Grinding Stress 1

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Understanding Grinding Stress
  In: Powder Metallurgical High Performance Materials, Volume 2, 16th International Plansee Seminar. Eds. G. Kneringer, P. Rödhammer, H. Wildner, Plansee, Reutte, Austria, 2005, pp. 1075-1085. 1075 Residual stresses in hardmetals caused by grinding and EDM machining and their influence on the flexural strength Dongtao Jiang a , Guy Anné a , Jef Vleugels a , Kim Vanmeensel a , Wesley Eeraerts b , Weidong Liu b , Bert Lauwers b , Omer Van der Biest a   a  Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven Kasteelpark Arenberg 44, B-3001 Leuven, Belgium b  Department of Mechanical Engineering, Katholieke Universiteit Leuven Celestijnenlaan 300 B, B-3001 Leuven, Belgium Summary Melting is observed to be the primary material removal mechanism during EDM machining of WC-Co hardmetals. Upon rapid solidification, considerable thermal residual stresses on the surface layer can be expected. There is however very little information in literature on the residual thermal stresses of EDMed hardmetals, which would inevitably influence the performance in service. In this study, experimental measurements revealed that a significant compressive stress is present in ground materials, whereas a tensile stress is measured on EDMed surfaces. Moreover, the compressive stress in the ground materials is strongly enhanced compared to that of polished materials. The high compressive stresses on the surface of the ground samples results in a higher bending strength whereas the tensile stress in the EDM samples deteriorates the strength, explaining the general strength reduction of EDMed samples compared to ground samples. The flexural strength reduction is found to be proportional to the thermal residual stress generated during EDM, and is strongly influenced by the EDM machining strategy applied. Comparing different WC-Co hardmetal grades, the relative amount of residual tensile stresses generated after EDM finish cutting and the strength reduction compared to ground samples increased with decreasing WC grain size due to a lower thermal conductivity. Keywords Hardmetal, thermal residual stresses, strength, electrical discharge machining, grinding  1076 1. Introduction Electro-discharge machining (EDM) is a cost-effective way of shaping complex geometry materials with high accuracy, and is widely used in the hardmetal industry. Melting is observed to be the main material removal mechanism in EDM machining of hardmetals. Upon rapid solidification, considerable thermal residual stresses can be expected on the surface layer, which will influence the component properties. Although there have been numerous studies on the EDM of hardmetals [1,2,3], there is very little information in literature on the residual stresses of hardmetals after EDM, which would inevitably influence its performances in service. In this study, 6 WC-Co hardmetal grades with different thermal conductivity were used to evaluate the effect of surface residual stresses after polishing, grinding and different EDM finishing regimes on the flexural strength. 2. Experimental The different CERATIZIT hardmetal grades used in this study are listed in Table 1. Representative scanning electron micrographs are given in Fig. 1. All Wire EDM finishing cuts were performed on a ROBOFIL 2030 (Charmilles Technologies, Switzerland) in demi-water with a dielectric conductivity of 5 µ S/cm, using a CuZn37 wire electrode ( ∅  = 0.25 mm, tensile strength of 500 N/mm 2 ). The height of the hardmetal starting material is 35 mm. The EDM parameters of the 4 consecutively performed finish cutting regimes are summarised in Table 2. CERATIZIT grade Co content (wt %)  Average WC intercept length (µm) Thermal conductivity (W m -1 k -1 ) *3-pt bending strength (MPa) GC20 12 0.85 95 4279 ±  61 MG12 6 0.55 90 3078 ±  295 GC32 10 2.17 105 3064 ±  91 MG18 10 0.32 85 3509 ±  168 GC20CR 12 0.93 95 2919 ±  101 SMG13 6.5 0.25 85 2632 ±  545 Table 1. Summary of the hardmetal grades used. *Measured on ground samples  1077 (a)(b) (c)(d) (e) (f) Fig. 1. Scanning electron micrographs revealing the microstructure of the different hardmetal grades: GC20 (a), GC20CR (b), GC32 (c), SMG13 (d), MG12 (e), MG18 (f) The residual stresses in the WC phase were measured by X-ray diffraction, using the d-sin 2 ψ  method. The (300) WC peak, corresponding with a diffraction angle 2 θ  = 133.31° was used to measure the residual stress. The sin 2 ψ  range was varied from 0  1078 to 0.6 in steps of 0.1. 2 θ  was varied between 130° and 136° at 0.02 O /steps of 5 s. As elastic constants ½ S 2  = 2.740.10 -6  Mpa -1  and -S 1  = 0.460.10 -6  Mpa -1  was used. The measurements were carried out on a Siemens D500 XRD. Microstructural investigation was performed by scanning electron microscopy (SEM, XL30-FEG, FEI, the Netherlands). The flexural strength at room temperature was measured in a 3-point bending test. All surfaces of the test specimens (25.0 x 4.7 x 1.7 mm) were ground with a Diamond Board MD40 75 B55 grinding wheel on a Jung grinding machine. The size of the EDM samples is exactly the same. The span width was 20 mm with a crosshead displacement of 0.1 mm/min. The reported values are the mean of at least five bending experiments. EDM regime E13 E21 E22 E23 Surface Roughness Ra ( µ m) ±  0.4 ±  0.2 ±  0.2 < 0.2 Material removed ( µ m) 2 1 0 0 Offset ( µ m) 134 131 131 131 Open voltage (V) 140 140 140 140 Pulse Ignition Height (A) 5 4.5 3.5 3.5 Pulse duration ( µ s) 3 1 1 1 Pulse interval ( µ s) 6.6 4 4 4 Maximum speed (mm/min) 6.1 6.1 6.4 8 Servo Reference Voltage (V) 7 6 6 0 Wire winding speed (m/min) 6.8 6.8 6.6 4.8 Wire Tension (N) 12 10 10 10 Table 2. Applied wire EDM finishing regimes. 3. Results and discussions Representative SEM micrographs of polished cross-sectioned EDM surfaces are presented in Fig. 2. After rough cutting, a 10-20 µm recast layer is formed on the sample surface (Fig. 2.a), which is almost completely removed after finish cutting regime E23 (Fig. 2.b). Small cracks inside the WC grains are frequently observed in the top layer carbide grains after finish cutting, as presented in Fig. 2.c and d. These cracks most probably have to be attributed to thermal cracking of the WC grains during EDM, since they were hardly found on cross-sectioned ground surfaces.


Jul 23, 2017
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