Electrically - Assisted Manufacturing (EAM)

EAM; It is one of the hybrid production processes aiming to increase productivity, efficiency and quality. Various Electrically - Assisted Manufacturing studies have shown that the material properties of a metal can be changed by applying electricity to the metal during deformation. The presence of a continuous electric current applied during plastic deformation of a metal can significantly reduce the flow voltage of the metal. Under compression, the formability of a metal increases significantly with the application of a continuous electric current. It is expected from Electrically - Assisted Manufacturing to reduce processing time and production cost in a variety of traditional manufacturing processes such as sheet metal forming or die forging. Since Electrically - Assisted Manufacturing is expected to be performed at relatively lower temperatures compared to hot working, common hot working problems such as thermal stress, poor controllability of warp and tolerance can be minimized and cost-effective. Therefore, Electrically - Assisted Manufacturing is a cost-effective and energy-saving manufacturing process that improves the quality of products.

The effects of electric current on the material properties of metals were observed by performing tensile, compression, bending and hardness tests. Looking at the results, it is seen that there are generally decreases in flow stress, plastic deformation energy and spring-back. You can see them in the table below.

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Springback Problem in Electrically - Assisted Manufacturing (EAM)

-       What is an incremental sheet forming?

Incremental sheet metal forming is a sheet metal forming technique in which a sheet is made into the final workpiece with a series of small incremental deformations. Generally, the sheet is formed with a round tip tool, typically 5 to 20 mm in diameter. To a CNC machine, a tool recesses the sheet and follows a path for the desired part. It then recesses more and pulls the next path for the part into the slab and continues to do so until the complete part is formed. While doing these, lubrication is done to reduce the friction between the metal and the forming tool.

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-       What is an Springback?

According to dictionary definition, spring back is a metal product condition that describes the degree which metal tends to try to recover to its previous shape after going through a forming process.

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The elasticity value is shown with the spring-back angle as shown in the picture. The straight line is the target shape of the incremental layer, while the dotted line is the true shape image after spring-back, α is the sidewall angle, before spring-back, α′ is the sidewall angle after spring-back, and Δα is the difference between α and α’, which ultimately represents the angle of spring-back of the workpiece.

-       Springback from Incremental Forming

Spring-back is one of the biggest hurdles in sheet metal forming. Current methods used to compensate for springback are costly and involve a process of trial and error or intensive mathematical modeling. Therefore, a method is desirable to eliminate or reduce the effects of spring-back. It has been found that spring-back can be prevented by straightening and shape preservation methods. When direct current and voltage are applied to metal, spring-back can be prevented with straightening and shape retention methods.

When creating parts incrementally, three different types of spring-back can occur. The first spring-back form occurs immediately after the tool is turned over any particular part of the machining workpiece. That is, as the forming tools move around the part, this tool will deform the material to match the tool's position.

The second form of springback occurs due to the geometric progression of forming tools and cross-piece interactions. That is, when the tool leaves any area, deformation of other parts of the workpiece causes some stretch along the part, which can cause permanent plastic deformation in previously deformed areas.

The third form of springback is known as global springback. It is caused by residual stresses that occur during the forming process. It causes large scale deterioration of the part.

Electric assisted forming (EAF) is seen as one of the most energy efficient methods for deforming sheet metal compared to heat assisted forming and traditional flat mechanical forming.

In the studies conducted while creating 2024-T3 aluminum, it has been shown that it is possible to reduce the spring-back by approximately 70 percent by using electrical point applications in the pyramids formed gradually after shaping.

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A schematic representation of the Electrically Assisted incremental forming process;

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Many methods have been tried so far to prevent springback. One of the first discoveries of Electirically-Assisted Manufacturing occurred in 1959 when Machlin discovered that the ductility of salts and the yield point of the salt could be changed by passing the current through rock salts.

 In researches, the required load for forming the metal dropped because the currby pulsing a large electric current density of very short duration through a meent increased the dislocation velocity in both the parallel and perpendicular directions with respect to the direction of forming. Since that time, electricity has been shown to reduce the flow stress and increase the ductility forvarious materials under both compression and tension and allows for brittle materials to be formed further than previously possible.

It was found that in general, higher current density (amount of current through cross sectional area of specimen (A/mm2), more frequent pulse period, and longer pulse duration all resulted in a greater degree of springback reduction. A microstructural evaluation showed no change in grain size in the presence of electric current.

References;

1) ELECTRICALLY ASSISTED GLOBAL SPRINGBACK ELIMINATION AFTER SINGLE POINT INCREMENTAL FORMING, Trey Neveux1 , Brandt J. Ruszkiewicz2 , Tyler Grimm1 John T. Roth1,* and Ihab Ragai1 1Penn State Erie, June 27-July 1, 2016, Blacksburg, Virginia, USA

2) A Review of Electrically-Assisted Manufacturing, Huu-Duc Nguyen-Tran1 , Hyun-Seok Oh1 , Sung-Tae Hong1 , Heung Nam Han2 , Jian Cao3 , Sung-Hoon Ahn4 , and Doo-Man Chun1, OCTOBER 2015.

3) Electro-Thermal Modeling and Design of High-Current Pulse Power Supply for Electrically Assisted Manufacturing, JIAWEI WU 1,2,4, XIAOTAO HAN 1,2, (Member, IEEE), AND HUAI WANG 3,4, (Senior Member, IEEE), November 14, 2019.

https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=8883260

4) Springback prediction for incremental sheet forming based on FEM-PSONN technology, Fei HAN1 , Jian-hua MO2 , Hong-wei QI1 , Rui-fen LONG1 , Xiao-hui CUI2 , Zhong-wei LI2 5 March 2012

5) Springback Prediction in Sheet Metal Forming, Based on Finite Element Analysis and Artificial Neural Network Approach, Stefanos C. Spathopoulos * and Georgios E. Stavroulakis, 17