone B- melted layer, (c) Zone C- heated affectedlayer and (d) Zone D- initial substrate of the as-received sample.

Fig. 13. EDX result of the cross-section of the polished sample with concave surface. (a) cross-section of the polished sample and (b) the main chemical compositionof the cross-section from the surface to the inside layers.

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polishing track in this zone is prominent. The thickness of the meltedlayer is about 77 μm. Zone C is the heat affected layer where the sub-strate material of the as-received sample was partially remelted, and thethickness of this section is around 65 μm. The section defined as Zone Dis the initial material with the typical structure of a SLM component,which is “finger-like”.

In regard to applications in the dental field, the testing was con-ducted to analyze the hardness of the polished CoCr alloys, calculatedby the Eqs. (1) and (2).

= × ≈ ×HV Fd

Constant Test load FSurface of the indentation

0.1891 2 (1)

=+d d d( )2

1 2(2)

where F is the test load, d1 and d2 are the diagonal lengths (shown inFig. 14), d is the mean diagonal length.

The hardness testing was conducted by using a “Mitutoyo” HM-221Series Micro Vickers Hardness. The hardness measurements were car-ried out every 100 μm from the surface to the inside. When conductingthe hardness measurement, experimental error was taken into con-sideration. The hardness measurement of five different points at thesame depth was made at a constant spacing of every 50 μm to avoid theerror. The average hardness values measured for each depth are shownin the Fig. 15. The hardness shows a decreasing tendency from thesurface to the inside. The shallowest surface, corresponding to themelting layer, shows the highest average hardness value of 413 HV. Thehardness values remain almost the same at 150 μm depths, mainly be-cause in this heat affected zone, partial remelting occurs in some areaswhich enhances the hardness. Then, the average hardness value is re-duced to 400 HV, and finally, at a depth of 350 μm from the surface,there is a consistent hardness with an initial value of 384 HV. Thesurface hardness is clearly enhanced after the laser polishing, comparedwith the initial hardness of the as-received samples.

The components fabricated by SLM have an unavoidable porousstructure, due to the unmelted powders and pores formed during theprinting process. By analyzing the metallographic and EDX results, thesurface microstructure of the SLM components was significantly en-hanced, which improved the hardness of the samples. The shallowsurface was remelted and resolidified, forming a more compact struc-ture.

Laser polishing is a process similar to laser peening without using acoating (LPwC) and can increase the surface hardness. Also, the surfaceroughness can decrease under optimum energy density (Karthik et al.,2015). It is obvious that laser polishing can improve the surface hard-ness of CoCr alloys fabricated by SLM. The surface properties weresignificantly improved, including the surface roughness and the hard-ness. However, in dental applications, the compatibility between den-tures and real teeth must be taken into consideration. The outer layer ofthe human tooth is the enamel with a maximum hardness of 367.8 HV(Zhang et al., 2002), which is the hardest and most calcified tissue inthe human body. Compared with this, the surface hardness of the laserpolished sample is higher. To achieve the final suitable hardness for thelaser polished SLM components, both the laser polishing strategy andthe hardness of the as-received SLM samples must be reasonably con-trolled. The initial hardness of the as-received SLM components can beadjusted by using the appropriate 3D printing parameters (Liveraniet al., 2016). It should be noted that high temperature gradients duringSLM processing can lead to high residual stresses. The residual stressesformed on the sample surface are expected to be tensile and in the di-rection of the scanning (Yadroitsev and Yadroitsava, 2015).

4. Geometric model for laser polishing

Based on previous research, layered polishing can be an effectiveway to conduct surface treatment for complex surface geometries, andan appropriate laser polishing strategy is very important. For laserpolishing, another key parameter is the defocusing distance, which has

Fig. 14. Size of indentation shape for hardness testing.

Fig. 15. Vickers hardness values at different depths of the polished sample.

Fig. 16. (a) The sample with a concave surface and (b) the geometric model for laser polishing.

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a significant effect on the final surface quality, meaning that the defo-cusing distance should remain unchanged during the polishing process.The surface would receive constant laser energy due to a stable defo-cusing distance during the process, and is beneficial in finally achievinga satisfactory and uniform surface. It is easy to obtain stable polishingparameters when the surface is planar, however, the circumstances aremore complicated when processing components with complex surfacegeometry. The most challenge problem is to keep the defocusing dis-tance constant along with the morphology of the complex surfacegeometry. However, constant changing of the defocusing distance isunavoidable when polishing nonplanar surfaces using a 2D laser device.The rate of the change depends on the curvature of the complex surfacegeometry. Thus, the most important thing is to discover the relationshipbetween the laser focus position and the surface to be polished. Thisobservation allows us to develop a simple and effective geometricpolishing model to analyze the relationship. In this study, the geometricmodel was built based on the samples with the concave surface to il-lustrate the relationship (Fig. 16(a)).

The first step is to develop an equation that matches the sample withthe concave surface. As it involves surface treatment, only an equationof the graphical outline is needed when developing the geometricmodel. The equation of the graphical contour of the spherical concavesurface is shown below in Eq. (3).

− + − + − =x a y b z c r( ) ( ) ( )2 2 2 2 (3)

In this equation, r represents the spherical radius and z< r, (a, b, c)represents the spherical coordinates. The next step is to set up themodel coordinate system, which is fairly important to facilitate thecalculation in the following work.

To simplify the calculation, the origin coordinates are set at thebottom center of the spherical concave surface in this case, so thespherical coordinate (0, 0, r) is on the z-axis. The Eq. (3) is simplified toEq. (4).

+ + − =x y z zr2 02 2 2 (4)

In order to precisely control the defocusing distance during thepolishing each time, the concave surface was divided into a number oflayers. The thickness of each layer (ti) should not be larger than thefocal depth of the laser (ti < focal depth). In this study, the thickness ofeach layer ti = 0.2 mm. Different heights on the concave surface cor-respond to the modified area to be polished. For each time, the laserheight and layer to be polished need to be adjusted (Fig. 16(b)). Only inthis way, can the defocusing distance remain constant, and the energyradiating on the surface remain stable. In the geometric model, a dif-ferent value of z corresponds to a different curved surface. Specifically,when the height of the right defocusing point is hi (z= hi), the corre-sponding area to be polished can be calculated by the Eq. (5), which is

+ = −x y h r h2 i i2 2 2 (5)

The radius of the corresponding circle ri is (2hir− hi2)1/2, and thearea to be polished is π(2hir− hi2). Through controlling the defocusingpoint, the polishing area extends gradually. Eventually, the wholeconcave surface can be polished based on a constant defocusing dis-tance. It is important to make sure that the polished area deviates fromthe focus point during the polishing process

Actually, this layered polishing method can also be applied on thesamples with convex surfaces and slant surfaces, where a similar geo-metric model can be developed based on the same mechanism. Themost fundamental principle in this layered polishing method is the re-lationship between the position of the laser focus point and the surfacearea to be polished for each layer.

The above analysis and modelling approach helps to simplify,benchmark and validate the changes of surface morphology and ele-mental composition for the CoCr alloy components with complex sur-face geometry, before and after laser polishing. This study also clarifiesthe interaction between the laser beam and the CoCr materials. The

overall results can facilitate the laser polishing process and minimizethe laser energy consumption, thereby achieving a superior surfacefinish and an enhanced surface hardness.

5. Conclusions

This paper presents the application of laser polishing on Cobalt-Chromium alloys with complex surface geometry. The laser polishingparameters were firstly optimized on a planar surface by using ortho-gonal testing methodology. A laser polishing strategy was developedand identified in order to conduct a further study on samples withcomplex surface geometry by using a 2D laser system.

The optical profiling and SEM results show that the developed laserpolishing strategy (called the layered polishing method) is an effectivemethod to reduce the surface roughness of CoCr alloys with complexsurface geometry using a laser. Surface roughness reductions up to 93%were achieved on complex surface geometry, which was, as expected,similar to the results obtained on a planar surface.

The surface hardness of the laser polished samples was enhanced by8%, compared with the as-received SLM components. The hardnessvalues exhibited a decreasing tendency from the polished surface to theinside layers.

Finally, a simple and effective geometric model was built to un-dertake the laser polishing of components with complex surface geo-metry by using a 2D laser system. Moreover, the layered polishingmethod can be developed into a generic one which can be suitable forlaser polishing of any freeform surface by establishing the corre-sponding model.

Acknowledgments

This work was funded by the Hong Kong Innovation andTechnology Fund (ITF) under project number ITS/369/15.

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