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On the Free-Surface Roughness in Incremental Forming of a Sheet Metal: A Study from the Perspective of ISF Strain, Surface Morphology, Post-Forming Properties, and Process Conditions

1
Department of Industrial Engineering, King Abdulaziz University, Jeddah 80200, Saudi Arabia
2
Faculty of Mechanical Engineering, GIK Institute of Engineering Sciences & Technology, Topi 23640, KPK, Pakistan
*
Author to whom correspondence should be addressed.
Metals 2019, 9(5), 553; https://doi.org/10.3390/met9050553
Received: 20 April 2019 / Revised: 7 May 2019 / Accepted: 8 May 2019 / Published: 12 May 2019
PDF [2613 KB, uploaded 12 May 2019]

Abstract

Due to absence of any supporting die, the free surfaces in incremental sheet forming (ISF) experience uneven deformation. This results in rough surfaces, possibly leading to the reduced service life of components. Therefore, it is necessary to analyze and quantify the effects of the application of strain on the free-surface roughness. Moreover, in order to control roughness, both on the free surface and the opposite contact surface, the nature of correlation between the two types of roughnesses needs to be identified by classifying the significance of different process conditions. The present work is a fundamental study to address these points. A series of specimens are produced by subjecting a metallic sheet to a range of ISF strains (13% to 98%). These specimens are then subjected to a number of characterization tests, namely roughness, uniaxial tension, and residual stress tests. The results reveal that the mean free-surface roughness increases non-linearly as the normal strain (stretching + bending) on the free surface increases (where strain state on the surface is as follows: ɛ1 = 0, ɛ2 > 0, γmax = ɛ2 and 1 and 2 are principal directions). The roughness also increases, although linearly, with the post-forming sheet strength, residual stress, and forming force, thereby showing that strain hardening has a direct influence on the roughness in a way that sheet strengthening is achieved at the cost of surface quality. The surface morphology reveals that the free surfaces contained orange peel, slip lines, and micro-voids, with density increasing with strain application, thus indicating the possible influence of tensile stresses on free surface deformation and roughening at an increasing degree with strain. Further analysis of roughness results discloses that the free-surface roughness and the contact-surface roughness are inversely related, because the responses of the two to ISF processing were mutually exclusive. Based on the obtained results, future research directions are also discussed.
Keywords: incremental forming; plastic strain; roughening; correlations; surface morphology incremental forming; plastic strain; roughening; correlations; surface morphology
This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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Al-Ghamdi, K.A.; Hussain, G. On the Free-Surface Roughness in Incremental Forming of a Sheet Metal: A Study from the Perspective of ISF Strain, Surface Morphology, Post-Forming Properties, and Process Conditions. Metals 2019, 9, 553.

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