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Fatigue crack propagation in friction stir welded and parent AA6082
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.ORCID iD: 0000-0002-8494-3983
2006 (English)In: Steel Research International, ISSN 1611-3683, Vol. 77, no 6, 450-455 p.Article in journal (Refereed) Published
Abstract [en]

The fatigue crack propagation characteristics of a friction stir welded AI-Mg-Si alloy, 6082, have been investigated. The electrical potential drop method was used for measurements. A low and a high load ratio (R) level were tested. At low load ratio (R=0.1) and a low stress intensity Delta K the propagation rate in the weld was higher than in the parent material by a factor of 3 to 5. However, the propagation rates were approaching each other close to fracture. At high load ratio (R=0.8) the propagation rate was similar in the parent material and weld. The weld crack growth rate was about the same at low and high R (except close to fracture), while the parent material growth rate increased at high R. Paris law was used to describe the measured crack propagation rates in the weld. In the case of the parent material, showing an R-dependence, Forman's law was used.

Place, publisher, year, edition, pages
2006. Vol. 77, no 6, 450-455 p.
Keyword [en]
friction stir welding, Al-Mg-Si alloy, fatigue crack propagation, load ratio, residual stress, aluminum-alloys, growth
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-15833ISI: 000238898200012Scopus ID: 2-s2.0-33745597337OAI: oai:DiVA.org:kth-15833DiVA: diva2:333875
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2010-10-08Bibliographically approved
In thesis
1. Fatigue Strength of Friction Stir Welded Joints in Aluminium
Open this publication in new window or tab >>Fatigue Strength of Friction Stir Welded Joints in Aluminium
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Solid state Friction stir welding (FSW) is of major interest in the welding of aluminium since it improves the joint properties. Many applications where Al-alloys are used are subject to varying load conditions, making fatigue failure a critical issue. In the scope of this thesis, the fatigue performance of friction stir welded AlMgSi-alloy 6082 has been investigated. Static and dynamic properties of different joint configurations and welds produced with varying process parameters have been determined. Microstructures of fractured surfaces have been studied to evaluate the effect of weld discontinuities on fatigue. The mechanical strength of the friction stir welds was set in relation to that of conventional fusion welds, and that of other FS welded Al-alloys.

The friction stir process produced aluminium butt welds with high and consistent fatigue strengths, which exceeded the strengths of similar fusion welded samples. A smooth weld geometry showed to be of great importance for the fatigue performance, favouring the friction stir welds. Welding speed in a tested range of 0.35-1.4 m/min had only a modest influence on the properties of the friction stir welds; properties were not deteriorating at the highest speed. The softening of the alloy around the weldline was modelled. A fair description of the hardness profiles across the weld was obtained. At a low and high welding speed a full and partial softening respectively was predicted, indicating that full softening is not required to obtain a flawless weld.

In case of friction stir overlap welds, tool design is even more important than in butt welding to secure weld quality. A broad tool shoulder with a concave pin end gave the best performance. In particular, the minimal influence on the sheet interface when welding with such a tool was beneficial for the fatigue strength. The stress distribution in overlap and T-type test specimens has been modelled. The stress intensity factors were determined. The corresponding crack propagation rates were in fair accordance with the experimental results. It was found that a simplified approach, developed to estimate ∆K for overlap spot welds, could be used also for friction stir overlap joints.

Place, publisher, year, edition, pages
Stockholm: KTH, 2005. 68 p.
Keyword
Materials science, Applied Materials Technology, friction stir welding, fatigue, mechanical properties, welding speed, softening, Materialvetenskap
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-160 (URN)91-7178-001-7 (ISBN)
Public defence
2005-04-13, Kollegiesalen, Valhallavägen 79, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20101008Available from: 2005-04-06 Created: 2005-04-06 Last updated: 2010-10-08Bibliographically approved

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Sandström, Rolf

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