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Research Papers

Characterization of Melting and Solidification Phenomena in Electromagnetic Punching of Aluminum Tubes

[+] Author and Article Information
De Waele Wim

Laboratory Soete,  Ghent University, Technologiepark 903, 9052 Zwijnaarde, Belgiumwim.dewaele@ugent.be

Faes Koen

 Belgian Welding Institute, Technologiepark 935, 9052 Zwijnaarde, Belgiumkoen.faes@bil-ibs.be

Van Haver Wim

 Belgian Welding Institute, Technologiepark 935, 9052 Zwijnaarde, Belgium

J. Manuf. Sci. Eng 134(1), 011010 (Jan 13, 2012) (10 pages) doi:10.1115/1.4005307 History: Received January 24, 2011; Revised September 26, 2011; Published January 13, 2012; Online January 13, 2012

Electromagnetic punching of tubular products is considered to be a promising innovative perforating process. The required punching energy decreases when using high velocities. Also, less tools are required when compared to conventional mechanical punching. However, the increase in punching speed can involve new strain and fracture mechanisms which are characteristic of the dynamic loading. In high energy rate forming processes the effect of temperature versus time gradient on the material properties becomes important due to the heat accumulated from plastic deformation and friction. The deformation induced heating will promote strain localization in it, possibly degrade its formability and cause premature failure in the regions of high localized strain. The feasibility of the electromagnetic pulse forming process for punching holes in aluminum cylindrical specimens has been investigated on an experimental trial-and-error basis. Experiments were performed using a Pulsar system (model 50/25) with a maximum charging energy of 50 kJ and a discharge circuit frequency of 14 kHz. Microscopic and metallographic inspection of the punched workpieces, together with hardness measurements, was performed to critically evaluate the quality of the cuts. It was observed that damage occurred at part of the edge of the punched hole during some of the perforation experiments. It was evidenced that in most workpieces, especially those performed at higher charging energy levels, a considerably high temperature must have been reached in the regions near the punched hole. The aluminum in this region was assumed to have melted and resolidified. These assumptions were affirmed by the following observations. Microscopic-size precipitates present in the unaffected base metal microstructure, had completely dissolved in that region; shrinkage cavities and dendrite rich regions were clearly visible. Next to this region, a heat affected zone was present where the grain boundaries had partially melted and precipitates partially disappeared. Considerably high temperatures, in the order of 520 to 660 °C, were reached in the regions around the punched holes, leading to melting and resolidification of the material. The total width of the thermally affected regions appeared to be larger at higher energy levels. The combination of heat generated by ohmic heating and by plastic deformation in a very short time interval is the most probable cause of the high peak temperatures that have occurred during the electromagnetic punching process.

Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Different zones at the perforation of the workpiece in the case of mechanical punching of aluminum sheet metal [9]

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Figure 2

Measured discharge current flowing through the coil. A peak amplitude of almost 200 kA is reached after approximately 18 μs.

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Figure 3

Photograph of the multi-turn coil used for the electromagnetic punching experiments. The insert is a detailed view of the fieldshaper.

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Figure 4

(a) Drawing of the supporting mandrel used (cutting hole diameter: 10 mm) and (b) Assembly of the punching setup: the punching hole of the mandrel is positioned at the center of the fieldshaper

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Figure 5

Result of a punching experiment from set 2 (a) and from set 4 (b)

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Figure 6

Stereomicroscopic images of an experiment of set 2. Left: overview; Middle: enlargement of the region with cracks; Right: enlargement of the shear zone. Note that the maximum crack length is less than 0.9 mm and the shear zone is about 1 mm.

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Figure 7

Stereomicroscopic images of an experiment of set 4. Top left: overview; Top right: magnification of the rough zone; Bottom: magnification of the cracks observed.

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Figure 8

Stereomicroscopic image of an experiment of set 5

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Figure 9

Sample from an experiment of set 2 at the 9 o’clock position. Different zones of the part edge: 1 = roll-over depth, 2 = fracture depth, and 3 = burr height.

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Figure 10

Left (3 o’clock position) and right (9 o’clock position) cross-section of the punched hole in the sample of set 2 (prior to etching)

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Figure 11

Detailed view of the left hand side of the punched hole in the sample of set 2 (prior to etching)

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Figure 12

Left and right cross-section of the punched hole in the sample of set 5 (prior to etching)

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Figure 13

Detailed image of the boxed area in Fig. 1 (prior to etching)

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Figure 14

Shrinkage cavities observed at the left side of the punched hole in the sample of set 5 (prior to etching)

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Figure 15

Left cross-section of the punched hole in the sample of set 2 (after etching)

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Figure 16

Enlargement of the boxed area in Fig. 1

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Figure 17

Cross-section at the 9 o’clock position of the punched hole in the sample of set 5 (after etching)

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Figure 18

More detailed image of zone 1 indicated in Fig. 1

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Figure 19

More detailed image of zone 2 in Fig. 1

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Figure 20

(a) Hardness profiles of the sample of set 2 (9 o’clock position); (b) hardness profile of the sample of set 2 (3 o’clock position); and (c) hardness profile of the sample of set 5

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Figure 21

Distribution of magnetic flux density along the axial length of the fieldshaper. The dashed lines indicate the extent of the punching hole.

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