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ScienceDirect Materials Today: Proceedings 2S (2015) S156 – S161

Conference MEFORM 2015, Light Metals – Forming Technologies and Further Processing

Effect of machining parameters on surface properties in slide diamond burnishing of aluminium matrix composites A. Nestler, A. Schubert* Professorship of Micromanufacturing Technology, Technische Universität Chemnitz, Reichenhainer Str. 70, 09126 Chemnitz, Germany

Abstract The implementation of light weight construction often requires the use of composites such as aluminium matrix composites. However, machining of suchlike materials is more difficult compared to unreinforced alloys. The heterogeneous structure of the metal matrix composites involves high tool wear and surface imperfections. For an increase of the fatigue strength, smooth surfaces and strongly compressive residual stresses in the surface layer are beneficial. The limits of cutting processes concerning surface properties can be overcome by using forming processes like slide diamond burnishing, which enables a highly efficient surface finishing of even thin-walled components. The effect of machining parameters and diamond sphere radius on surface properties are analysed. Experimental investigations in slide diamond burnishing of AA2124 with 25 % volume proportion of SiC particles show that surface roughness values and imperfections like voids can be reduced significantly. Furthermore, absolute values for residual stresses generated in the surface layer are in the order of the yield strength of the aluminium alloy. The application of slide diamond burnishing offers high potential for an increase of the tolerable load or a reduction of the mass of reliable components. © 2014 The Authors. Published by Elsevier Ltd.

© 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license Selection and peer-review under responsibility of the Conference Committee of Conference MEFORM 2015, Light Metals – (http://creativecommons.org/licenses/by-nc-nd/3.0/). of the Conference Committee Conference MEFORM 2015, Light Metals – Forming Selection peer-review under open access articleofunder the CC BY-NC-ND license Formingand Technologies andresponsibility Further. This is an Technologies and Further

(http://creativecommons.org/licenses/by-nc-nd/3.0/).

Keywords: Aluminium; Aluminium matrix composite; Finishing; Forming; Machining; Metal Forming; Metal matrix composite; Residual stress; Roughness; Slide diamond burnishing; Surface integrity

* Corresponding author. Tel.: +49-371-531-34580; fax: +49-371-531-23549. E-mail address: [email protected]

2214-7853 © 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of the Conference Committee of Conference MEFORM 2015, Light Metals – Forming Technologies and Further doi:10.1016/j.matpr.2015.05.033

A. Nestler and A. Schubert / Materials Today: Proceedings 2S (2015) S156 – S161

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1. Introduction Aluminium matrix composites (AMCs) are lightweight materials, which consist of a comparatively soft aluminium alloy as base material and a reinforcing component like particles, fibres or whiskers. Composites offer the advantage of tailored properties due to an appropriate selection of their constituents and determination of the proportion. In most cases, AMCs are characterised by a higher Young’s modulus, yield strength, fatigue strength, creep strength and resistance to abrasive wear compared to unreinforced aluminium alloys. However, despite this for many applications beneficial performance market share of AMCs is still relatively low. In addition to increased costs of suchlike composites, machining of AMCs involves a higher tool wear and surface imperfections like voids, microcracks and fractured particles. But, surface properties have high effect on components’ reliability concerning for instance tribological or corrosion behaviour and fatigue strength. Light weight components with a high fatigue strength require surfaces with low roughness values, strongly compressive residual stresses and an absence of significant imperfections. In machining of AMCs, these surface properties can be influenced markedly by an appropriate tool design and cutting parameters [1]. For overcoming technological boundaries of cutting processes regarding surface roughness and residual stresses a forming process can be added subsequently. An emerging finishing process is slide diamond burnishing, which enables machining of a variety of geometries and even thin-walled components. Kinematics of this process corresponds to turning, but there is no chip removal. Korzynski et al. investigated slide diamond burnishing of a heat treatable steel 42CrMo4 with a cylindrical and spring supported sliding component made of PCD [2]. The results show that surface roughness values could be reduced significantly. The surface layer was characterized by an increased microhardness and strongly compressive residual stresses. These surface properties involved a higher fatigue strength. Luo et al. examined slide diamond burnishing of aluminium and copper alloys using a cylindrical, but rigidly mounted PCD sliding component [3]. Surface roughness values revealed a markedly reduction due to burnishing. However, burnishing depth, feed and the radius of the sliding component have to be chosen carefully to avoid material adhesion on the tool and minimize roughness values. The effect of machining parameters on the surface roughness of an aluminium alloy burnished with a PCD spherical sliding component was analysed by Yu and Wang [4]. This research indicated that the reduction of surface roughness values can be maximized by using a small feed and a larger sphere radius of the diamond tool. However, there is no research in slide diamond burnishing of heterogeneous materials and in particular AMCs. The aim of the investigations is the determination of appropriate machining parameters for slide diamond burnishing of AMCs considering surface roughness, imperfections and residual stresses in the surface layer. Furthermore, basic mechanisms and special characteristics in burnishing of suchlike heterogeneous materials shall be identified. Nomenclature ap f F Ra Ratheor Rz Rztheor

depth of cut feed burnishing force arithmetic mean surface roughness theoretic arithmetic mean surface roughness surface roughness depth theoretic surface roughness depth

SR v vc

Vrs, axial direction Vrs, circumf. direction Vrs, 1

sphere radius of the diamond burnishing speed cutting speed residual stress in the axial direction residual stress in the circumferential direction first principal residual stress

2. Experiment The AMC used consist of an aluminium alloy AA2124 as matrix material and SiC particles with a volume proportion of 25 % and a dominant size of two to three microns. This composite is manufactured by a powder metallurgy route including a high energy mixing process and subsequent hot isostatic pressing for powder consolidation. In order to increase the strength of the material, billets are extruded and heat treated to the condition T4 (solution annealing, quenching and natural ageing). Premachined specimens used for slide diamond burnishing have a diameter of 23 mm and a length of 20 mm. Both faces exhibit a chamfer with the size of 1 mm and an angle of

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45°. The values for the surface roughness depth Rz of the specimens before burnishing were in the range of about 6 μm to 8 μm. For clamping purposes, one face of each specimen comprise a blind hole with a diameter of 8 mm. The experiments in slide diamond burnishing were carried out on a SPINNER precision lathe of the type PD 32. Specimens were clamped to one side with a mandrel, which allows burnishing of the whole cylindrical length. Turning as premachining and slide diamond burnishing were done in one clamping process to minimize the concentric run-out in burnishing. The turning tool was held in the disc turret and the burnishing tool was mounted on a three-axis force dynamometer of the type Kistler 9257A. Because of the highly abrasive effect of the silicon carbide particles, CVD (chemical vapour deposition) diamond tipped indexable inserts were used for turning. The inserts have a polished rake face and a very sharp cutting edge with a radius of about 3 μm. The type of the inserts for finally premachining was VCGW 110304 screwed on a tool holder of the type SVVCN 1212 F11. Finally premachining was done with constant feed f = 0.14 mm, cutting speed vc = 150 m/min and depth of cut ap = 0.25 mm. For slide diamond burnishing a tool of the kind VKT20LH delivered by Baublies AG was used. The tool head was adjusted to an angle of 90° between the burnishing force direction and the feed direction, which corresponds to the axial direction. The fixture for the diamond tipped body is mounted on a laminated disc spring with a certain preload. The monocrystalline diamonds are characterized by a polished contact area and a spherical shape. In order to analyse the effect of different diamond sphere radii, the diamond tipped body can be changed. Machining parameters were varied within the ranges recommended by the burnishing tool manufacturer. Diamond tipped bodies with different sphere radii SR were used to examine the effect of the contact conditions on surface properties. Machining parameters and diamond sphere radii are presented in Table 1. For planning of the investigations a fractional factorial design of experiments has been used. The influence of machining parameters on the surface properties was analysed using the diamond tipped body with the medium diamond sphere radius (SR = 2 mm). In addition, diamond sphere radius was varied for different feeds leaving the other parameters unchanged. The experiments were done using emulsion cooling with a concentration of approximately 5 % to reduce the tendency for adhesion of the aluminium based material on the turning and burnishing tools. All parameter combinations defined by design of experiments were tested thrice for statistical validation. During slide diamond burnishing force components were monitored and recorded. Table 1. Machining parameters for slide diamond burnishing (bold type: base level) Process parameter

Levels

Burnishing force F [N]

50, 150, 250

Burnishing speed v [m/min]

50, 100, 150

Feed f [mm]

0.05, 0.1, 0.15

Sphere radius of the diamond SR [mm]

1, 2, 2.75

The surface roughness in the axial direction was measured with a stylus instrument of the type Mahr LD 120. The measuring length was 4 mm and filtering of the profile is in accordance to ISO 11562. Because of the comparatively high influence of surface imperfections on the roughness values each specimen was measured thrice at different positions. In addition, three-dimensional surface profiles were generated using the same measurement equipment including a supplemental cross table. For these measurements, the distance of the measuring points was 1 μm in both circumferential and axial direction. The measuring field was 2 mm in the axial direction and 0.5 mm in the circumferential direction. For the representation of the surface structure, details with a size of 500 μm in the axial direction and 300 μm in the circumferential direction were chosen. Subsequently, form was removed using a polynomial of the second degree. Furthermore, SEM (scanning electron microscope) micrographs of the specimens’ surfaces were generated to analyse the surface structure and imperfections. The residual stresses in the surface layer were determined by X-ray diffraction analysis. The measurements were done with a cobalt anode in the lattice planes {420} of the aluminium alloy using sin² \ method. Thereby, an area with a diameter of about 2 mm was detected.

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3. Results and discussion 3.1. Influence of machining parameters and diamond sphere radius on surface structure A detailed estimation of the results in slide diamond burnishing requires sufficient information about the structure of the premachined surface. According to typical roughness values for medium machining and preferred numbers of surface roughness depth Rz a theoretical value of Rz = 6.3 μm was defined. This corresponds to a feed of about 0.14 mm under consideration of the tool corner radius, which amounts 0.4 mm. However, because of surface imperfections measured roughness values Rz were in the range of approximately 6.3 μm to 7.7 μm. Details of the surface structure of a premachined specimen are presented in Fig. 1. The three-dimensional surface profile shows distinct feed marks whose distance in the axial direction complies with the feed f = 0.14 mm. Furthermore, form deviations of the arc-shaped tool corner and cutting edge chipping reflect in the surface profile. In addition to the three-dimensional surface profile, SEM micrographs reveal numerous voids with different size and shape. 10

a)

μm

c)

b)

6 4 2

300 μm

25 μm

0

Fig. 1. Surface structure of a premachined specimen (ap = 0.25 mm, f = 0.14 mm, vc = 150 m/min): (a) 3D surface profile; (b) SEM micrograph (SEM magnification 100); (c) SEM micrograph (SEM magnification 1000).

Referring to Table 1 burnishing force, burnishing speed, feed and the sphere radius of the diamond were varied. For each process variable three levels were tested using a fractional factorial design of experiments. The influence of each machining parameter was analysed separately keeping the other variables constant at their base levels. The effect of the diamond sphere radius on the surface properties was investigated for all feeds tested using the base levels of the burnishing force and speed. The influence of the feed and the diamond sphere radius on the surface roughness values Ra and Rz is presented in Fig. 2. The bars represent the scattering of the measured values, starting at the minimum value and ending at the maximum value. Each bar incorporates nine measurements (three measurements per specimen at different locations). For arithmetic mean surface roughness Ra measured values approximately correspond to the calculated values using the equation

0.5 0.4 0.3

f2 . 31.2 SR

(1) b)

SR SR = 2 mm (Measured values) SR = 2 mm (Calculated values) SR = 2.75 mm (Measured values) SR = 2.75 mm (Calculated values)

0.2 0.1 0

0.05 mm

0.1 mm Feed f

0.15 mm

Surface roughness Rz (μm)

a)

Surface roughness Ra (μm)

Ra |

2 1.6 1.2

SR SR = 2 mm (Measured values) SR SR = 2 mm (Calculated values) SR SR = 2.75 mm (Measured values) SR SR = 2.75 mm (Calculated values)

0.8 0.4 0 0.05 mm

0.1 mm Feed f

0.15 mm

Fig. 2. Influence of burnishing feed and diamond sphere radius on surface roughness (F = 150 N, v = 100 m/min): (a) Arithmetic mean surface roughness Ra; (b) Surface roughness depth Rz.

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a)

3.5 μm 2.5 2 1.5 1 0.5 0

c)

b)

Fig. 3. Influence of burnishing feed on surface structure (F = 150 N, v = 100 m/min, SR = 2 mm): (a) f = 0.05 mm; (b) f = 0.1 mm; (c) f = 0.15 mm.

The measured values for Ra consider the complete sampled data, which leads to a relatively small scattering. In contrast, measured values of surface roughness Rz show a large scattering. This can be attributed to the definition of Rz representing the mean value of the five maximum heights of the roughness profile within the sampling lengths. Consequently, large voids detected by the stylus lead to a significant increase of the roughness values Rz. Furthermore, the average values of the measurements are higher than the calculated values referring to the equation

Rz |

f2 . 8 SR

(2)

Both diagrams show that an increase of the diamond sphere radius and a decrease of the feed result in a reduction of the surface roughness values for Ra and Rz. This is in accordance with the calculations. Slide diamond burnishing with feeds of 0.1 mm and 0.15 mm led to a kinematic roughness represented in Fig. 3. The distance between the valleys and the ridges respectively corresponds to the feed. The 3D surface profiles reveal a smooth and waved surface structure for the axial direction with rounded valleys and ridges because of the polished diamond tool surface and the higher diamond radius in comparison to the turning tool. These are significant differences to the premachined specimens. The surface profile for a feed of only 0.05 mm shows a smooth surface without a distinct kinematic roughness. Using a diamond with a sphere radius of only 1 mm led to a very high surface pressure resulting in a deep penetration of the tool in the surface layer and adhesion of workpiece material on the diamond tool, especially for the feeds 0.1 mm and 0.15 mm. The resulting surface structure is characterized by a distinct and irregular burr formation at the feed marks. Suchlike surfaces exhibit only a few small voids, but surface roughness values are increased significantly. For this reason, the illustration of surface roughness values is dispensed. The influence of burnishing force on surface roughness values is presented in Fig. 5. It can be seen that the lowest average surface roughness values were obtained with medium burnishing force. This can be explained with the help of SEM micrographs (Fig. 4). In diamond slide burnishing with a force of 50 N a lot of voids remain in the surface. The burnishing force is too small for a sufficient closure of the voids, which results in higher surface roughness values. Using a burnishing force of 150 N results in smooth surfaces with only a few small voids. Consequently, surface roughness values can be reduced. Slide diamond burnishing with a force of 250 N entail a closure of the voids, too. But, the very high force and the resultant contact pressure lead to a scaling of the surface caused by material fatigue. This involves slightly higher surface roughness values. For the variation of burnishing speed, no significant influence regarding surface structure could be revealed. a)

b)

25 μm

c)

25 μm

25 μm

Fig. 4. SEM micrographs of surfaces generated with different burnishing forces (f = 0.1 mm, v = 100 m/min, SR = 2 mm): (a) F = 50 N; (b) F = 150 N; (c) F = 250 N.

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Burnishing force F

2

50 N

1.6

Residual stresses (MPa)

Surface roughness Rz (μm)


1.2 0.8

0.4 0 50 N

150 N Burnishing force F

250 N

Fig. 5. Influence of burnishing force on surface roughness (f = 0.1 mm, v = 100 m/min, SR = 2 mm).

150 N

250 N

0

-100 -200

-300 -400

-500

σrs, circumferential direction

σrs, axial direction

σrs, 1

Fig. 6. Influence of burnishing force on residual stress state (f = 0.1 mm, v = 100 m/min, SR = 2 mm).

3.2. Influence of machining parameters on residual stresses Fig. 6 shows the influence of the burnishing force on the surface residual stresses. The column groups in the diagram represent the results of in each case one measurement. For a comparison of the residual stress state the residual stresses in the circumferential direction and in the axial direction as well as the first principal residual stress were chosen. All measured values are negative and consequently residual stresses in the aluminium alloy are compressive. The absolute values of the residual stresses in the axial direction are higher than the absolute values of the residual stresses in the circumferential direction. In comparison to the stress state in the surface layer of a premachined specimen (Vrs, axial direction ≈ -32 MPa, Vrs, circumferential direction ≈ -95 MPa) absolute values of burnished surfaces are much higher for all burnishing forces. The largest absolute values of residual stresses were obtained with a burnishing force of 150 N because of a strong deformation of the surface layer. The absolute values of the first principal residual stress are in the order of the yield strength of the aluminium alloy. For specimens burnished with a force of only 50 N absolute values are slightly lower except residual stresses in the circumferential direction. The largest burnishing force investigated led to the lowest absolute values of the residual stresses. This may be explained by material fatigue resulting in a scaling at the surface layer, which allows a stress relaxation. 4. Summary and conclusions The experimental results show that slide diamond burnishing of AMCs leads to a significant reduction of surface roughness values and voids. The main influential variable for surface roughness is the burnishing feed. An appropriate determination of the burnishing force is very important for the closure of voids and avoidance of material fatigue. Furthermore, slide diamond burnishing of AMCs involves strongly compressive residual stresses in the order of the compressive strength. Consequently, this process enhances the performance of components with vibrational loading. However, residual stress distribution in the surface layer has to be examined more in detail. Acknowledgements The authors acknowledge the DFG (German Research Foundation) for supporting this work carried out within the framework of Project CRC 692. References [1] A. Schubert, A. Nestler: Enhancement of Surface Integrity in Turning of Particle Reinforced Aluminium Matrix Composites by Tool Design. Procedia Engineering 19 (2011), 300–305. [2] M. Korzynski, J. Lubas, S. Swirad, K. Dudek, Journal of Materials Processing Technology 211 (2011), 84-94. [3] H. Luo, J. Liu, L. Wang, Q. Zhong,. International Journal of Advanced Manufacturing Technology 25 (2005), 454-459. [4] X. Yu, L. Wang, International Journal of Machine Tools & Manufacture 39 (1999), 459-469.