Failure Modes of Friction Stir Spot Welds in Lap-Shear Specimens of Dissimilar Advanced High Strength Steels under Quasi-Static and Cyclic Loading Conditions

Although new structural and advanced materials have been used in the automotive and aircraft industries, especially lightweight alloys and advanced high strength steels, the successful introduction of such materials depends on the availability of proven joining technologies that can provide high quality and performance joints. Solid-state joining techniques such as Friction Stir Welding (FSW) are a natural choice since their welds are produced at low temperatures, so the low heat input provides limited, slight distortion, microstructural and mechanical degradation. Great effort has currently been devoted to the joining of Al-Cu-Mg and the Al-Mg-Si alloys because of their high strength, improved formability, and application in airframe structures. FSW is a continuous, hot shear, autogenous process involving a non-consumable and rotating tool plunged between two abutting workpieces. The backing bar plays an important role in heat transfer from stir zone (SZ), which can influence the weld microstructure as well as the consolidation of material in the root of the join. This study aims at investigating issues concerning heat generation, within the SZ of friction stir welded aircraft aluminium alloys.

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Dissimilar Material Joint Strength and Structure for Friction Stir Forming Process

Volume 2: Processing ◽

10.1115/msec2014-4044 ◽

2014 ◽

Cited By ~ 1

Author(s):

Kenneth A. Ogata ◽

Sladjan Lazarevic ◽

Scott F. Miller

Keyword(s):

Failure Modes ◽

Dissimilar Materials ◽

High Strength Steels ◽

High Strength ◽

Forming Process ◽

Experimental Conditions ◽

Friction Stir ◽

Automotive Body ◽

Neck Fracture ◽

Ultra High Strength Steels

Mass reduction of automotive body structures is a critical part of achieving reduced CO2 emissions in the automotive industry. There has been significant work on the application of ultra high strength steels and aluminum alloys. However, the next paradigm is the integrated use of both materials, which creates the need to join them together. Friction stir forming is a new environmentally benign manufacturing process for joining dissimilar materials. The concept of this process is stir heating one material and forming it into a mechanical interlocking joint with the second material. In this research the process was experimentally analyzed in a computer numerical controlled machining center between aluminum and steel work pieces. The significant process parameters were identified and their optimized settings for the current experimental conditions defined using a design of experiments methodology. Three failure modes were identified (neck fracture, aluminum sheet peeling, and bonding delamination i.e. braze fracture). The overall joint structure and grain microstructure were mapped along different stages of the friction stir forming process. Two layers were formed within the aluminum, the thermo-mechanical affected zone that had been deformed due to the contact pressure and angular momentum of the tool, and the heat affected deformation zone that deformed into the cavity.

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Spot Weld Strength Determination Using the Wedge Test: In Situ Observations and Coupled Simulations

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.24-25.299 ◽

2010 ◽

Vol 24-25 ◽

pp. 299-304 ◽

Cited By ~ 7

Author(s):

Rémi Lacroix ◽

Joël Monatte ◽

Arnaud Lens ◽

Guillaume Kermouche ◽

J.M. Bergheau ◽

...

Keyword(s):

Energy Analysis ◽

High Strength Steels ◽

High Strength ◽

Spot Weld ◽

Weld Strength ◽

Spot Welds ◽

Local Loading ◽

Advanced High Strength Steels ◽

Strength Determination

This paper describes an innovative way to characterize the strength of spot welds. A wedge test has been developed to generate interfacial failures in weldments and observe in-situ the crack propagation. An energy analysis quantifies the spot weld crack resistance. Finite Element calculations investigate the stresses and strains along the crack front. A comparison of the local loading state with experimentally observed crack fronts provides the necessary data for a failure criterion in spot weld fusion zones. The method is applied to spot welds of Advanced High Strength steels.

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Investigations on the Mechanical Behavior of Advanced High Strength Steels Resistance Spot Welds in Cross Tension and Tensile Shear

Advanced Materials Research ◽

10.4028/www.scientific.net/amr.89-91.130 ◽

2010 ◽

Vol 89-91 ◽

pp. 130-135 ◽

Cited By ~ 6

Author(s):

Sylvain Dancette ◽

Véronique Massardier-Jourdan ◽

Jacques Merlin ◽

Damien Fabrègue ◽

Thomas Dupuy

Keyword(s):

Three Dimensional ◽

High Strength Steels ◽

Complex Loading ◽

High Strength ◽

Weld Strength ◽

Tensile Shear ◽

Spot Welds ◽

Advanced High Strength Steels ◽

Cross Tension ◽

Failure Types

Advanced High Strength Steels (AHSS) are key materials in the conception of car body structures, permitting to reduce their weight while increasing their behavior in crash conditions. Nevertheless, the weldability of AHSS presents some particular aspects, in that complex failure types involving partial or full interfacial failure can be encountered more often than with conventional mild steels during destructive testing, despite high spot weld strength levels. This paper aims at characterizing the behavior of different AHSS spot welds under two quasi-static loading conditions, tensile shear and cross tension, often used in the automotive industry for the determination of their weldability. Interrupted cross tension and tensile shear tests were performed and spot welds failure was investigated with optical micrographs, SEM fractography and 3D-tomography in order to follow the three-dimensional crack paths due to the complex loading modes. A limited number of failure zones and damage mechanisms could be distinguished for all steel grades investigated. Moreover, numerical simulation of the tests was used to better understand the stress state in the weld and the influence of geometrical features such as weld size on the occurrence of the different failure types.

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Influence of Microstructure on Damage in Advanced High Strength Steels

Materials Science Forum ◽

10.4028/www.scientific.net/msf.706-709.925 ◽

2012 ◽

Vol 706-709 ◽

pp. 925-930 ◽

Cited By ~ 12

Author(s):

Frank Hisker ◽

Richard Thiessen ◽

Thomas Heller

Keyword(s):

Work Hardening ◽

Tensile Testing ◽

Failure Modes ◽

Tensile Elongation ◽

High Strength Steels ◽

High Strength ◽

Body Parts ◽

Hole Expansion ◽

Advanced High Strength Steels ◽

Dp Steels

AHSS (Advanced High Strength Steels) combine high strength and good ductility. Their outstanding forming and work-hardening behavior predestines these steels for fabrication of strength relevant structural elements and automobile body parts. To characterize a material, not only tensile, but also hole-expansion and bending behavior are important and help predict the stretch-flange-formability. In this study, detailed analyses of the correlation between these three tests and the damage mechanisms during forming have been performed for selected steels. The results show that for AHSS one should differentiate between “local” and “global” failure. Furthermore, not only are certain materials more sensitive to local or global damage, but also various testing methods tend to provoke either local or global damage. Tensile testing provokes global failure whereas hole-expansion tends to induce local failure. A specimen fails during bending with a mixture of local and global modes. These failure modes are strongly attributed to the microstructure. DP-steels yield high elongation during tensile testing and poorer hole-expansion values. High-resolution EBSD has revealed that the microstructure of DP-steels is sensitive to localized damage, which is compensated by work-hardening around damaged regions and thus shifts the loading to un-hardened regions. This makes DP-microstructures well-suited to tensile loading but sensitive to hole-expansion. CP-steels of comparable strength show poorer tensile elongation and higher hole-expansion ratios due to a microstructure which is not sensitive to localized failure (but has limited capacity for work-hardening). The failure mode in TRIP-steels exhibits a similar character as in DP-steels, but only after the martensitic transformation of retained austenite.

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