Fracture and fatigue behavior of electrical-discharge machined cemented carbides

Fracture and fatigue behavior of electrical-discharge machined cemented carbides

International Journal of Refractory Metals & Hard Materials 24 (2006) 162–167 www.elsevier.com/locate/ijrmhm Fracture and fatigue behavior of electri...

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International Journal of Refractory Metals & Hard Materials 24 (2006) 162–167 www.elsevier.com/locate/ijrmhm

Fracture and fatigue behavior of electrical-discharge machined cemented carbides B. Casas, Y. Torres, L. Llanes

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Departament de Cie`ncia dels Materials i Enginyerı´a Metallu´rgica, ETSEIB, Universitat Polite`cnica de Catalunya, 08028 Barcelona, Spain Received 11 November 2004; accepted 12 April 2005

Abstract Electrical discharge machining (EDM) is an alternative shaping route for manufacturing complex component shapes of hard and brittle materials such as WC–Co cemented carbides (hardmetals). However, it results in a poor surface integrity that often leads to mechanical degradation of these materials. In this investigation, the influence of multi-pass sequential EDM on the fracture and fatigue behavior of a WC–10wt%Co cemented carbide is studied. It is compared with the behavior exhibited by a reference condition, attained through conventional mechanical grinding and polishing using diamond as abrasive. Considering that rupture is related to existing defects, either introduced during sample fabrication or induced by machining, a detailed fractographic examination is conducted to discern failure origins. The experimental findings indicate that flexural strength of WC–Co hardmetals, under both monotonic and cyclic loading, is strongly affected by EDM. An analysis of the results using a linear-elastic fracture mechanics approach allows to establish a clear connection between surface integrity and mechanical strength. Quantitative discrepancies between the estimated and the experimentally measured critical flaw sizes for all the EDM samples are rationalized through the existence of residual tensile stresses of considerable magnitude at the shaped surface. From a fatigue viewpoint, these residual stresses are even more detrimental because they imply an additional mean stress; thus, higher effective load ratio at the EDMed surface. As a consequence, fatigue sensitivity of the EDMed specimens is higher than for the reference condition. Relief of these stresses through annealing treatments is assessed and shown to be a relatively effective alternative for improving the fracture and fatigue behavior of WC–Co cemented carbides shaped by EDM.  2005 Elsevier Ltd. All rights reserved. Keywords: Fracture; Fatigue; Electrical-discharge-machining; Cemented carbides

1. Introduction WC–Co cemented carbides, usually referred to as hardmetals, are materials of choice for use as tools and structural components by virtue of their unique combination of hardness, strength and wear resistance. Many of these applications often involve extreme mechanical demands at the surface, and therefore, quality and integrity of surface finish resulting from manufacturing processes are of critical interest. Although

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Corresponding author. Tel.: +34 934011083; fax: +34 934016706. E-mail address: [email protected] (L. Llanes).

0263-4368/$ - see front matter  2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2005.04.007

there have been many research studies on surface integrity effects on the mechanical response of these materials, they have been mainly limited to diamond-based machining routes and very little prior work has addressed non-abrasive shaping processes. Considering that non-contact material removal techniques (e.g. electrical discharge machining, laser machining, water jetcutting, etc.) are increasingly approached for overcoming technical difficulties and high costs associated with the elevated hardness and intrinsic brittleness of hardmetals, the referred lack of knowledge may be described as relevant, particularly for applications where dimensional accuracy combined with complex geometries are primary requirements.

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Electrical discharge machining (EDM) is one of the most important abrasionless machining methods for hard and brittle materials [1–3]. The feasibility of implementing EDM for conforming WC–Co cemented carbides has been proven in terms of machining performance indexes [4,5]. However, even for the cases where optimum surface conditions are achieved, EDM of hardmetals usually yields a thermally affected zone, just beneath the shaped surface, with poor surface integrity including unfavorable residual stresses, cracks and craters [6–9]. Although it may be speculated that these features would decrease the strength of cemented carbides, as evidenced for other structural ceramic composites (e.g. Refs. [10–12]), information on mechanical degradation of hardmetals due to such EDM-induced damage is scarce [4,13]. This is especially true regarding fatigue behavior for which there does not exist any previous report in the open literature. It seems clear that such knowledge is essential if the performance reliability associated with the structural application of EDMed cemented carbides is to be improved. Following the above ideas, this paper reports on a systematic experimental test program developed for assessing the influence of EDM, corresponding to optimal surface integrity, on the fracture and fatigue characteristics of a fine-grained hardmetal. In doing so, flexural strength changes under both monotonic and cyclic loading are used as discriminative parameters and surface integrity effects are evaluated with respect to the behavior exhibited by the ground and polished material [14], the surface finish here used as reference condition.

2. Experimental procedure A commercial fine-grained WC–10wt%Co hardmetal grade produced by DURIT Metalurgia Portuguesa do Tungste´nio was used as base material. Two different surface finish variants were investigated. The first one, referred to as EDM, was accomplished by multi-sequential and finely executed EDM using a commercial wiremachine with an advanced pulse-type power supply. The second surface condition, designated as P and used as reference in this work, was attained by conventional grinding and polishing with diamond as abrasive. Surface integrity features for both conditions are different, as it has been reported in detail elsewhere [8], and may be summarized in terms of: (1) presence of 5–10 lm indepth microcracks at the EDM-shaped surface (Fig. 1); (2) average roughness value (Ra) several times higher for the EDM variant (between 0.05 and 0.10 lm) as compared to that determined for the P one (0.01 lm). Fracture and fatigue behavior was evaluated in fourpoint bending using a standard 20 · 40 mm fully articulating fixture. Rectangular bars with dimensions 4 ·

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3 · 45 mm and chamfered edges were tested with the machined surface on the tensile side. Flexural strength was measured using a servohydraulic testing machine at an applied loading rate of 200 N/s. At least ten specimens were tested for each surface condition. Regarding cyclic loading, testing was aimed to determine fatigue limit, defined as the fatigue strength corresponding to an ‘‘infinite’’ life of 2 · 106 cycles. It was done following the staircase or up-and-down method [15] using a large enough number of specimens. Fatigue experiments were carried out employing a resonant testing machine at working frequencies of about 170 Hz and a load ratio [R = (rmin/rmax)] of 0.1. Fracture surfaces for selected broken samples were inspected through SEM. For each specimen examined, possible fracture initiation sites were first traced back, at low magnification, from sequential sets of fracture surface markings. Particular areas of interest were then observed at higher magnification in order to identify and measure strength-limiting flaws.

3. Results and discussion 3.1. Flexural strength degradation associated with EDM-induced residual stresses Data for the flexural strength (rr) of the surface variants investigated are given in Table 1.1 It is clear that the average strength value for the EDM-shaped specimens is considerably lower (more than half) than the one determined for the P surface variant. In agreement with previous literature findings [4,13], these results indicate a pronounced detrimental effect of the EDM process, even after following multi-pass and fine-executed sequences, on the fracture resistance of WC–Co cemented carbides. After mechanical testing, an extended and detailed fractographic analysis of a large number of broken specimens indicated a close relationship between machining route and fracture origin nature. While fracture-controlling defects for the conventionally machined samples were always processing heterogeneities contained within the material volume (Fig. 2a), for the EDMed samples they were surface flaws associated with microcracks and craters induced during the shaping process (Fig. 2b). In order to rationalize the experimental findings, flexural strength results were analyzed on the basis of linearelastic fracture mechanics (LEFM). Brittle materials are known to fail from pre-existing flaws, induced during

1 Table 1 also includes data for a third condition, EDM + TT, corresponding to EDM specimens subjected to further polishing followed by a high temperature annealing treatment, the reasoning behind its study being detailed in Section 3.2.

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Fig. 1. Top- (a) and cross- (b) views of EDM-shaped surface.

Table 1 Nomenclature, flexural strength and critical flaw size, both experimental and estimated (on the basis of LEFM) for the different surface conditions investigated Surface condition

Flexural strength, rr [MPa]

EDM EDM + TT P

1337 ± 25 2284 ± 331 2869 ± 234

Critical flaw size, ac Experimental [lm]

Estimated [lm]

4–13 4–11 7–9

25 8 8

either material manufacturing or subsequent machining processes, and a fracture mechanics approach is generally accepted as the best criterion for failure. This criterion simply states that fracture resistance of a particular material is given by the maximum stress that it may withstand (rr) which depends on its fracture toughness (KIc, an intrinsic material parameter) and the maximum size of the controlling flaw (ac), according to a generic equation of the form KIc = Yrr(pac)1/2, where Y is a geometry factor that depends on the configuration of the flawed sample and the manner in which the loads are applied. For the P specimens, where critical defects were always found within the bulk, the Y factor was chosen as that corresponding to the solution of an embedded circular flaw, i.e. 2/p. Meanwhile, a geometrical factor Y calculated from the empirical equations proposed by Newman and Raju [16] for a semi-elliptical surface crack of depth (a) and width (2c) was used for the EDMed samples. Information on the average crack geometry aspect ratio (a/c) required for estimating ac in this case was directly obtained from the corresponding experimentally measured values. Independent of the nature, geometry andplocation of the critical flaw, a KIc ffiffiffiffi value of 9.21 MPa m, determined following the single edge notched bend (SENB method), was used for the calculation. A detailed description of the SENB technique for evaluating fracture toughness of hardmetals may be found elsewhere [17]. Values corresponding to the calculated and fractographically observed flaw depths (ac) are also included in Table 1. Within the framework of all the experimental

Fig. 2. SEM micrographs of failure initiation sites: (a) internal defect for a P sample, and (b) EDM-induced surface flaw for an EDM specimen.

findings above discussed, a lack of 1–1 correspondence between these values may be attributed to residual stress effects [18]. The good agreement among experimental and predicted values for the hard machined surface variant, where failure origins were discerned to be subsurface intrinsic defects, established the suitability of the above LEFM approach for rationalizing the fracture

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behavior of cemented carbides. On the other hand, for the EDM condition, experimentally determined critical flaw sizes are much smaller than the estimated ones, i.e. EDM-induced residual stresses play an important role on the failure behavior found for this surface condition. Based on the observed discrepancies, the referred residual stresses may be roughly accounted by using the principle of superposition of both applied far-field stress and residual stress intensity factors for the real critical flaw system and equating the resulting total stress intensity factor to the fracture toughness of the material [19]. Implementation of this LEFM-based analysis yields estimated values, rres, ranging from 1 to 3 GPa, in accordance with previous literature works [5]. Considering that these values are of similar order of magnitude to the nominal flexural strength determined for the EDM condition, relief of the EDMinduced residual stresses is an immediate action to attempt if the mechanical strength of EDMed cemented carbides is to be enhanced. 3.2. Strength improvement of EDMed hardmetals through secondary thermal treatments It is well-established for hardmetals that residual surface stresses, either induced by conventional abrasive grinding or associated with mechanical pre-cracking procedures, may be released through high temperature annealing [17,19–21]. Within this context, thermal treatment was pursued as post-EDM procedure for increasing the flexural strength of cemented carbides shaped by EDM [19]. It was carried out by means of in vacuo annealing at 920 C during 1 h, and the corresponding heat treated EDM condition will be referred to as EDM + TT. In addition, and previously to heat treatment, EDMed samples were slightly polished in order to attain a surface texture aspect (Ra) similar to that exhibited by the reference P condition. Mean and standard deviation values for the flexural strength of the EDM + TT condition are also included in Table 1. It is observed that fracture resistance increases with high temperature annealing up to values close to 80% of the one measured for the surface variant P. As before, several EDM + TT broken samples were fractographically examined and critical flaw sizes were both experimentally measured and analytically (on LEFM basis) estimated. Although failure origins were still found to be affiliated to EDM-induced flaws, there was a significant reduction on the relative differences between observed and calculated critical flaw sizes for the thermally treated condition. It directly translates into lower residual stresses, as discerned from the estimated range, corresponding to values always lower than 1 GPa. Hence, the relevant strength improvement achieved for the EDM + TT condition, as compared to the EDM one, may then be rationalized by recourse

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to residual stress relief through high temperature annealing. 3.3. Fatigue behavior of EDMed cemented carbides: effective mean stress effects Assessment of the influence of EDM on the fatigue behavior of the hardmetal studied was conducted in terms of fatigue limits (rf). It was experimentally determined following the up-and-down test method and using groups of more than 15 samples per condition. Additionally, failure origins of selected broken samples were examined by means of SEM. The complete testing sequences are shown in Fig. 3 and the corresponding mean and standard deviation values for the 95% confidence fatigue limit (rf), in terms of the maximum applied stress (rmax), are listed in Table 2. On the other hand, the fractographic inspection allowed one to discern fatigue failure origins of same type, shape and size as those already observed as strength-controlling flaws under monotonic conditions for each particular case. The fatigue study shows that EDM effects on fatigue limit follow similar, and even more pronounced, qualitative trends as those determined on flexural strength. As a consequence of this increased relevance of EDM influence, lower fatigue ratios [(rf/rr)] for the EDM-shaped specimens, including the heat treated ones (Table 2), and a more significant departure from the baseline value measured for the P condition are discerned. Attempting to analyze and understand the experimental findings, the average alternating [ra = (rmax  rmin)/2] and mean [rm = (rmax + rmin)/2] stress data corresponding to fatigue failure for each condition have been plotted in Fig. 4. Experimental results under different load ratio values (0.4 and 0.7) and the predicted fatigue failure loci given by the corresponding Goodman-like relationship for the reference P condition, from a previous work by the authors [14], are also included for comparison. Assuming that fatigue failure at lives lower than 2 · 106 cycles requires at least a combination of alternating and mean stress laying on the predicted failure loci, the data included in Fig. 4 indicate that broken EDM-shaped samples were exposed to higher effective load ratios, i.e. to higher mean stress, than the experimentally imposed R = 0.1. Under these conditions, residual stress effects under fatigue could be simply rationalized in terms of an additional mean stress that for the conditions here studied, EDM and EDM + TT, should imply effective load ratios close to 0.7 and 0.4, respectively. If this were the case, and recalling again the superposition principle above used, the fatigue failure experimental data could then be plotted in terms of an effective mean stress that would take into account the existing residual stresses. The correspondingly modified data is included in Fig. 4. In general, a quite

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1000

Table 2 EDM effects on the fatigue limit and fatigue ratio of the hardmetal grade studied

EDM

Stress (MPa)

800

Surface condition

Fatigue limit, rf [MPa]

Fatigue ratio, rf/rr

EDM EDM + TT P

689 ± 14 1170 ± 58 1827 ± 92

0.52 0.51 0.64

600

Test starts here

1200

400

P EDM+TT EDM EDM+TT ( σm + σres)

R = -1

failure run-out (2 x 106cycle)

1000

R = 0.1

800

0

3

6

9

12

15

18

Number of specimen in sequence

σ a (MPa)

200

EDM ( σm + σres) R = 0.4

600

R = 0.7

400

1600

EDM+TT 200 σ

1400

0

Stress (MPa)

0

500

1000

1500

2000

2500

r

3000

3500

σ m (MPa)

1200 Fig. 4. Goodman-like diagrams for describing mean (and residual) stress effects on the determined fatigue limit for the different surface conditions studied.

Test starts here 1000

failure run-out (2 x 106cycle) 0

3

6

9

12

15

18

Number of specimen in sequence 2200

P

Stress (MPa)

2000

1.00 Fatigue sensitivity (1-σ σ f /σ σr )

800

EDM P EDM+TT

0.75

0.50

0.25

1800

0.00 0.0 Test starts here

failure run-out (2 x 106cycle)

1400 0

3

0.5

1.0

1.5

2.0

2.5

3.0

3.5

σ res / σ f

1600

6

9

12

15

Fig. 5. Relative EDM-induced residual stress effects on the fatigue sensitivity of the hardmetal investigated.

18

Number of specimen in sequence Fig. 3. Up-and-down fatigue tests used to determine mean fatigue limit for the different surface conditions investigated.

satisfactory agreement between it and the predicted failure loci for the hardmetal studied is evidenced, yielding strong support to the above ideas. Moreover, such a finding also underlines the variable relevance of the EDM-induced residual stresses depending upon the level

and type of service condition under consideration. Indeed, the fact that for a given surface condition, the absolute value of EDM-induced residual stresses is independent of the far-field applied stress (i.e. rres/rf will always be higher than rres/rr) is here postulated as the main reason for the lower efficiency of the thermal treatment assessed under fatigue, as compared to its influence on fracture, and consequently for the higher fatigue sensitivity, given by the parameter [1  (rf/rr)], associated

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with EDM-related surface conditions (Fig. 5). From this perspective, optimal performance of EDMed hardmetals under fatigue seems to require either alternative or complementary means to high temperature annealing for residual stress relief and/or final surface integrity optimization. 4. Summary The following summary statements can be made based on the present work. (1) The flexural strength of the hardmetal investigated, under both monotonic and cyclic loading, is strongly affected by EDM. The main reason for such strength degradation is discerned to be associated with the existence of significant EDMinduced residual tensile stresses at the shaped surface. (2) The strength decrease induced under fatigue is found to be even more detrimental, in terms of both fatigue limit and fatigue sensitivity, because the corresponding residual stresses imply an additional mean stress; thus, higher effective load ratio at the EDMed surface. (3) Relief of the EDM-induced residual stresses through annealing treatments is found to be an effective route for particularly improving the fracture behavior of WC–Co cemented carbides shaped by EDM. However, alternative or complementary approaches to these high temperature treatments seem to be required if similar success is to be attained under cyclic loading conditions. Acknowledgements This work was supported by the Spanish Ministerio de Ciencia y Tecnologı´a (Grant No. MAT2002-00368) and the Catalonian Departament dÕUniversitats, Recerca i Societat de la Informacio´ (Grant No. ACI200331). Research work was conducted within a cooperative effort among DURIT Metalurgia Portuguesa do Tungste´nio, Mecanizados Gine´s, AMP—Tyco Electronics and Universitat Polite`cnica de Catalunya. The authors gratefully acknowledge all the above support and stimulating collaboration. They also extend their sincere thanks to M. Marsal (UPC) for experimental assistance in SEM examination. Finally, one of the authors (B.C.) acknowledges the scholarship received from the Spanish MCYT.

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