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Title: The Influence of Strain Rate Variations on the Appearance of Serrated Yielding in 2024-T3 Al-Clad Aluminium Alloy

Abstract

To avoid failure during the stretch forming process using manual control, machine operators tend to achieve the final form using a stop-start approach. It was observed that when approaching full form, stretcher-strain marks appeared on the surface of the part if the operator stopped and restarted the forming operation. In order to investigate this phenomenon, a series of tensile tests was conducted using two batches of 2024-T3 aluminium alloy. The specimens were tested using several different strain rates, representative of those used on the shop floor. Additional tests were conducted involving a series of pauses under displacement control at differing levels of strain and strain rate. In the uninterrupted tests for the two batches of 2024-T3 material tested, serrated yielding was observed just prior to failure. However for the tests in which there was a pause in displacement, the material consistently exhibited serrated yielding when the crosshead began to move again. These results indicate that the pause provides an opportunity for strain ageing and pinning of the dislocations resulting in serrated yielding of this alloy. In order to avoid serrated yielding, stretch forming operations using 2024-T3 aluminium should be conducted at a constant strain rate without interruption. This also hasmore » far reaching implications for those involved in the production and testing of these alloys. The test programme described represents an initial attempt to investigate a phenomenon noted during an industrial forming process and should be extended to analyse the affect of strain path changes on the occurrence of serrated yielding.« less

Authors:
; ;  [1];  [2]
  1. Advanced Metal Forming Research Group, Nanotechnology and Advanced Materials Research Institute, University of Ulster, Shore Road, Newtownabbey, Co. Antrim, N. Ireland, BT370QB (United Kingdom)
  2. Bombardier Aerospace, Airport Road, Co. Antrim, N. Ireland, BT3 9DZ (United Kingdom)
Publication Date:
OSTI Identifier:
21057040
Resource Type:
Journal Article
Resource Relation:
Journal Name: AIP Conference Proceedings; Journal Volume: 907; Journal Issue: 1; Conference: 10. ESAFORM conference on material forming, Zaragoza (Spain), 18-20 Apr 2007; Other Information: DOI: 10.1063/1.2729545; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; ALUMINIUM; ALUMINIUM ALLOYS; DISLOCATIONS; FAILURES; STRAIN AGING; STRAIN RATE; STRAINS; SURFACES; TESTING; VARIATIONS

Citation Formats

Leacock, Alan G., McMurray, Robert J., Brown, D., and Poston, Ken. The Influence of Strain Rate Variations on the Appearance of Serrated Yielding in 2024-T3 Al-Clad Aluminium Alloy. United States: N. p., 2007. Web. doi:10.1063/1.2729545.
Leacock, Alan G., McMurray, Robert J., Brown, D., & Poston, Ken. The Influence of Strain Rate Variations on the Appearance of Serrated Yielding in 2024-T3 Al-Clad Aluminium Alloy. United States. doi:10.1063/1.2729545.
Leacock, Alan G., McMurray, Robert J., Brown, D., and Poston, Ken. Sat . "The Influence of Strain Rate Variations on the Appearance of Serrated Yielding in 2024-T3 Al-Clad Aluminium Alloy". United States. doi:10.1063/1.2729545.
@article{osti_21057040,
title = {The Influence of Strain Rate Variations on the Appearance of Serrated Yielding in 2024-T3 Al-Clad Aluminium Alloy},
author = {Leacock, Alan G. and McMurray, Robert J. and Brown, D. and Poston, Ken},
abstractNote = {To avoid failure during the stretch forming process using manual control, machine operators tend to achieve the final form using a stop-start approach. It was observed that when approaching full form, stretcher-strain marks appeared on the surface of the part if the operator stopped and restarted the forming operation. In order to investigate this phenomenon, a series of tensile tests was conducted using two batches of 2024-T3 aluminium alloy. The specimens were tested using several different strain rates, representative of those used on the shop floor. Additional tests were conducted involving a series of pauses under displacement control at differing levels of strain and strain rate. In the uninterrupted tests for the two batches of 2024-T3 material tested, serrated yielding was observed just prior to failure. However for the tests in which there was a pause in displacement, the material consistently exhibited serrated yielding when the crosshead began to move again. These results indicate that the pause provides an opportunity for strain ageing and pinning of the dislocations resulting in serrated yielding of this alloy. In order to avoid serrated yielding, stretch forming operations using 2024-T3 aluminium should be conducted at a constant strain rate without interruption. This also has far reaching implications for those involved in the production and testing of these alloys. The test programme described represents an initial attempt to investigate a phenomenon noted during an industrial forming process and should be extended to analyse the affect of strain path changes on the occurrence of serrated yielding.},
doi = {10.1063/1.2729545},
journal = {AIP Conference Proceedings},
number = 1,
volume = 907,
place = {United States},
year = {Sat Apr 07 00:00:00 EDT 2007},
month = {Sat Apr 07 00:00:00 EDT 2007}
}
  • Much attention has been paid to the rather special stress-strain characteristics of Al-Mg alloys. Beginning with the pioneering work of Portevin and LeChatelier in 1923, the focus has been on the appearance of tensile band-type deformation markings that are observed coincident with the appearance of serrated flow in stress-strain curves. The serrations are the result of dynamic strain aging (DSA). A major influence of the DSA phenomenon is to produce both a higher flow stress and, very importantly, greater strain hardening at lower strain rates than for higher ones at which serrations do not appear. Physically-based constitutive equations, for example,more » of Zerilli-Armstrong type, that are derived for thermally-activated dislocation-lattice or dislocation-dislocation interactions, are susceptible to interference from DSA. For this reason, clarification of the importance of strain aging in accounting for laboratory tests of the stress-strain behavior of the alloy has been investigated. Of special interest was the solute influence on the combined yield and strain hardening behavior.« less
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  • The propensity for hot tearing and porosity in pulsed Nd-YAG laser beam welding of 2024-T3 aluminum was observed to vary with beam on'' duty cycle. High average laser output power of 1 kW and variation in duty cycle were achieved by multiplexing a system of three 400-W average power Nd:YAG lasers with fiber-optic delivery. Operating this system with a greater than 60% duty cycle eliminated the severe hot tearing and gross porosity that occurred in lower duty cycle welds made under otherwise identical conditions. Microstructural characterization of the low and high duty cycle welds identified differences in weld morphology directlymore » related to hot-crack susceptibility. Solute segregation and the distribution of low melting point phases are dependent upon the solidification history. In addition, manipulating the input power alters the solidification-related stress state developed in the weld fusion zone. Appropriate adjustment of the process parameters, e.g., duty cycle, optimizes the solidification velocity, thermal gradients, and stress state in the weld fusion zone in order to prevent hot tearing in this crack-susceptible aluminum alloy.« less
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