**3. Conclusion**

Some welding process can be employed to weld aluminum alloys. In this chapter the fundamental characteristics of the most common welding processes have been presented, such as: shielded metal arc welding (SMAW), gas metal arc welding (GMAW), gas tungsten arc welding (GTAW), friction stir welding (FSW), and a new welding technique named modified indirect electric arc (MIEA). Special attention has presented on welding of 6061-T6 aluminum alloy welded by MIEA and FSW. In the case of MIEA welds important microstructural characteristics in terms of morphology and grain size has been observed with respect to those obtained by a multi-pass welding process (GMAW). It means that when

Welding of Aluminum Alloys 85

Branza, T., Deschaux-Beaume, F., Velay, V. & Lours, P. (2009). A microstructural and low-

Chiarelli, M., Lanciotti, A. & Sacchi, M. (1999). Fatigue resistance of MAG welded steel elements. *International Journal of Fatigue*, Vol. 21, No. 10, pp. 1099-1110. Davies, G.J. & Garland J.G. (1975). Solidification structures and properties of fusion welds.

Dawes, C.J. & Thomas, W.M. (1996). Friction stir process welds aluminum alloys. *Welding*

Dutta, I. & Allen, S.M. (1991). Calorimetric study of precipitation in commercial Al alloys.

Easterling, K. (1992). *Introduction to the Physical Metallurgy of Welding* (Second Edition),

Grong, O. (1997). *Metallurgical Modeling of Welding*, (Second Edition), The Institute of

James, M.N. & Paterson, A.E. (1997). Fatigue performance of 6261-T6 aluminium alloy –

Kou, S. (2003). *Welding Metallurgy* (Second Edition), John Wiley and Sons, ISBN 0-471-43491-

Li, Y., Murr, L.E. & McClure, J.C. (1999). Flow visualization and residual microstructures

Lin, D.C., Wang, G.X. & Srivatsan T.S. (2003). A mechanism for the formation of equiaxed

Liu, G., Murr, L.E., Niou, C.S., McClure, J.C. & Vega, F.R. (1997). Microstructural aspects of

Mahoney, M.W., Rhodes, C.G., Flintoff, J.G., Spurling, R.A. & Bingel, W.H. (1998). Properties

Matic, T. & Domazet, Z., (2005). Determination of structural stress for fatigue analysis of

Mishra, R.S. & Ma, Z.Y. (2005). Friction stir welding and processing. *Materials Science and*

Moreira, P.M.G.P, de Jesus, A.M.P., Ribeiro, A.S., & Castro, P.M.S.T. (2008). Fatigue crack

Myhr, O.R., Grong, O., Fjaer, H.G., & Marioara, C.D. (2004). Modelling of the microstructure

comparison. *Theoretical and Applied Fracture Mechanics*, Vol. 50, pp. 81-91. Moreira, P.M.G.P, Figueiredo, de M.A.V. & Castro, P.M.S.T. (2007). Fatigue behaviour of

*Engineering Materials and Structures*, Vol.28, No. 9, pp. 835-844.

constant and variable amplitude loading of parent plate and welded specimens.

associated with the friction-stir welding of 2024 aluminum to 6061 aluminum.

grains in welds of aluminum-lithium alloy 2090. *Materials Science and Engineering A*,

the friction-stir welding of 6061-T6 aluminum. *Scripta Materialia*, Vol. 37, No. 3, pp.

of friction-stir-welded 7075 T651 aluminum. *Metallurgical Materials Transactions*,

welded aluminium components subjected to bending. *Fatigue and Fracture*

growth in friction stir welds of 6082-T6 and 6061-T6 aluminium alloys: as a

FSW and MIG weldments for two aluminium alloys. *Theoretical and Applied Fracture*

and strength evolution in Al-Mg-Si alloys during multistage thermal processing.

*Materials Processing Technology*, Vol. 209, No. 2, pp. 944-953.

*International Metals Review*, Vol. 20, pp. 83-106.

*Journal of Materials Science Letters*, Vol. 10, pp. 323-326.

Butterworth Heinemann, ISBN<PHONE_NUMBER>, Oxford.

*International Journal of Fatigue*, Vol. 19, No. 93, pp. 109-118.

*Materials Science and Engineering A*, Vol. 271, pp. 213-223.

*Journal*, Vol. 75, 3, pp. 41-45.

4, United States of America.

Vol. 351, pp. 304-309.

Vol. 29, pp. 1955.

*Enginnering R*, Vol. 50, pp. 1-78.

*Mechanics*, Vol. 48, pp. 169-177.

*Acta Materialia*, Vol. 52, pp. 4997-5008.

355-361.

Materials, ISBN<PHONE_NUMBER>, London.

cycle fatigue investigation of weld-repaired heat-resistant cast steels. *Journal of*

MIEA is used, the solidification process tends to promote a heterogeneous nucleation, thus an auto-refinement of the grain size is promoted. However, when multi-pass welding process is employed (GMAW) columnar-epitaxial solidification prevails causing an increase in terms of grain size. On the other hand, the grain structure in the fusion zone produced by FSW has the better characteristics in terms of grain size (∼10 μm).

A few mechanical properties after a welding process of 6061-T6 aluminum alloy have been presented. It is observed that quasi-static mechanical properties decrease in a dramatic manner in MIEA as well as in FSW, this aspect is totally related to the micro-structural transformation in the heat affected zone of very fine needle shape β'' precipitates to coarse bar shape β' precipitates produced by the thermal effect during the welding process (thermodynamic instability). This micro-structural transformation has been quantified by means of a micro-hardness map from which is possible to observe the soft zone formation where the failures are presented after a monotonic load (tension load).

Fatigue crack growth behaviors in weld metal, HAZ and base metal of 6061-T6 welded joints obtained by MIEA were quantified. It was observed that the worst crack growth conditions are presented in the fusion zone (weld metal), which are related to brittle microstructure characteristics due to abundant presence of eutectic Si. A comparison between weld metal for FSW and the MIEA indicates that fatigue crack growth rate in the MIEA is higher than that in FSW; it means that for a critical crack length, the ΔK represents a 57% of the base material, whereas in the case of FSW it reaches a 79%. In addition, it was observed that the fatigue crack growth rate in the HAZ tends to be similar in both welding processes.


**4. Materials and Methods**

## *4.1. Bacterial Strains, Plasmids, and Reagents*

*Pseudomonas* sp. 7HK4 bacterial strain capable of using 7-hydroxycoumarin as the sole source of carbon and energy was isolated from soil by enrichment in mineral medium containing 0.05% of 7-hydroxycoumarin. For cloning purposes, *E. coli* DH5α bacteria (ϕ80 *lacZ*ΔM15 Δ(*lacZY-argF*)*U169 deoR recA1 endA1 hsdR17*(rK-mK+) *supE44 thi-1 gyrA96 relA1*) (Thermo Fischer Scientific, Lithuania) were used. *E. coli* BL21 (DE3) bacteria (F- *ompT gal dcm lon hsdS*B(rB-mB-) λ(DE3) [*lacI lacUV5-T7* gene 1, *ind1*, *sam7*, *nin5*]) (Novagen, Darmstadt, Germany) were used for gene expression studies.

All reagents used during this study are listed in Table S1 and plasmids are described in Table S2 in the Supplementary Material.

## *4.2. Bioconversions with Whole Cells*

*Pseudomonas* sp. 7HK4 bacteria were grown in mineral medium supplemented with 0.05% (*w*/*v*) of 7-hydroxycoumarin or glucose, as the sole carbon and energy source, at 30 ◦C with rotary aeration (180 rpm) for 48 h. *E. coli* BL21 (DE3) bacteria containing recombinant genes were grown in 30 mL of Brain Heart Infusion (BHI) medium at 30 ◦C and 180 rpm overnight. High density bacterial culture was centrifuged and resuspended in 30 mL of minimal C-750501 medium, in which the synthesis of proteins was induced with 1 mM of isopropyl-*β*-D-1-thiogalactopyranoside (IPTG) after 1.5 h of incubation at 20 ◦C and 180 rpm [65]. Incubation at 20 ◦C was continued for another 24 h. Both *Pseudomonas* sp. 7HK4 and *E. coli* cells were sedimented by centrifugation (3220× *g*, 15 min). The collected cells were washed twice with 15 mL of 0.9% NaCl solution. For whole-cell conversion experiments, cells from 20 mL of culture were resuspended in 1 mL of 50 mM potassium phosphate buffer (pH 7.2). All small-scale bioconversions with whole cells were made in 50 mM potassium phosphate buffer, pH 7.2, which contained 1–2 mM of the substrate. The reaction mixtures were kept in a thermoblock at

30 ◦C and 500 rpm. Bioconversion mixtures were centrifuged for 2 min at 10,000× *g*, and 100 μL of the supernatant were analyzed by UV-VIS spectroscopy (range 200–600 nm). Measurements were repeated to record the changes in the absorption intensity over time. All measurements were performed with PowerWave XS microplate reader.