Friction stir welding (FSW) is a relatively new solid-state joining process. This joining technique is energy efficient, environment friendly, and versatile. In particular, it can be used to join high-strength aerospace aluminum alloys and other metallic alloys that are hard to weld by conventional fusion welding. FSW is considered to be the most significant development in metal joining in a decade. Recently, friction stir processing (FSP) was developed for microstructural modification of metallic materials. In this review article, the current state of understanding and development of the FSW and FSP are addressed. Particular emphasis has been given to: (a) mechanisms responsible for the formation of welds and microstructural refinement, and (b) effects of FSW/FSP parameters on resultant microstructure and final mechanical properties. While the bulk of the information is related to aluminum alloys, important results are now available for other metals and alloys. At this stage, the technology diffusion has significantly outpaced the fundamental understanding of microstructural evolution and microstructure–property relationships.

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Friction Stir Welding and Processing

Recent advances in friction-stir welding : Process, weldment structure and properties

The comprehensive body of knowledge that has built up with respect to the friction stir welding (FSW) of aluminium alloys since the technique was invented in 1991 is reviewed The basic principles of FSW are described, including thermal history and metal flow, before discussing how process parameters affect the weld microstructure and the likelihood of entraining defects After introducing the characteristic macroscopic features, the microstructural development and related distribution of hardness are reviewed in some detail for the two classes of wrought aluminium alloy (non-heat-treatable and heat-treatable) Finally, the range of mechanical properties that can be achieved is discussed, including consideration of residual stress, fracture, fatigue and corrosion It is demonstrated that FSW of aluminium is becoming an increasingly mature technology with numerous commercial applications In spite of this, much remains to be learned about the process and opportunities for further research a

Friction stir welding of aluminium alloys

Microstructural investigation of friction stir welded 7050-T651 aluminium

Microstructure, mechanical properties and residual stresses as a function of welding speed in aluminium AA5083 friction stir welds

The microstructural distribution associated with a hardness profile in a friction-stir-welded, age-hardenable 6063 aluminum alloy has been characterized by transmission electron microscopy (TEM) and orientation imaging microscopy (OIM). The friction-stir process produces a softened region in the 6063 Al weld. Frictional heating and plastic flow during friction-stir welding create fine recrystallized grains in the weld zone and recovered grains in the thermomechanically affected zone. The hardness profile depends greatly on the precipitate distribution and only slightly on the grain size. The softened region is characterized by dissolution and growth of the precipitates during the welding. Simulated weld thermal cycles with different peak temperatures have shown that the precipitates are dissolved at temperatures higher than 675 K and that the density of the strengthening precipitate was reduced by thermal cycles lower than 675 K. A comparison between the thermal cycles and isothermal aging has suggested precipitation sequences in the softened region during friction-stir welding.

Microstructural evolution of 6063 aluminum during friction-stir welding

In order to characterize plastic flow during friction-stir welding, the microtextures in a friction-stir weld of the precipitation-hardened aluminum alloy 6063 have been analyzed by orientation imaging microscopy (OIM). The base-material plate has a Goss orientation. The weld center region, except for the upper surface, takes a typical shear texture component with two types of orientations. The orientations have a pair of common {111} and 〈110〉 parallel to the cylindrical pin surface and transverse direction of the plate, respectively. The typical texture component is also observed around the weld center on the midsection, although it rotates about the plate normal direction. A microtexture analysis after postweld heat treatment has suggested that dynamic recrystallization during friction-stir welding generates the recrystallized grains at the weld center.

Microtexture in the friction-stir weld of an aluminum alloy

The precipitation sequence in friction stir weld of 6063 aluminum during postweld aging, associated with Vickers hardness profiles, has been examined by transmission electron microscopy. Friction stir welding produces a softened region in the weld, which is characterized by dissolution and growth of the precipitates. The precipitate-dissolved region contains a minimum hardness region in the aswelded condition. In the precipitate-dissolved region, postweld aging markedly increases the density of strengthening precipitates and leads to a large increase in hardness. On the other hand, aging forms few new precipitates in the precipitate-coarsened region, which shows a slight increase in hardness. The postweld aging at 443 K for 43.2 ks (12 hours) gives greater hardness in the overall weld than in the as-received base material and shifts the minimum hardness from the as-welded minimum hardness region to the precipitate-coarsened region. These hardness changes are consistent with the subsequent precipitation behavior during postweld aging. The postweld solution heat treatment (SHT) and aging achieve a high density of strengthening precipitates and bring a high hardness homogeneously in the overall weld.

Precipitation sequence in friction stir weld of 6063 aluminum during aging

Retention of fine grained microstructure of equal channel angular pressed aluminum alloy 1050 by friction stir welding

PURPOSE:To continuously manufacture an expected oxide dispersed Al alloy by continuously feeding a metal or the like as a modifying material into an Al material melted continuously by using a high energy density heat source, uniformly dispersing the modifying material into the molten Al material and preparing a molten metal. CONSTITUTION:At first, an MIG welding machine 1 is operated and an arc 3 is generated between an Al material electrode wire 13 and a metallic sheet electrode 12 to melt the wire 13, and the droplets are fallen into a mold 5. At the same time, a modifying material 4 (such as Ni, Fe, B, Si, TiC and WC) is fed into the arc 3 from a modifying material feeding apparatus 2, is melted and is fallen into the mold 5, and the above operation is continuously executed to form a molten metal 7 constituted of the aggregation of droplets in the mold 5. The molten metal 7 is subjected to electromagnetic stirring by an electromagnetic coil 6 to uniformly disperse the modifying material 4 into the Al molten metal, which is successively cooled, by which an Al alloy ingot 8 is continuously cast. In this way, the oxide dispersed Al alloy having expected properties such as high hardness, heat resistance and wear resistance can be obtd.

Masatoshi Enomoto

Papers

Microstructural evolution of 6063 aluminum during friction-stir welding

Microtexture in the friction-stir weld of an aluminum alloy

Precipitation sequence in friction stir weld of 6063 aluminum during aging

Retention of fine grained microstructure of equal channel angular pressed aluminum alloy 1050 by friction stir welding

Manufacture of oxide dispersed aluminum alloy by using high energy density heat source