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Fatigue Strength of Friction Stir Welded Joints in Aluminium
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
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 [en]
Materials science, Applied Materials Technology, friction stir welding, fatigue, mechanical properties, welding speed, softening
Keyword [sv]
Materialvetenskap
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-160ISBN: 91-7178-001-7 (print)OAI: oai:DiVA.org:kth-160DiVA: diva2:7526
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
List of papers
1. Fatigue performance of friction stir welded AlMgSi-alloy 6082
Open this publication in new window or tab >>Fatigue performance of friction stir welded AlMgSi-alloy 6082
2001 (English)In: Aluminium, ISSN 0002-6689, Vol. 77, no 7-8, 572-575 p.Article in journal (Other academic) Published
Abstract [en]

Friction stir welding is a relatively new solid-state welding method, ideal to join large, thin-walled aluminium profiles. In this paper results from tensile and fatigue testing of friction stir welds of the hardenable Al-Mg-Si alloy AA6082 is presented. For temper T4, post weld age treated (PWAT), the fatigue strength was found to be lower than for temper T6, and with a steeper slope of the stress-life curve. The lower fatigue strength for the statically stronger T4 + PWAT material was unexpected. Fracture in T6 initiated and propagated in the vicinity of the weak weld/HAZ border, directly outside the stirred up edge on the advancing (or shear-) side of the weld. For T4 + PWAT many fractures occurred in the weld, often initiated on the advancing side.

National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-25117 (URN)
Note
QC 20101008Available from: 2010-10-08 Created: 2010-10-08 Last updated: 2017-12-12Bibliographically approved
2. Influence of welding speed on the fatigue of friction stir welds, and comparison with MIG and TIG
Open this publication in new window or tab >>Influence of welding speed on the fatigue of friction stir welds, and comparison with MIG and TIG
2003 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 25, no 12, 1379-1387 p.Article in journal (Refereed) Published
Abstract [en]

The objective of this investigation was to determine whether the fatigue strength of friction stir (FS) welds is influenced by the welding speed, and also to compare the fatigue results with results for conventional arc-welding methods: MIG-pulse and TIG. The Al-Mg-Si alloy 6082 was FS welded in the T6 and T4 temper conditions, and MIG-pulse and TIG welded in T6. The T4-welded material was subjected to a post-weld ageing treatment. According to the results, welding speed in the tested range, representing low and high commercial welding speed, has no major influence on the mechanical and fatigue properties of the FS welds. At a significantly lower welding speed, however, the fatigue performance was improved possibly due to the increased amount of heat supplied to the weld per unit length. The MIG-pulse and TIG welds showed lower static and dynamic strength than the FS welds. This is in accordance with previous comparative examinations in the literature on the fatigue strength of fusion (MIG) and FS welds. The TIG welds had better fatigue performance than the MIGpulse welds. The softening, of the alloy around the weldline has been modelled. Using a model without adjustable parameters, a fair description of the hardness profiles across the weld as a function of welding speed was obtained. The softening in front of the Friction Stir Welding tool was also estimated. At the low and high welding speeds a full and partial softening is predicted, respectively.

Keyword
friction stir welding, aluminium alloy, welding speed, fatigue, tensile properties, microstructure, MIG-pulse and TIG welding
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-22992 (URN)10.1016/s0142-1123(03)00059-8 (DOI)000186843200007 ()
Note
QC 20100525Available from: 2010-08-10 Created: 2010-08-10 Last updated: 2017-12-12Bibliographically approved
3. Fatigue crack propagation in friction stir welded and parent AA6082
Open this publication in new window or tab >>Fatigue crack propagation in friction stir welded and parent AA6082
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.

Keyword
friction stir welding, Al-Mg-Si alloy, fatigue crack propagation, load ratio, residual stress, aluminum-alloys, growth
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-15833 (URN)000238898200012 ()2-s2.0-33745597337 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2010-10-08Bibliographically approved
4. Fatigue properties of friction stir overlap welds
Open this publication in new window or tab >>Fatigue properties of friction stir overlap welds
2007 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 29, no 1, 57-68 p.Article in journal (Refereed) Published
Abstract [en]

Friction stir welding (FSW) is currently used for many applications involving lap or T-joints, e.g. hermetically closed boxes such as cooling elements and heat exchangers. The frequent pressure changes in these make them susceptible to fatigue. The fatigue characterization of lap joints involves a combination of shear and bending. Forces applied to the ends of lap joints result in non-axial stresses in the connection area. FSW lap joints of Al-Mg-Si alloy 6082 in the artificially aged condition T6 were studied. A pin (probe) based on the Triflute (TM) concept was used with two modifications to the pin, the pin end being either convex or concave. Tool shoulders of 15 and 18 turn respectively were utilized, producing four different weld series. Fracture was initiated in the highly stressed area where the weld cuts through the interface between the two sheets. The cracks typically propagated through the weld in the upper sheet (tool side). The broadest tool shoulder with a concave end of pin design gave the best fatigue performance. This was due to an improved flow path provided by the hollowed out end of the pin; allowing material flow around the pin which resulted in minimal hooking of the sheet interface adjacent to the weld nugget. Additionally heat energy was supplied by the increased contact area. The stress intensity factor Delta K was determined. It was found that a simplified approach, developed to estimate Delta K for overlap spot welds, could be applied to friction stir overlap joints. The corresponding crack propagation rates were in fair accordance with the experimental results.

Keyword
friction stir welding, overlap joints, tool design, fatigue, mechanical properties, stress intensity factor
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-16290 (URN)10.1016/j.ijfatigue.2006.02.052 (DOI)000242511000006 ()2-s2.0-33748955722 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
5. Fatigue of friction stir welded T-joints
Open this publication in new window or tab >>Fatigue of friction stir welded T-joints
2005 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452Article in journal (Other academic) Submitted
Keyword
Friction Stir Welding, T-joints, tool design, fatigue, mechanical properties, stress intensity factor
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-25124 (URN)
Note
QS 2010 QS 20120319Available from: 2010-10-08 Created: 2010-10-08 Last updated: 2017-12-12Bibliographically approved

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