Epitaxial Grain Growth during Surface Modification of Friction Stir Welded Aerospace Alloys by a Pulsed Laser System

Abstract

The liquid film re-growth behaviour resulting from pulsed laser surface melting (LSM) has been investigated for typical 2xxx, and 7xxx aerospace alloys, both on parent plate and friction stir welded (FSW) joints. In Zr free alloys, as a result of the high growth rate and steep thermal gradient, the melted layer re-grew with a stable front, epitaxially, from the parent subsurface grains. This caused a thin coarse grained solidified layer to form over the parent material, thermomechanically affected zone (TMAZ) and heat affected zone (HAZ), and fine columnar grains to develop over the FSW nugget zone of the same order in width as the nugget grain size. In the case of the Zr containing alloys, a very fine columnar grain structure was found over the entire surface, independent of the subsurface grain structure. This has been shown to occur by growth selection from a band of nanoscale Al grains epitaxially nucleated on Al3Zr dispersoids, at the melt solid interface, that had not fully dissolved in the melt.

Corrosion protection of AA2024-T351 friction stir welds by laser surface melting with Excimer laser

“…2b), as in these area the constituent particles are fragmented into smaller pieces by the action of the FSW tool. 17,18 The morphology of the laser treated layer observed in this study is consistent to that observed by other studies after LSM aluminium alloys with Excimer lasers, [39][40][41][42][43][44][45][46][47][48][49][50][51] which showed only submicrometre precipitation (usually observed with TEM or high resolution SEM techniques) in laser treated aluminium alloys. [47][48][49][50][51] Electrochemical measurements indicated that LSM with an Excimer laser can improve the corrosion resistance of AA2024-T351 friction stir welds by decreasing cathodic reactivity and increasing the breakdown potential in weld region and parent material (Figs.…”

“…The laser treated layer delamination might be related to the formation of fine bands of precipitate in the LSM layer. [45][46][47][48] …”

“…The reduction in cathodic reactivity produced with LSM is achieved through the dissolution of constituent particles that, being rich in noble elements such as Cu or Fe, act as catalytic sites for oxygen reduction. [39][40][41][42][43][44][45][46][47][48][49][50][51] The increase in breakdown potential is likely to be related to the dissolution of constituent particles, which can be preferential sites for pitting initiation, 35,36 but also to the formation of a homogeneous Cu rich laser treated layer: Cu is known to increase the breakdown potential of aluminium alloys when in solid solution 65 and might enhance the dissolution resistance of the LSM layer. The increase in breakdown potential and the decrease in cathodic reactivity found in this study on AA2024-T351 are consistent with the results found by Viejo et al [49][50][51] on AA2050-T8 and by Chan et al 40 on AA6013 after LSM with an Excimer laser.…”

“…The constituent particles, rich in relatively noble elements such as Cu and Fe, increase the cathodic reactivity by acting as catalytic sites for oxygen reduction 35,36 and at the same time act as preferential sites for pitting initiation 37 so that their removal can produce a noticeable increase in corrosion resistance. A review of the application of LSM to aluminium alloys was produced by Watkins et al 38 On parent material, increase in corrosion resistance has been obtained by treating aluminium alloys with pulsed lasers such as Excimer lasers, [39][40][41][42][43][44][45][46][47][48][49][50][51] in which the short duration of the thermal cycle induced by laser irradiation leads to limited microsegregation in the molten and resolidified layer. [47][48][49][50][51] In contrast, relatively low corrosion resistance has been obtained with continuous wave (CW) lasers such as CO 2 and CW-Nd:YAG lasers, as, in this case, the longer interaction times of the laser beam with the surface produces significant microsegregation of the alloying elements.…”

Efeito do número de passes e do tratamento térmico pós-soldagem de liga de alumínio AA 6063 soldada por atrito linear com mistura (FSW).

“…Um processo conhecido é o SALM assistido por LASER ou plasma, que aquece o material na frente da ferramenta. Este calor adicionado por um processo secundário somado ao calor gerado pelo atrito do ombro da ferramenta eleva a temperatura do material permitindo a sua mistura (35,36) . A figura 9 ilustra este processo onde ocorre a adição de um terceiro metal na soldagem.…”