DEVELOPMENT OF RAPID DIE WEAR TEST METHOD FOR ASSESSMENT OF DIE LIFE AND PERFORMANCE IN STAMPING OF ADVANCED/ULTRA HIGH STRENGTH STEEL (A/UHSS) SHEET MATERIALS

Automotive companies are actively pursuing to increase the use of high-strength-lightweight alloys such as aluminum, magnesium, and advanced/ultra high-strength steels (A/UHSS) in body panel and structural part applications to achieve fuel efficiency while satisfying several environmental and safety concerns. A/UHSS sheet materials with higher strength and crashworthiness capabilities, in comparison to mild steel alloys, are considered as a near-term (i.e., ~5 years) choice of material for body and structural components due to their relatively low cost when compared with other lightweight materials such as aluminum and magnesium. However, A/UHSS materials present an increased level of die wear and springback in stamping operations when compared to the currently used mild steel alloys due to their higher surface hardness and high yield strength levels. In order to prevent the excessive wear effect in stamping dies, various countermeasures have been proposed such as alternative coatings, modified surface enhancements in addition to the use of newer die materials including cast, cold work tool, and powder metallurgical tool steels. In this study, a new die wear test method was developed and tested to provide a cost-effective solution for evaluating various combinations of newly developed die materials, coatings and surfaces accurately and rapidly. A new slider type of test system was developed to replicate the actual stamping conditions including the contact pressure state, sliding velocity level and continuous and fresh contact pairs (blank-die surfaces). Several alternative die materials in coated or uncoated conditions were tested against different AHSS sheet blanks under varying load, sliding velocity circumstances. Prior to and after wear tests, several measurements and tribological examinations were performed to obtain a quantified performance evaluation using commonly adapted wear models. Analyses showed that (1) the rapid wear method is feasible and results in reasonable wear assessments, (2) uncoated die materials are prone to expose severe form wear (galling, scoring, etc.) problems; (3) coated samples are unlikely to experience such excessive wear problems, as expected; (4) almost all of the the recently developed die materials (DC 53, Vancron 40, Vanadis 4) performed better when compared to conventional tool steel material AISI D2, and (5) in terms of coating type, die materials coated with thermal diffusion (TD) and chemical vapor deposition (CVD) coatings performed relatively better compared to other tested coating types; (6) It was seen that wear resistance correlated with substrate hardness

An investigation of contact interactions in powder compaction process through variable friction models

guner, faruk/0000-0002-3438-0553; SOFUOGLU, HASAN/0000-0002-8433-4045WOS: 000371100800001In this study, multiparticle finite element (MPFE) approach was used to analyze contact interactions of spherical copper particles in powder compaction process. To this goal, 74 spherical copper particles of 200 in diameter were modeled as individual elastic-plastic bodies, and randomly filled into a die cavity. Interparticle and die-wall-particle contact interactions were investigated, and coefficients of friction were obtained using variable friction models; Wanheim-Bay's general friction model and Levanov's friction model. Variable friction models were incorporated into FE analyses through user subroutines. It was found that the variation of contact stresses inside the die leads to different contact conditions at different zones. The range of coefficient of friction encountered in the analysis was found to be slightly higher in Levanov's friction model than that for Wanheim-Bay's general friction model. On the other hand, Levanov's model was found to be more appropriate for elevated temperature analyses. (C) 2015 Elsevier Ltd. All rights reserved

Effects of friction models on the compaction behavior of copper powder

guner, faruk/0000-0002-3438-0553; SOFUOGLU, HASAN/0000-0002-8433-4045WOS: 000430522500015A comparative numerical and experimental analysis of metal powder compaction processes was presented. Closed-die compaction of spherical copper particles with a nominal diameter of 200 pm was analyzed using Multi Particle Finite Element Method (MPFEM). The von Mises material model associated with contact sensing algorithms was employed to investigate variation of coefficient of friction, and contact interactions between powder particles as well as particles with the die walls. Three different friction models (Amonton-Coulomb, Wertheim Bay, and Levanov) were used to provide a better insight and the latter two were integrated into the commercial finite element package via user-subroutines. To verify the established model, some compaction experiments were carried out. Optical, and scanning electron microscopy analyses were performed, and images obtained were compared with the numerical results. The values of the coefficient of friction obtained using Wanheim-Bay and Levanov friction models fall into the range of 0.04-0.07. From the stress distribution perspective, it was observed that the results obtained with Wanheim-Bay friction model were more conforming to experimental cases where high relative density compaction takes place while Levanov friction model was found to be preferable at low relative density compaction process

Numerical Investigation of Coefficient of Friction in Copper Powder Compaction Process at Micro Scale

7th International Conference on Mechanical and Aerospace Engineering (ICMAE) -- JUL 18-20, 2016 -- London, ENGLANDguner, faruk/0000-0002-3438-0553; SOFUOGLU, HASAN/0000-0002-8433-4045WOS: 000390417000023Multi Particle Finite Element Method (MPFEM) which is a power full approach for particle systems analyzes particle interactions via different friction models. In this study, Amontons-Coulomb constant friction (ACM), Wanheim/Bay generalized friction (WBM) and Levanov's friction models (LFM) are utilized in MPFEM in order to obtain coefficient of friction at particle-particle and particle- die wall interactions in spherical copper powder compaction. Friction models are introduced into the analysis by user subroutines. Compaction processes at room temperature and at 270 degrees C were investigated by terms of coefficient of friction, shear stress and equivalent strain. Although equivalent strain curve of WBM and LFM are in good agreement, ACM resulted in higher equivalent strain and shear stress values. Coefficient of friction those were obtained with WBM and LFM varies in a reasonable range.IEE

Microindentation on the porous copper surface modulations

Ekmekci, Dursun/0000-0001-9045-7909WOS: 000413562200034This study aimed to investigate the mechanical properties of a surface modulation which was realized through compaction, and then sintering of copper powders. To this goal, Berkovich type of indenter and depth-sensing indentation technique were used in microindentation to measure the hardness and modulus of elasticity values at different features of compact. Indentations were performed with a peak force of 50 mN. Hardness values were obtained in 0.88-1.12 GPa range while the modulus of elasticity was recorded in the 70-111 GPa interval. Even though both modulus of elasticity and hardness values were noted to be different for copper powders and substrate, one-way ANOVA analyses showed that the differences in both modulus of elasticity and hardness values are insignificant. FE modeling of microindentation was also performed and validated. It was shown that the force-displacement values obtained from FE analyses are quite well in agreement with the experimental data