drawing process through drawing brass 26000. ½ hard (H02) with 100 µm, 150 µm, and 200 µm thickness. The as-received materials were used and their grain ...
STUDYING THE MICRO DEEP DRAWING PROCESS THROUGH DRAWING BRASS MICRO CUPS
Jenn-Terng Gau Department of Mechanical Engineering Northern Illinois University DeKalb, IL
Chi-Han Chen Department of Mechanical Engineering National Cheng Kung University Tainan, Taiwan
Zhong-Yi Yang Micro/Meso Mechanical Manufacturing R&D Department Metal Industries Research & Development Center (MIRDC) Kaohsiung, Taiwan
KEYWORDS
INTRODUCTION
Micro Deep Drawing, Grain Size, Micro Sheet Forming, Size Effects, Limit Drawing Ratio
Microforming is the most cost-effective process among micromanufacturing to produce micro metallic parts in mass production [Geiger et al. 2001]. However, due to size effects [Vollertsen 2008], the knowledge and concepts for macro-scale sheet metal forming cannot be directly used through simply tooling and process miniaturizations [Vollertsen 2001]. That is reason many researchers have been working on the experimental studies on the size effects for microforming. For example, in micro bulk forming, experiments on micro forward extrusion [Cao et al. 2004; Krishnan et al. 2005] and micro forward-backward extrusions were conducted [Chen et al. 2008]. For studying the size effects on micro sheet forming, some experiments for investigating the influence of size effects on flow stress [Raulea et al. 2001]; formability [Gau, Principe, and Wang 2007]; friction [Tiesler, Engel, and Geiger 1999]; and springback [Gau, Principe, and Yu 2007; Chen, Gau, and Lee 2008a] were conducted.
ABSTRACT A set of micro deep drawing dies was designed and made for studying the micro deep drawing process through drawing brass 26000 ½ hard (H02) with 100 µm, 150 µm, and 200 µm thickness. The as-received materials were used and their grain sizes and T/D ratios (thickness/average grain size) were obtained through microstructure analysis for understanding the influence of T/D ratio, thickness, and blankholder force on limit drawing ratio (LDR), earring, and depth of cup. The purpose of this study is to draw micro cups as deep as possible and with uniform cup heights in one stroke when cup inner diameter is defined. In this study, it was found that the thicker blank (higher T/D) will have larger LDR but with more noticeable earring phenomenon. However, the thinner material (smaller T/D) or/and drawn by the punch with higher Pr/Dp (punch radius/punch diameter) has more uniform drawn cup height and better material utilization.
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For micro deep drawing, the thin steel with 0.05 mm to 1 mm thickness were studies to investigate the limit drawing ratio (LDR) with respect to the relative punch diameter (punch diameter/thickness of sheet material) [Saoteme et al. 2001]. The influence of friction coefficient
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and applied pressure on LDR was also studied [Vollertsen et al. 2004]. Recently, the authors has conducted micro deep drawing on stainless steel 304 foils with four different thicknesses (150 µm, 100 µm, 50 µm, and 20 µm) for obtaining the LDR of the as-received materials [Lee, Chen, and Gau 2008]. In addition, the authors also conducted heat treatments on the as-received stainless steel 304 foils with four thicknesses (150 µm, 100 µm, 50 µm, and 20 µm) to obtain a variety of T/D ratios for micro deep drawing study; they concluded the macro scale empirical equations for deep drawing can be used to calculate the maximum draw load and LDR on the stainless steel 304 foils which are not thinner than 150 µm [Chen, Gau, and Lee 2008b].
For microforming, high precision and fine surface roughness for punch and die are necessary. To achieve these requirements, the manufacturing processes for making punch and dies for this study are as follows.
Manufacturing Process for Punch. As the flowchart shows in Figure 2, if the punch diameter is larger than 1 mm, a lathe can be used to pre-shape the punch profile by turning. However, if the punch diameter is smaller than 1 mm, the turning force can destroy or distort the punch. Therefore, a different process will be used to cut the micro punch of which diameter is smaller than 1 mm. After pre-shaping, heat treatment is needed to eliminate the residual stress of the machined part. For this study, the pre-shaped punches were annealed twice. After annealing, profile grinding machining was used for grinding and polishing the punches.
A set of micro deep drawing dies were designed and cut for studying the micro deep drawing process through drawing brass 26000 ½ hard (H02) with 100 µm, 150 µm, and 200 µm thickness. The as-received materials were used and their grain sizes and T/D ratios (thickness/average grain size) were obtained through microstructure analysis for understanding the influence of T/D ratio, thickness, and blankholder force on limit drawing ratio (LDR), earring, and depth of cup. EXPERIMENTAL MICRO DIE AND MICRO PUNCH
FIGURE 2. FLOWCHART FOR PUNCH MANUFACTURE.
Three punches with 2 mm diameter (Dp) and three different punch radii (Pr: 0.25 mm, 0.5 mm, and 1 mm) were used for this study, while a series of the female dies with different die diameters (Dd) and die shoulder radii (Sr) were used. The dimensions of the female die diameter (Dd) and die shoulder radius (Sr) are based on the specimen’s thickness (T); they are Dd = 2 + 1.1*T and Sr = 4*T. Figure 1 shows where Dp, Pr, Dd, Sr, and Db (blank diameter) are in experimental tooling.
Figure 3 shows an optical profile grinding machine used to grind the punches for this study and the concept of this grinding process. Besides, the diameter of the punch was measured in real time. The final step is to polish the punches.
FIGURE 3. OPTICAL PROJECTOR GRINDING PROCESS. FIGURE 1. DRAWING OF TOOLING.
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Manufacturing Process for Die. The manufacturing process flow for the female dies can be seen in Figure 4. Two types of WEDM (traditional wire EDM and micro WEDM) were used to cut micro dies. The reason is that the traditional wire EDM can be used to remove the most metal while the micro WEDM was used to make a very high precision (±2 µm) hole with fine wall roughness (
