MATERIAL BEHAVIOR IN MICROFORMING AT ELEVATED TEMPERATURE

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1 MATERIAL BEHAVIOR IN MICROFORMING AT ELEVATED TEMPERATURE Emil Egerer, Ulf Engel LFT Chair of Manufacturing Technology, Erlangen, Germany Abstract The application of metal forming to the manufacturing of microparts is still not widespread. One reason is that the know-how of conventional forming cannot be simply transferred to the microscale. This is due to the so called size effects, which in cold forging can be observed in terms of effects on material flow, on tribological behavior and on working accuracy. Investigations at cylindrical microparts at stainless steel (X4CrNi18-1) produced by cold upsetting reveal that there is a transition in the observed material behavior from homogeneous to inhomogeneous continuum being one of the reasons for a very unstable processes. The forming behavior mainly depends on the part s individual grain constellation leading to a rather unpredictable forming result and making a process design impossible. One possibility to counteract the inhomogeneous material character is to process at elevated temperature. During forming at elevated temperature additional slip systems are activated with the effect to reduce the dependency on single grains. This improves not only the material flow but also the reproducibility of process variables like the flow stress. Introduction The trend towards higher integrated functional density and miniaturization in electronic and micro mechanical products is still unbroken causing an increased demand for metallic parts of smallest dimensions and closest tolerances. Many modern products like mobile phones, hand held computers or navigation systems contain such small parts. According to the actual NEXUS market analysis the world wide total turnover of microsystems for the year 25 is estimated to be in the range of 68 billion US$ [1]. The production of these components demands for technologies which enable to produce high numbers of pieces economically, environmentally friendly and reliably. Metal forming technology meets these demands and is already occasionally applied for the production of smallest parts like springs, screws, gears, rivets, contacts and shafts. Microforming is defined as the production of metallic parts by forming with at least two part dimensions in the sub millimeter range, which in the following will be termed by microparts [2]. However, facing the fact how many of those parts are produced today by machining, the break through of the new technology of microforming is still missing. One reason for this is the lack of an appropriate system technology. Another reason concerns the process itself and is due the fact that the know-how of conventional forming cannot be simply transferred to the microscale because of so called miniaturization effects influencing the material flow behavior, tribological conditions and working accuracy. Up to now the realization of microforming processes is still characterized by empirical process design, which especially in this case is confronted with problems regarding tool design and process control. It is well recognized that in order to facilitate a more extensive use of the microforming technology, an improved understanding of the mechanisms causing the size effect is indispensable. Therefore a fundamental basis for the design of micro metal forming processes has been created. This paper takes as a basis that in microforming the continuum cannot be regarded as a homogeneous one as it is done in the case of conventional forming, being the main reason for the observed scatter of process variables [3]. In order to counteract this problem, warm forging is investigated expecting an ho- American Society for Precision Engineering 37

2 mogenizing effect. First results will show that in fact scatter can be reduced by forging at elevated temperature, which is regarded as an important presupposition for improved working accuracy. Furthermore the investigation shows a temperature influence on mechanical properties like work hardening. In addition a distinct temperature effect on the homogeneity of deformation is observed. Experimental Methods and Set-Up All the investigations reported here are done on cylindrical specimens of stainless steel (X4CrNi18-1, diameter.48 mm, height.72 mm). This alloy has an high industrial relevance since it is quite frequently used for microparts in the field of fine mechanics and medicine technology due to its favorable mechanical properties (e.g. strength, machinability). In terms of the experimental investigations it is important that the grain size of the material can be varied by by heat treatment in a range of approximately 1 µm to 1 µm. Specimens were machined from drawn wire and finished by the highest possible accuracy in order to minimize geometrical effects. For the cold and warm forging experiments a mechanically driven material testing system (type MTS Synergy 1 (.5 kn nominal press load)) has been used (Fig. 1a). The procedure of material tests especially in warm microforming necessitates a manipulation unit for the handling of microparts. In this case a properly designed automatic handling system is used, which is first of all due to the necessity of the operator safety considering the high process temperatures. Besides, the automated operating sequence guarantees the reproducibility of heating conditions i. e. temperature and heat-up time. Furthermore, an exact positioning of the specimen guaranteed by this handling stage leads to a reduction of perturbations, which e.g. lowers scattering of the flow stress evaluated by upsetting tests. The technology of the handling system used in this experimental setup was already developed by the authors as a so called cross transport system in an former project and successfully implanted in a prototype of a micro former. The handling system consists of a linear translation stage for the part transfer and a special designed vacuum pipette for the gripping of the part (Fig. 1c). The advantages of this system are the excellent dynamic behavior, the high speed and the high precision [4, 5]. upsetting die blank magazine 25 a) b) blank shaped vacuum.5 pipette c) Fig. 1: different views of the experimental set-up. a) total view, b) tool area with dies, blank magazine and handling unit and c) vacuum pipette with a blank In contrast to macro forging processes the micro part cannot be heated up outside of the tool area, because the high ratio of surface to volume favors the heat dissipation with the result that the micropart will be cooled down before starting the forming process. In order to reach a controllable temperature within the part the tools are heated up by special heating elements so that the micropart itself is warmed up purely by conduction within seconds to get a sufficient ho- 23 Winter Topical Meeting - Volume 28 38

3 mogenous temperature distribution. Finally, hardness measurements used to identify the inhomogeneity of work hardening on a micro structural scale were done with a special set-up to measure the so called universal hardness HU.1 (Fischerscope H1VP, indentation force.1 N) with a scanning facility [6]. Scatter of process variables The flow curve is one of the most important criterions for the design of bulk metal forming processes and is gained by upsetting tests. One result of the investigations on geometrically similar scaled upsetting tests at room temperature is that the scatter of the flow stress increases with decreasing specimen size [7]. With respect to the process stability and reliability the increase of scatter is rather problematic. The reason is the following: Depending on the grain size, in microforming the whole work piece or single work piece structures to be shaped may consist of only a few grains. This means that the forming result is strongly depending on the orientation even of single grains. Former investigations showed that the material cannot be considered as a homogeneous continuum like in conventional forming but as a inhomogeneous one. One countermeasure to this material behavior is to perform the forming process at elevated temperature as done in warm forming. In this context warm forming is defined as - considering economical reasonable terms - forming above room temperature while hardening effects still occur [8] and recrystallization may not take place during forming. As shown in Fig. 2 the investigations of micro upsetting tests at different temperatures bring up two results. flow stress σ f coefficient of variation 16 MPa grain size: L K 11 µm machined blanks upsetting speed: v =.5 mm/s no lubricant material X4CrNi18-1 (1.431) 4 2 C % 1 C 2 2 C 1 3 C log. strain ε C The first result is the reduction of the flow stress with increasing temperature as expected from macro forming. The second effect is the reduction in scatter of the flow stress. As a measure for the scatter the coefficient of variation (standard deviation in relation to mean value) was calculated and is depicted in the lower diagram of Fig. 2. Compared to room temperature the reduction of the coefficient of variation of the flow curve at a moderate temperature level of 2 C is approximately 7% at a strain of.6. 2 C 2 C 3 C.72 upsetting test Fig. 2: flow stress and scatter at different temperatures gained by upsetting test Mechanical properties A further effect regarding the mechanical properties of cold microformed parts is the inhomogeneous forming behavior characterized by distinctly different deformations of neighbored grains. To get knowledge about the local forming status hardness measurements at forged specimen were performed. Actually the cross sections of cylindrical specimens were scanned with micro American Society for Precision Engineering 39

4 hardness measurements. Fig. 3 shows three horizontal measurement profiles taken in the middle of the specimens which were upset at cold and warm forging conditions, room temperature and 3 C, respectively. The analysis yields two results. The first one shows that the average specimen hardness decreases with increasing temperature which also can be expected as known from conventional forming. The second effect shows, as can be seen at the spread of the hardness measurement profiles, that in both strain cases (ε =.2 and ε =.6) the scatter of the single hardness spots decrease at elevated temperature. The temperature effects that additional slip systems are activated in the material, so that the dependency of the material flow on the grain orientation and location is reduced. This influences the material behavior positively also in unfavorable orientated grains. Therefore, the temperature homogenizes the overall deformation in that way as the neighbored deformation zones are evened out. upset specimen ε =.2/.6 aver. grain size 65 µm cold (2 C) ε =.6 = ε 5 1PPð 3 2 hardness HU.1 Fig. 3: profiles of hardness measurements at specimen of different strain and temperatures Regarding the entire cross section area of the specimen a characteristic value is derived by statistical methods in order to characterize the degree of inhomogeneity. Starting from two adjacent points all the hardness gradients (hardness difference related to the distance of in mean 3 µm) are calculated line by line and column by column, corrected by the macroscopic contour of hardness which is approximated by a polynomial of 4 th degree, and finally weighted by its significance dependent on its absolute value. The mean value of the resulting distribution is taken as average hardness gradient AHG characterizing the homogeneity. In Fig. 4 these values are shown for two cases of strain (ε =.2 and ε =.4) as function of the temperature which confirm the effect of temperature towards a more homogeneous deformation. The temperature increase leads to a decrease of approximately 5% at the average hardness gradient which means that homogeneity improvement is in evidence. Additionally, the average of the absolute hardness is dis- average hardness gradient AHG x 3 µm 1PPð initial hardness: 25 N/mm² 1 2 C 4 temperature Fig. 4: Average hardness gradient AHG and universal hardness HU.1 of upset cylinder specimens. universal hardness HU.1 23 Winter Topical Meeting - Volume 28 4

5 played, indicating that even though forming at high temperature work hardening still occurs. Compared to the initial hardness of 25 N/mm² (at room temperature) the hardening effect at 3 C is approximately 2%. As it also can be seen at both the hardness as well as the hardness gradient the main step of these values takes place when increasing the temperature level from room temperature up to 1 C. A further increase of the temperature does not influence these parameters significantly although the flow stress is still decreasing at these temperatures. This result is correlating well to the investigations done by the authors on cylindrical specimens made of CuZn15 (diameter.5 mm, upset ratio 1.5) also processed by upsetting. Thus, it can be concluded, that by elevating the operating temperature on a moderate warm forging level of 1 C the desired effect of homogenization is reached without losing the effect of work hardening. Conclusion and Outlook Because of the fact that microforming processes are characterized by the existence of only a few grains in the deformed area, the material cannot be considered as a homogeneous continuum anymore. The inhomogeneous material flow is mainly caused by the random size, location and orientation of individual grains. Therefore cold microforming processes, inter alia, are characterized by two effects. One effect is the increasing scatter of such important material parameters like the flow stress, another effect is the inhomogeneous local deformation behavior. These effects are leading to a more or less random forming behavior and almost unpredictable mechanical properties of the produced micropart and, thus, are making a process design as applied in conventional forming impossible. As a countermeasure forging at elevated temperature reduces the observed unfavorable characteristics of cold micro forming yielding a more homogeneous deformation. The different formability of the grains is evened out by thermally activating those slip systems which are not active at cold conditions. It is shown that even temperatures of moderate level are sufficient to improve the homogeneity and to reduce distinctly the scatter without losing the benefits of cold forging such as work hardening. In contrast to upsetting where the share of free surfaces is very high future investigations will be performed at the cup backward extrusion process with elevated temperature. The cup backward extrusion process is characterized by a high share of material with tool contact so that the material flow is increasingly influenced by the complex tribological conditions. Acknowledgement All the research done by the Chair of Manufacturing Technology was supported by the German Research Foundation (DFG). References 1 WECHSUN, R. et al.: Nexus Market Analysis for microsystems Nexus Task Force Market Analysis. Wicht Technologie Consulting. Munich, GEIGER, M.; KLEINER, M.; ECKSTEIN, R.; TIESLER, N.; ENGEL, U.: Microforming. Keynote Paper. Annals of the CIRP, 5(21)2, ENGEL, U.; EGERER, E.: Basic research on cold and warm forging of microparts. In: Key Engineering Materials, (22), GEIGER, M.; EGERER, E.; ENGEL, U.: Cross Transport in a Multi-Station Former for Microparts. In: WGP (eds.): Production Engineering, 9(22), American Society for Precision Engineering 41

6 5 EGERER, E.; ENGEL, U.; VAN DER HEYD, G.: Handling of Microparts in a Multi-Station Former. In Balsamo, A. et al. (eds.); 2nd International Conference euspen, May 27th-31st, 21. Turin (Italy): Augusta Edizione Mortario, 21, pp Norm: ISO/DIS , edition 21-3: Paints and varnishes - Determination of microindentation hardness - Part 3: Determination of universal hardness and related properties using a Vickers indentor (Revision of ISO 6441:1984). 7 Tiesler, A.: Mikroumformtechnik: Miniaturisierungseffekte beim Fließpressen. In: Geiger, M.; Feldmann, K. (eds.): Reihe Fertigungstechnik Erlangen Nr. 12. Meisenbach, Bamberg, ICFG Document No. 12/1: Warm Forging of Steel. In: International Cold Forging Group. Meisenbach, Bamberg, 21, pp Winter Topical Meeting - Volume 28 42