Influence of workpiece hardness on EDM performance

(BQ) The aim of this research is to show the influence of the hardness of the alloy steel on the material removal rate and on the workpiece surface roughness.
Nội dung trích xuất từ tài liệu:
Influence of workpiece hardness on EDM performanceARTICLE IN PRESSInternational Journal of Machine Tools & Manufacture 49 (2009) 744–748Contents lists available at ScienceDirectInternational Journal of Machine Tools & Manufacturejournal homepage: www.elsevier.com/locate/ijmactoolShort CommunicationInfluence of workpiece hardness on EDM performance´´Jose Duarte Marafona Ã, Arlindo Araujoˆ˜Departamento de Engenharia Mecanica e Gestao Industrial, Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugalsteel AISI/SAE-D2,a r t i c l e in f oa b s t r a c tArticle history:Received 22 October 2008Received in revised form13 March 2009Accepted 13 March 2009Available online 27 March 2009The aim of this research is to show the influence of the hardness of the alloy steel on the materialremoval rate and on the workpiece surface roughness.The Taguchi methodology was used to study that influence. The result of the verification test forworkpiece surface roughness was a strong confirmation. This type of outcome allows the use of theadditive model to predict the workpiece surface roughness with an average error of 0.4%.The result of the verification test for material removal rate was a poor confirmation due to aninteraction of parameters. This type of outcome does not allow the additive model to predict thematerial removal rate with accuracy. Therefore, a linear regression model was developed for materialremoval rate using workpiece hardness and its interactions, among other variables. This model predictsthe material removal rate with an average error of 1.06%.These results show that workpiece hardness and its interactions have influence on the materialremoval rate and on the workpiece surface roughness.& 2009 Elsevier Ltd. All rights reserved.Keywords:Electrical discharge machining (EDM)Alloy steel hardnessMaterial removal rate (MRR)Workpiece surface roughnessTaguchi methodologyLinear regression model1. IntroductionElectrical discharge machining (EDM) is a non-traditionalmanufacturing process where the material is removed by asuccession of electrical discharges, which occur between theelectrode and the workpiece. These are submersed in a dielectricliquid such as kerosene or deionised water. The electricaldischarge machining process is widely used in the aerospace,automobile and moulds industries to machine hard metals and itsalloys.During the electrical discharge, a discharge channel is createdwhere the temperature reaches approximately 12,000 1C [1],removing material by evaporation and melting [2–4] from boththe electrode and the workpiece. When the discharge ceases thereis a high cooling on the surface of the workpiece creating a zoneaffected by the heat that contains the white layer. This layercontains several hollows, spheroids, fissures and micro fissures.Carbon is the main element of the white layer compositioninfluencing simultaneously its hardness and thermal conductivity[5]. The white layer thickness depends on the workpiece material,on the power used to cut the workpiece and on the appliedelectrical polarity.Electrical discharge machining is governed by a thermalphenomenon [6,7], therefore not only removes material fromthe workpiece but also changes the metallurgical constituents inthe zone affected by the heat. Thus, during machining by EDM theà Corresponding author. Tel.: +351 225 081 520; fax: +351 225 081 445.E-mail address: jdmar@fe.up.pt (J.D. Marafona).0890-6955/$ - see front matter & 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.ijmachtools.2009.03.002surface of the workpiece is submitted to a heat treatment(locally), where the time of stage is the pulse duration and thetemperature reached by the workpiece is due to the appliedcurrent intensity being followed by a quick cooling of theworkpiece. These variables affect the metallurgic constituent ofthe white layer and consequently its hardness [8–10] and itsthermal conductivity.It is also known that during the cut by EDM the materialremoval rate (MRR) decreases, which is due to process instabilityaccording to [11]. However, the decrease of material removalrate is due to the change of metallurgic constituent in the zoneaffected by the heat, according to the authors. Therefore, theauthors have decided to investigate the effect of the initialworkpiece hardness on the material removal rate and workpiecesurface roughness.2. Experimental methodologyThe effect of the workpiece hardness on the material removalrate and workpiece surface roughness was studied using theTaguchi method, which is generally applied to improve the qualityof a product. The Taguchi method is mainly used to optimise asingle output. However, some authors [12] use the orthogonalarray L18 and the grey relational analysis to optimise variousoutputs simultaneously. The authors used the orthogonal arrayL18, the analys ...