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Fatigue properties of friction stir overlap welds
KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Materialteknologi.
KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Materialteknologi.
KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Materialteknologi.ORCID-id: 0000-0002-8494-3983
2007 (Engelska)Ingår i: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 29, nr 1, s. 57-68Artikel i tidskrift (Refereegranskat) 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.

Ort, förlag, år, upplaga, sidor
2007. Vol. 29, nr 1, s. 57-68
Nyckelord [en]
friction stir welding, overlap joints, tool design, fatigue, mechanical properties, stress intensity factor
Nationell ämneskategori
Materialteknik
Identifikatorer
URN: urn:nbn:se:kth:diva-16290DOI: 10.1016/j.ijfatigue.2006.02.052ISI: 000242511000006Scopus ID: 2-s2.0-33748955722OAI: oai:DiVA.org:kth-16290DiVA, id: diva2:334332
Anmärkning
QC 20100525Tillgänglig från: 2010-08-05 Skapad: 2010-08-05 Senast uppdaterad: 2017-12-12Bibliografiskt granskad
Ingår i avhandling
1. Fatigue Strength of Friction Stir Welded Joints in Aluminium
Öppna denna publikation i ny flik eller fönster >>Fatigue Strength of Friction Stir Welded Joints in Aluminium
2005 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Stockholm: KTH, 2005. s. 68
Nyckelord
Materials science, Applied Materials Technology, friction stir welding, fatigue, mechanical properties, welding speed, softening, Materialvetenskap
Nationell ämneskategori
Materialteknik
Identifikatorer
urn:nbn:se:kth:diva-160 (URN)91-7178-001-7 (ISBN)
Disputation
2005-04-13, Kollegiesalen, Valhallavägen 79, Stockholm, 10:00
Opponent
Handledare
Anmärkning
QC 20101008Tillgänglig från: 2005-04-06 Skapad: 2005-04-06 Senast uppdaterad: 2010-10-08Bibliografiskt granskad

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