ECASIA special issue paper

Received: 2 August 2011 Revised: 19 December 2011 Accepted: 20 December 2011 Published online in Wiley Online Library

(wileyonlinelibrary.com) DOI 10.1002/sia.4840

Surface modification of 7075-T6 aluminiumalloy by laser melting†

A. Benedetti,c M. Cabeza,a* G. Castro,b I Feijoo,b R. Mosqueraa andP. Merinoa

A high power diode laser has been used for surface melting of a 7075-T6 aluminium alloy in order to induce changes in the micro-structure, which could lead to an improvement of its corrosion performance. The treatment produces a fine dendriticmicrostructure region at the surface, whose depth depends mostly on the temperature at the surface of the material and, onlymarginally, on the scanning speed of the laser beam. An analysis of the microstructure and second phases before and after thesurface treatment is presented in this paper. Copyright © 2012 John Wiley & Sons, Ltd.

Keywords: laser surface melting; AA7075-T6; microstructure; TEM; HRTEM

Introduction

Laser surface melting (LSM) treatments have been used inrecent years to improve the resistance to stress corrosioncracking (SCC) of the 7075-T6 aluminium alloy. The rapidmelting of the surface and its rapid solidification, typical ofthis treatment, result in a fine, homogeneous surface whichis free from the undesirable micrometric primary intermetalliccompounds. Most studies use CO2,

[1] Nd : YAG[2–4] and excimerlasers.[5,6] The aim of this piece of work is to investigate theapplicability of high power diode lasers (HPDLs), whichpresents some advantages over the aforementionedmethods.[7,8] Furthermore, the changes induced in the micro-structure have been analysed, too. Specific attention has beenpaid to the distribution and identification of the intermetalliccompounds in the surface layer, because it greatly affects the SCCresistance of these aluminium alloys.[9]

* Correspondence to: M. Cabeza, ENCOMAT Group, Vigo University, E.T.S.E.I.,Campus Universitario, Vigo, Spain. E-mail: mcabeza@uvigo.es

† Paper published as part of the ECASIA 2011 special issue.

a ENCOMAT Group, Vigo University, E.T.S.E.I., Campus Universitario, Vigo, Spain

b Technological Centre AIMEN, Relva 27A Torneiros, Porriño, Pontevedra, Spain

c C.A.C.T.I, Vigo University, Campus Universitario, Vigo, Spain

Experimental procedure

The analysed samples are 8-mm-thick sheets of AA 7075-T6,with the following nominal composition (wt%): 5.79% Zn,2.65% Mg, 1.59% Cu, 0.09% Si, 0.15% Fe, 0.02% Mn, 0.19%Cr and 0.04% Ti. The sample’s surface, once polished withSiC (grade 1200) paper, was treated using a continuous-waveHPDL with 3.3-kW maximum power and emission wavelengthsof 940 and 980 nm. The laser spot size was 3� 3mm[2] on thesample’s surface. The laser treatments were performed atdifferent surface temperatures, ranging from 720 to 780 �C.The temperature was controlled by a close-loop system,integrating a two-wavelength pyrometer that coaxiallymeasured the temperature to the laser beam on the incidentlaser spot. The control software allowed adjusting the controlresponse to temperature deviations from the set value, chang-ing the laser power to account for these temperaturevariations. A lateral gas protection was used to prevent

Surf. Interface Anal. (2012)

sample surface oxidation, with 20 l/min Ar gas flow. Scanningspeeds of 2–6mm/s were tested for each temperature.

Before and after laser melting, the microstructure and phasecomposition of the surface were characterised by light microscopy(LOM), SEM [JEOL 5410 microscope equipped with energydispersive spectrometry (EDS)], X-ray diffraction (XRD; Panalytical,X’Pert PRO model) and transmission electron microscopy (TEM;(JEOL JEM-2010 field emission gun). TEM specimens were preparedwith a FEI Helios NanoLab 600 focused ion beam, using the in situlift-out method.

Results and discussion

LSM treatment

The LSM treatment produces microstructural changes in thealloy surface layer.[10] For all the considered conditions, theresulting microstructures present three different zones incross-sectional view, as can be seen in Fig. 1A. The treatmenttemperature and the cooling rate are responsible for themicrostructural differences between them.

The depth of the melted region increases significantly (from240 to 381 mm) with the surface temperature (from 720 to780 �C), whereas the speed of the laser beam does notsignificantly influence this value, which ranges from 381, fora speed of 2mm/s, to 358 mm, using 6mm/s at 780 �C. Thisbehaviour is due to the very high thermal conductivity of

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Figure 1. LOM images of the cross-section after laser melting (Keller’s etchant): (A) laser-melted zone (MZ), partially melted zone (PMZ) and heat-affected zone (HAZ) and (B) Detail of the melting zone (MZ).

A. Benedetti et al.

the material, which allows to transfer more energy deeperinto the material, even though the duration of the interactionbetween the beam and the metal is reduced.Laser melting of the samples produces similar microstructures at

the surface for all the considered conditions (Fig. 1B): fine dendrites,about 10mm in size (MZ1) grown from the equiaxed cell structure(MZ2) located at the bottom of the melted zone.

Microstructural analysis

The microstructure of the alloy before LSM shows different types ofintermetallic particles uniformly distributed in the aluminiummatrix: large primary particles that were not dissolved during thesolubilisation treatment, as well as submicrometre particles, some

Figure 2. (Left) LOM and SEM images of the surface of 7075-T6 alloy befor

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of which are the result of the secondary hardening precipitationduring the artificial ageing.[9]

Figure 2 shows the primary particles of this alloy: uniformlydistributed within the matrix with dimensions ranging from 1to 10 mm. SEM/EDS (Fig. 2) and XRD (Fig. 3A) analyses showtheir composition: Mg2Si (ICDD card 35-0773) for the blackparticles and AlCuMg (ICDD card 28-0013) for the white roundones. A second type of white particles is also present, formingclusters of irregular shape and identified as Fe24Cu4Al72 (ICDDcard 6-0665).

The STEM image in Fig. 4 shows a different type of compo-nents, small particles incoherent with the matrix, with a sizebetween 50 and 200 nm, uniformly distributed in the matrixand along grain boundaries. STEM and XRD results indicatethat these are agglomerates of different particles. Some

e laser melting. (Right) EDS analysis of the different particles.

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Figure 3. XRD patterns (CuKa radiation): (A) 7075-T6 alloy and (B) LSM 7075-T6 alloy.

Surface modification of an aluminium alloy by laser melting

round, dark compounds have been identified as Al18Mg3Cr(ICDD card 29-0018), associated with round bright particlesof Mg(ZnCuAl)2 (ICDD card 34-457). There are also some rect-angular, bright ones, identified as MgZn2 (ICDD card 34-457).Figure 4 also shows smaller precipitates, responsible of the re-sistance of the material, with a specific orientation in relationto the aluminium matrix.

After LSM treatment, the microstructure at the surface(MZ1) is fine and dendritic (Fig. 1B). The second phases aredistributed at the dendrite boundary (Fig. 5B) in a discontinu-ous network. Some microsegregation of the elements insidethe dendrites is visible in Fig. 1B and is confirmed by EDS(Fig. 5B). Both effects result from the non-equilibriumsolidification of the metal pool after the laser surfacetreatment. The solidification of the last supersaturated liquidin alloy elements at the boundaries leads to the formationof agglomerates of several second phases, indistinguishableby SEM. STEM/EDS and XRD analyses (Figs 6, 7 and 3B,respectively) show that the main constituent at the grainboundary is AlCuMg (ICDD card 28-0013), always associatedwith bright white MgZn2 (ICDD card 34-457). There are someareas where Al7Cu2Fe (ICDD 25-1121) is also formed by a lossof solubility of these elements in the solidification of thealuminium alloy. In Fig. 7, much smaller precipitates are also

Surf. Interface Anal. (2012) Copyright © 2012 John Wiley

visible, with a specific orientation with respect to thealuminium matrix. The alloy’s rapid cooling results in a super-saturated solid solution of the elements alloys on the alumin-ium matrix, which leads to the formation of this submicronprecipitates by segregation at room temperature.

All in all, the second phases are fewer and smaller in the LSM-treated material than in the bulk material. The corrosionperformance of the alloy after LSM should improve as a result ofthe microstructure refinement.[8]

Conclusions

(1) The results confirm the applicability of diode laser for surfacetreatment of the 7075-T6 aluminium alloy. The treatmentproduces a fine dendritic microstructure at the surface. Thedepth of the melted zone depends mainly on the tempera-ture at the surface of the material.

(2) Several second phases can be observed before and afterLSM, with different size and distribution in the matrix. Micro-metric-sized Mg2Si, Fe24Cu4Al72 and AlMgCu are found to beuniformly distributed in the matrix of the blank alloy,

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Figure 5. (A) SEM image showing a discontinuous network of intermetallic particles. (B) EDS distribution profile of the elements of the alloy along thewhite line in image A.

Figure 4. STEM image showing submicron particles uniformly distributed within the AA7075-T6 alloy matrix and EDS elemental maps.

A. Benedetti et al.

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Figure 6. STEM image of the agglomerate of particles in the interden-drite spaces and EDS elemental maps.

Figure 7. STEM image of the agglomerate of particles in the interden-drite spaces and EDS elemental maps.

Surface modification of an aluminium alloy by laser melting

whereas after LSM treatment, the second phases with smallsize are located at the boundaries as agglomerate of submi-cron particles, mainly AlCuMg associated with MgZn2.

Acknowledgement

This work was carried out as part of Xunta de Galicia Project08TMT014CT.

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