Understanding formability and geometrical accuracy of SPIF process used as Reshaping approach

Putting in place Circular Economy strategies is an urgent action to be undertaken. Manufacturing processes play a relevant role as efficient material reuse enabler. Scientists have to make an effort either to find new process or to rethink old process to reprocess End-of-life (EoL) components to recover both material and functions. In this paper, Single Point Incremental Forming (SPIF) process is used for reshaping sheet metal EoL components. Deep drawing process as well as uniaxial pre-straining (to imitate the End-of-Life component) followed by SPIF operations (to obtain the reshaped components) are setup and implemented to form and reform aluminum sheet metal components. As the authors have already proved the technical feasibility of such an approach, the present paper aims at a better understanding of the formability and geometrical accuracy performance of SPIF process as used to reform components. Specifically, an experimental campaign varying kind and extent of restraining is developed and the formability and geometrical accuracy of the subsequent SIPF operations is analyzed. Results proves that SPIF process is a promising approach for reshaping purpose.


Intr Introduction oduction
Reducing anthropogenic environmental impact is an urgent issue to deal with. One of the main contributors to yearly CO2 emissions is materials production, accounting for about 25% of the global CO2 emissions [1]. Reduction of the environmental impact of material production could be achieved by the implementation of a Circular Economy paradigm. Intense usage, product repair and upgradation, remanufacturing, component re-use and open/closed loop recycling are some of the strategies that could contribute towards reducing the environmental impact of raw material production [2]. The main principle focuses on turning an EoL product/component directly either into a reusable material or, better yet, into new products/components. As far as metals are concerned, recycling is still the most applied strategy as it provides environmental, technological and economic benefits. Nevertheless, it is, by now, urgent turning to more virtuous circular economy strategies, such us product/component reuse. Besides material itself, reuse strategies would also allow functions recovery from EoL components. In this scenario manufacturing scientists play a relevant role as they are called to think of new processes or to rethink conventional processes to be applied as circular economy enabler. As far as metals EoL components are concerned, a "Reuse" framework was proposed by Cooper and Allwood [3]. To be more specific, in this framework four main strategies are identified; two of them rely on superficial reconditioning and the product/component are reused either for the same type of function (in the case of Relocate) or for a less demanding use (in case of Cascade). The other two envisage "The component(s) undergo extensive reconditioning" and these are: 1/ Remanufacturing, where inspection, disassembly, re-drilling, and metallic spraying/ thermal techniques are the process to be applied (typical remanufacturing applications for metals concern automotive engines and dies ); and 2/Reform/Reshape: where manufacturing approaches (additive, subtractive, mass conserving) are applied to obtain a new, more useful, geometry reprocessing the returned EoL component changing its shape. While Remanufacturing has been widely analyzed by manufacturing scientist over the last years [3] the Reshaping strategy has been overlooked by the scientific community so far. Looking specifically forming processes applied specifically as Reuse strategy for sheet metal components, very little scientific research has been developed so far. Brosius et al. [4] in a review paper describe how a demounted automotive engine-hood can be reshaped into a rectangular sheet metal component by sheet hydroforming process. Takano et al. [5] applied Single Point Incremental Forming (SPIF) on a flattened sheet. In fact, the Reshaping they propose includes the flattening of a previously bent sheet and a subsequent incremental forming step. Abu-Farha and Khraisheh [6] proposed the application of super plastic forming for applying Reshaping strategies on magnesium-based sheet components. These studies, although have the merit to propose the idea of Reshaping, they are preliminary ones and the potential of forming process in this new domain is not explored yet. The authors of the present paper have recently successfully applied Single Point Incremental forming SPIF to reshape sheet metal based EoL components [7]. Specifically, SPIF was used to change the shape of deep-drawn (DD) square box part. Along with the technical feasibility, the authors have proved the better energy efficiency with respect conventional and solid state recycling route [8]. Some issues characterizing such an approach are still to be analyzed, in fact there is a lack of process windows as well as of for process parameters influence analyses on formability accuracy.
In this paper the change in formability and in the geometrical accuracy of SPIF when applied as Reshaping approach have been analyzed. Specifically, formability performance as well as geometrical accuracy with varying restraining types and level are analyzed. This paper, therefore, aims at analyzing such aspects and to provide guidelines to bring this approach closer to an industrial applicability.  In order to replicate the reshaping process chain, different Primary process and Reshaping ones were tested in the present research. Concerning the Primary process (the process to turn the virgin blank into the EoL components),

Mat Materials and Methods erials and Methods
For the experimental phase of the research, an AA5754 aluminum alloy was selected as the material to be studied. The  For the uniaxial pre-straining step, a virgin blank was subjected to two levels of uniaxial pre-straining, 15% and 10% of its original length. After the pre-straining step, a similar process was performed to determine the α max. Lastly, the effects of a deep drawing on the α max of a virgin blank were studied. For the deep drawing process, the blank was cut into a circular sheet, onto which a deep drawing process was performed to obtain a square cup 16mm in height, having a side of 50 mm and a base fillet radius of 3mm. The flange from the deep drawing process was left in order to assist the clamping of the parts for SPIF. Similar to the first two cases, SPIF process was performed to identify the α max obtainable on the base of the square cup. Figure 2 depicts the three case studies carried out along with the geometrical dimensions of the parts.
The next step was the study of strain history of the critical area of the parts obtained at α max. For this, small circles 1.5mm in diameter were laser incised on the samples. In the first case study of SPIF directly on a virgin blank, the change in the circles shape was recorded and used for the calculation of the occurring major and minor strains on the edge of the pyramid. In the cases of SPIF on pre-strained sheets, the strain history after the pre-straining steps was recorded along with the histories after the performing of SPIF process on at α max. Furthermore, with the aim to understand the effects of pre-straining process on the geometrical accuracy of SPIF process, some shape comparisons were developed. Specifically, the geometry obtained after the SPIF process compared to a reference CAD model of the

R Results esults
In figure 3 the obtained α max with varying the primary process is reported. As far as the reshaping samples with uniaxial prestraining are concerned, a slight decreasing trend is visible. Actually α max decreases by 1°moving from the as received conditions up to the case with 15% of uniaxial pre-straining. These results prove that, although a decrease in formability is visible, the change is very limited and the SPIF process can still be successfully applied even after a significant amount of uniaxial pre-straining. Concerning the case with the DD used as primary process, no change in α max value was observed both in inward and in outwards case study. This can be explained considering that, in this case, the SPIF process is applied on zone characterized by a very limited (almost zero) degree of deformation, as a consequence the SPIF process can take advantage of the entire original material formability.  The results of the analysis are depicted in figure 5. For the sake of clarity only the deviation of the SPIFed area is analyzed, the error along the pyramid walls is visible on the three analyzed case studies. In any case, the higher rigidity of the pre-strained sheet reduces the rigid motion occurring due to SPIF. These early results prove the SPIF based Reshaping approach to be a promising one.

Conclusions Conclusions
This paper contributed to lay the ground for a better understanding of a novel circular economy strategy for metal recovery. Two different aspects of SPIF based Reshaping processes have been studied, namely, the change in formability as well as the shape accuracy performance of EoL components. To this aim process chains made of different prestraining levels followed by SPIF were replicated, strain path, α max and, final shape accuracy have been determined.
Results revealed that SPIF, used for Reshaping, performed well for both the analyzed criteria. In fact, concerning the accuracy even an improvement was observed for a higher extent of deformation. Concerning the formability, although a slight reduction of the α max value was observed, the reshaping process can rely on satisfactory amount of available formability.
Next research step will concern a better understanding of the formability and accuracy change with varying both the Primary as well as the Reshaping process, for example, the analysis of Reshaping a biaxial pre-strained blank or forming a different geometry during the reshaping step.