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Solar Energy Materials & Solar Cells 86 (2005) 145–163

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doi:10.1016/j

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On the investigation of 7075 aluminum alloywelding using concentrated solar energy

D.G. Karalis, D.I. Pantelis�, V.J. Papazoglou

Shipbuilding Technology Laboratory, School of Naval Architecture and Marine Engineering, National

Technical University of Athens, 9 Iroon Polytechniou Avenue, Zografos, Athens GR-157 73, Greece

Received 4 May 2004; accepted 1 July 2004

Available online 11 September 2004

Abstract

The application of concentrated solar energy for the welding of aluminum alloy 7075 was

attempted in the present work, by employing the installation of the CNRS Solar Furnace at

Odeillo, Pyrenees, southeast France. The characteristics of the solar treated specimens

(microstructure, hardness, SEM-EDS analysis) were fully investigated and correlated with

thermal numerical results using the finite element method.

r 2004 Elsevier B.V. All rights reserved.

Keywords: Concentrated solar energy; Welding; 7075 aluminum alloy; FEA thermal modeling

1. Introduction

The advantages of concentrated solar energy, as compared to other high-energydensity beams, are many and well known: they stem from a free natural andinexhaustible energy source, the sun. Sun does not pollute the environment, whereasthe wide spectrum of its light allows the increased absorption of energy. In addition,the cost of the installations used for its concentration is not very high, especially whenthese are also going to be used for other applications. However, the discontinuity inthe sun’s appearance does not permit its use in industrial scale. The introduction of

see front matter r 2004 Elsevier B.V. All rights reserved.

.solmat.2004.07.007

nding author. Tel.: +302-10-772-3691; fax: +302-10-772-1412.

dress: [email protected] (D.I. Pantelis).

www.elsevier.com/locate/solmat

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D.G. Karalis et al. / Solar Energy Materials & Solar Cells 86 (2005) 145–163146

concentrated solar energy in the field of heat treatments of metals, has been receivingincreased interest in the past 15 years. Surface transformation of steels throughquenching [1], as well as through formation of coatings [2–5] using concentrated solarenergy have been attempted, providing good results with respect to the tribologicalbehavior, as compared to other surface treatment techniques, i.e. laser [6–9].

More specifically:

�
The cladding of Ni superalloy powders on AISI 4140 steel, by use of concentratedsolar energy, gave treated zones with microstructure typical of rapid solidification,free of pores or cracks. However, the low achieved dilution resulted in the presenceof a large amount of primary carbides of very high hardness (850HV) in thetreated zone [2].
�
The melting of pre-deposited WC powder on cast iron, using solar energy, hasformed alloyed layers of high hardness (1000HV0.2) and improved wear andtempering resistance [3].
�
TiN hard coatings on ASP23 steel have also been successfully oxidized andtherefore hardened, by use of concentrated solar energy [4].
�
The possibility of realizing surface alloying of ceramic (SiC) and ceramic/metallic(Cermet: WC+17%Co) powders on ferrous substrates (Ck60, St52.3 steels), usingconcentrated solar energy has also been examined [10]. In all cases, the obtainedtreated zones were deep, of high hardness, well adhered to the substrate and ofimproved tribological behavior.
In general, no other research work has been officially reported as far as welding isconcerned. In the present work, concentrated solar energy is employed for thewelding of the aluminum alloy. This alloy is extensively used for the construction ofthin welded structures and has an outstanding mechanical behavior compared toother aluminum alloys. The characteristics of the formed welds are correlated to theheating process and the optimum combination of these parameters is determined.Experimental results are further compared with results obtained from thermalnumerical modeling, in the case of the optimum combination.

2. Experimental procedure

During the experimental procedure extensive bead-on-block experiments on thealuminum alloy were carried out in order to understand how the blocks behave,while treated with concentrated solar energy, with respect to the process parameters.Further research was focused on the estimation of the optimum parameters for thewelding procedure, followed by a few plate-on-block weld experiments.

2.1. Materials

All block specimens used for the experiments were made of aluminum 7075-T6alloy. Block dimensions were 56� 36� 26mm3. The plates used for welding were

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Table 1

Chemical composition of the aluminum alloys used (% w/w)

Material Al Zn Mg Cu Cr

Al 7075 bal 5.6 2.5 1.6 0.3

D.G. Karalis et al. / Solar Energy Materials & Solar Cells 86 (2005) 145–163 147

also made of aluminum 7075-T6 alloy with dimensions 56� 36� 2mm3. Thechemical composition of the materials used is presented in Table 1. The aluminumalloy blocks had a micro-hardness value of 200HV0.3, whereas that of the aluminumplates was 167HV0.3.

2.2. Solar processes

The experiments were carried out at the CNRS Solar Furnace installation inOdeillo, South France, within the framework of the ‘‘Training Mobility Resear-ch–Large Scale European research facilities’’ European program. Part of the 1MWinstallation is shown in Fig. 1a while a sketch of the solar installation is shown inFig. 1b. Experiments were carried out at intermediate 2 kW concentrators as alsoshown in Fig. 1c.

The rate at which the solar energy reaches a unit area on earth is called ‘‘solarirradiance’’ or ‘‘insolation’’ (W/m2). In the case of the solar furnace, this direct solarirradiance is reflected by heliostats. The sun’s rays are reflected onto a parabola thatconcentrates them at a specific focal point with a diameter of 20mm. The energydistribution within the focal area is Gaussian, with maximum flux density obtainedapproximately 16MW/m2. The latter can be expressed as

FS ¼ F0S � e�ðr=dÞ2

and

F0S ¼ 16000� IS;

where FS (W/m2) is the Gaussian distribution of energy, F0S the maximum value of

the distribution, IS the insolation and d (mm) equal to 5.1mm, the distribution’sconcentration factor. This flux ends on the sample to be treated, which is heldhorizontally on a three-axis table that is controlled from a computer unit. The wholearrangement—in case required—is covered by a glass chamber, filled with Argon at1.7 bars pressure. The latter value was selected based on experimental and empiricalsources. The specimen is laid on the table and is also water-cooled on its bottomsurface. In order to reduce solar reflection, the specimen is black coated on itssurface. The coating type usually used has the identification code PIN 20164A L/N10354-D417-2-2, a Medtherm Corporation product. The coated surface is placed atfocusing level. The table is coupled with a small motor device controlled by acomputer unit in order to achieve motion of the table along the longitudinaldirection with constant speed. A typical sketch of the experimental setup is shown inFig. 2.

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Fig. 1. (a) General view of the solar installation at Odeillo in South France (Pyrenees). (b) Schedule of the

operation principle of a Solar Furnace. (c) Heliostat and Parabola for the experimental work.


During the study a total number of 41 experiments were carried out. The variationof solar and welding process parameters of the experimental study are presented inTable 2.

2.3. Characterization techniques

The specimens were cut at several sections vertical to their longitudinal axis. InFig. 3, a typical sketch of a bead on block specimen and two sections vertical to itslongitudinal axis are shown. Optical observation of the microstructure of the treatedspecimens was carried out with the aid of a MZ6 Leica stereoscope and a DMILMLeica microscope. Vickers micro-hardness measurements, with a load of 3N/20 s

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Fig. 2. A typical sketch of the solar treatment set up.

Table 2

Solar treatment parameters of the study

Kind of process Velocity (mm/s) Solar insolation (W/m2) Chamber atmosphere (bars)

Bead-on-block 0.3–1.2 909–980 Open air or Argon (1.7 bar)

Plate-on-block 0.45 958–960 Argon (1.7 bar)

Fig. 3. Two typical sections vertical to the bead on block specimen’s longitudinal axis.


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(HV0.3), were carried out using a SHIMADZU micro-hardness tester. Furthermicrostructural observations have taken place using SEM-EDS technique.

3. Results and discussion

3.1. Macroscopic observations

3.1.1. Bead-on-block experiments

The best specimens in the as treated conditions, namely (a), (b), (c) and (d), onwhich a weld bead was clearly developed along their longitudinal axis followed bylocal melting at the end of the specimens, are shown in Fig. 4. Specimens (a), (b), (c)and (d) were treated with the same velocity value of 0.6mm/s, held at 1.7 bar Argonatmosphere and with solar insolation of 900, 950, 974 and 980W/m2, respectively.

3.1.2. Plate-on-block welds

Only the best plate-on-block weld specimens are shown (Fig. 5). Treatmentparameters were the same for both specimens (see Table 2). In specimen (a), Fig. 5a,local over-melting of the plate was observed. The latter was in bad contact with theblock substrate after the experiment, as the plate was simply held on the block withtwo steel springs. As far as specimen (b) is concerned (Fig. 5b), a weld bead was laidalong the longitudinal axis of the plate, which was held onto the substrate with sixscrews before the commencement of the experiments, in order to maintain goodcontact.

3.2. Micro-structural and numerical studies

3.2.1. Bead-on-block treatments

In order to investigate the micro-structural changes due to solar treatment, allspecimens were cut vertically to their longitudinal axis. A metallurgical study wascarried out focusing on the microstructure of the melted and heat affected zone. As

Fig. 4. Specimens in the as-treated condition.

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Fig. 5. Plate-on-block specimens.


far as the microstructure of the different sectioned planes is concerned, theirappearance was of similar type with differences mainly on the sizes of the meltedzone and the heat affected zone. In the present paper, the specimen cross sectionpresented in Fig. 4a (see Section 3.1.1) is going to be analyzed. The solar treatmentparameters used were: velocity equal to 0.6mm/s, 1.7 bar Argon atmosphere and900W/m2 solar insolation. The specimen was selected among all others as the weldmetal and the heat affected zone were free of pores, the morphology of these zoneswas clearly developed and easily recognized, and the penetration was considered tobe satisfactory.

3.2.1.1. Microstructure. The microstructure of a section of the treated zone of thisspecimen is shown in Figs. 6a and b, while a more detailed analysis of itsmicrostructure is shown schematically in Fig. 6c. The microstructure can be dividedinto three different zones (Fig. 6c), the melted zone MZ (areas 1 and 2), the zone ofpartial melting PMZ (area 3) and the heat affected zone, HAZ (area 4). The sectionunder investigation was taken from a distance 2mm from the edge of the specimen atwhich treatment had started (x ¼ 2mm). Two different types of grains form themelted zone: coarse basaltic grains are observed in the middle of the melted zone(area 1, Figs. 6 and 7a), while at larger depths, grains are equiaxial and finer (area 2,Figs. 6 and 7b). The former provides evidence of slow solidification, while the latterof a relatively higher solidification rate. In the basaltic zone, the mean grain sizemeasured was 250 mm, containing a lot of precipitates; on the other hand, in theequiaxial zone the mean grain size was 55 mm. Absence of precipitates in theequiaxial zone is also evidence of rapid solidification. The equiaxial zone is the limitbetween the melted zone and the partially melted zone (areas 1 and 3, see Fig. 7b).The coarse basaltic zone has maximum width w1 ¼ 8:08mm and depth z1 ¼

0:97mm; while the maximum width and depth of the equiaxial zone is w2 ¼ 8:62mmand z2 ¼ 1:23mm; respectively (see Fig. 6c).

The zone of partial melting has maximum width w3 ¼ 10:92mm and depth z3 ¼

2:65mm (see Fig. 6c). It contains longitudinally oriented grains, with mean

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Fig. 6. (a) Microstructure of a section of the treated specimen (general view). (b) Microstructure of a

section of the treated specimen. (c) Schematic microstructure of the solar treated area of the specimen.

Fig. 7. Melted Zone: (a) Microstructure of the basaltic zone (area 1), and (b) microstructure of the

equiaxial zone (area 2).


longitudinal dimension of 210 mm (see Figs. 7b and 8a). In this zone liquation, partialmelting and post-solidification has taken place while the number of precipitatescontained is decreased. As far as the heat affected zone is concerned (area 4, seeFig. 6c), microscopic observation revealed that the specimen was heat affected

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Fig. 8. (a) Microstructure of the partially melted zone (area 3). (b) Microstructure of the heat affected

zone (area 4).


through its total width and that solution of precipitates has taken place (see Fig. 8b).In order to implement the above microstructural observations on each zone, micro-hardness measurements were used and further finite element analysis was carriedout.

In Figs. 9a–c, micro-hardness measurements results are shown for the specimentreated under the optimum conditions (see Fig. 4a). Measurements (at x ¼ 2mm)were taken across the width (w) of the treated zone, at depth levels (z) of 300 mm (Fig.9a), 1000 mm (Fig. 9b) and 1600 mm (Fig. 9c). Measurements were symmetrical totreatment axis that in case of Fig. 9, is considered to be at the position of 16mm. Inthese figures, HAZ, PMZ and MZ refer to heat affected zone, partially melted zoneand melted zone, respectively.

As shown in Figs. 9a–c, the micro-hardness of the heat affected zone is about140–145HV0.3. In the partially melted zone the micro-hardness increases up to amean value of 170HV0.3, while in the melted zone hardness values reach 180HV0.3

(o190HV0.3). The micro-hardness values near the bottom side of specimen reacheda mean value of 145HV0.3, evidence that almost the whole width of the block hasbeen affected by the heat treatment, compared to the initial hardness value of200HV0.3. Lower micro-hardness values, compared to the values of the aluminumalloy in the as-received condition (200HV0.3), are due to melting, solidification andpartial solution of precipitates that was carried out on specimen treated zone duringsolar treatment.

Further SEM-EDS investigation was carried out on several areas on all differentmetallurgical zones of the best treated specimen. A typical grain boundaryprecipitate of the MZ and a SEM-EDS scanning on the same precipitate is shownin Fig. 10. SEM-EDS analysis showed that the amount of Zn contained on grainboundary precipitates was decreasing moving from MZ to the HAZ from 26% to8%. The same behavior was observed in case of Cu, decreasing from 7% to 1.5%.These observations are also listed in Table 3.

3.2.1.2. Numerical simulation. In order to calculate the thermal cycle of the bead-on-block specimen, a finite element model was set up, using Algor’s Heat Transfer

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200180160140120100806040200

0 2 4 6 8 10 12 14 16 18

(HV

)

200

180

160

140

120

100

80

60

40

20

0

(HV

)(H

V)

HAZ PMZ MZ

Distance (mm)

0 2 4 6 8 10 12 14 16 18

Distance (mm)

0 2 4 6 8 10 12 14 16 18

Distance (mm)

175

170

165

160

155

150

145

140

135

(c)

(b)

(a)

HAZ PMZ MZ

HAZ PMZ

Fig. 9. Micro-hardness values at different depths: z ¼ 300mm; (b) z ¼ 1000mm; and (c) z ¼ 1600mm:


Finite Element code. One half of the cross section, taken from a distance 2mm fromthe specimen edge, was modeled, in order to use the measured heat treatment zonessizes for model calibration (see also Fig. 6c).

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5 10Energy (keV)

0

5000

10000

15000

20000

Cou

nts

CrCuZnCuZnMg

Al

SCl

Ar

Ca Fe CuZnCu

Zn

Fig. 10. SEM-EDS analysis.

Table 3

SEM-EDS quantitative results on grain precipitates

Area % (Zn) % (Cu)

MZ, Basaltic 26 7

MZ, Equiaxial 15 5.3

PMZ 13 5

HAZ 8 1.7


As far as the heat transfer analysis is concerned, 1391 two-dimensional thermalelements were used to model half of the specimen. The material model utilizedassumes temperature dependent physical properties. Initial nodal temperature wasset to be 25 1C while the ambient temperature due to water cooling of the bottomsurface was set to be 15 1C. On the top surface of the plate the thermal loadingconsidered was the surface solar heat flux, together with convection and radiationboundary conditions, while the bottom side of the specimen convection due to watercooling was applied. Convection coefficient on the top surface was set to be5� 10�6W/mm2

1C. Black coating absorptivity was treated by setting surfaceradiation blackness degree equal to � ¼ 95� 10�1: In order to estimate theconvection coefficient between the block surface and the water cooling surface,data from the above presented micro-structural analysis were used. The convectioncoefficient was estimated, so that the temperature value measured at the end of theheating phase at a depth of 1.23mm ðz2Þ was equal to the liquidus temperature,635 1C (see Figs. 6b and c).

The temperature distribution at the cross section examined (x ¼ 2mm) is shown inFig. 11 during heating (t ¼ 7 s), at the end of the heating step (t ¼ 15 s) and at thebeginning of cooling (t ¼ 16 s).

Time–temperature plots of the melted zone, partially melted zone and heataffected zone, corresponding to the plane of symmetry (x ¼ 2mm; w ¼ 0) are shownin Fig. 12, as derived from the finite element analysis. Melted zone plots refer to adepth of z ¼ 0mm from the treated surface, while the partially melted zone and heat

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Fig. 11. Temperature distributions during solar treatment: (a) heating phase at t ¼ 7 s; (b) end of heating

phase at t ¼ 15 s; and (c) beginning of cooling at t ¼ 16 s:

Fig. 12. Numerical time–temperature distribution at four depth levels: (a) MZ, z ¼ 0mm; (b) PMZ,

z ¼ 2mm; (c) HAZ, z ¼ 6mm; and (d) HAZ, near the bottom side of specimen.


affected zone plots refer to a depth of z ¼ 2 and 6mm; respectively. The maximumtemperature reached in case of melted zone at z ¼ 0mm was 705 1C. As far as thepartially melted zone is concerned (z ¼ 2mm; see Fig. 12b), temperatures up to500 1C are attained for about 12 s while the maximum temperature reached at thisdepth was 615 1C. The width of the melted zone as derived from the thermal finiteelement analysis was w ¼ 8:5mm; a value that is in good agreement with

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experimental measurements (w2 ¼ 8:62mm). The maximum temperature reached atdepth of z3 ¼ 2:65mm (the bottom border of the partially melted zone) was 575 1Cwhile the maximum temperature calculated at the maximum width (w3 ¼ 10:92mm)was 565 1C. This temperature value presents a deviation of about 5% fromsolidification temperature (535 1C) and is considered to be accepted in case of thenumerical study.

At depth of z ¼ 6mm (heat affected zone), the maximum temperature calculatedby the finite element analysis was 486 1C while temperatures higher than 100 1C wereattained for more than 18 s. A time–temperature plot referring to the bottom side ofthe specimen is also shown in Fig. 12d. The maximum temperature reached in thiscase was 111 1C and the total cooling time to room temperature was about 15 s.

3.2.1.4. Discussion. The time–temperature transformation diagram for ageing ofAl7075 (Al–6.1Zn–2.4Mg–1.6Cu) is indicated in Fig. 13a, [11,12]. Generally, theobtaining of the high strength in alloys of Al–Zn–Mg–(Cu) system is ensured by alarge volume fraction of fine Zi-(equilibrium precipitate MgZn2) and Z0-(metastableprecipitate with composition and structure similar to MgZn2) phases precipitateparticles. In case of single-stage ageing regimes such structure of precipitates isformed during long exposures, for example, 24 h at 120 1C, or 16 h at 140 1C. In Fig.13a, the shaded B area corresponds to a maximum yield strength while the A areacorresponds to first-stage ageing regimes with minimum duration, at whichprecipitates of GP2 (Gunier Preston zones) and Z0-phase, are forming. Two stageageing regimes, with low temperature first stage and higher temperature second stagecan also be applied, e.g. 3 h at 120 1C, or 3 h at 160 1C in order to accelerate thetreatment process. Retrogression treatment (160–220 1C) on the T6 condition

Fig. 13. (a) Al 7075 TTT ageing diagram. (b) 460–635 1C temperature intervals.

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followed by re-aging can also be applied (RRA). During retrogression, solution ofthe precipitates that occurs, results in low mechanical properties while in the secondstage, the solute re-precipitates and the mechanical properties increase again [13].

In case of bead on block experiments, all treated zones experience similar thermalcycles, but with different maximum temperatures and cooling rates, depending onthe distance from the treated surface. Liquid phase due to heating abovesolidification temperature has been solidified producing coarse basaltic and equiaxialgrains. Solid phase due to heating above 460 1C and below liquidation temperature,has been turned into supersaturated solid solution [14]. In contrast, regions treatedbetween 103 and 460 1C have been partially solution treated or retrograded. Fromthe finite element analysis it was derived that regions up to z ¼ 7mm of depth at theplane of symmetry and at maximum width of w ¼ 19mm on the treated surface(z ¼ 0) were heated to a temperature higher than 460 1C, see Fig. 13b. Post-quenching to room temperature has taken place within approximately 12–14 s, asshown in Figs. 12b and c. As the cooling rate was rather slow, appreciable rapidprecipitation of the supersaturated solid regions has taken place between 398 and260 1C. As a result, supersaturated solutions that are considered to be the optimumcondition for subsequent precipitation and hardening were not fully produced atroom temperature. In retrograded or partially solution treated zones, dissolution ofthe initial—less stable—precipitation phases has taken place. The latter retrogressionprocess has resulted in lower hardness values than those of the initial T6 heattreatment condition as this is described in microstrucutre section (Section 3.2.1.1).

3.2.2. Plate-on-block welds

As far as the plate-on-block experiments is concerned, these were carried out withtreatment parameters similar to those for the bead-on-block experiments (see alsoSection 2.2). A weld zone was observed on both plate and block along thelongitudinal axis only in one specimen that was treated with the optimum parameters(see Fig. 5b). In Fig. 14, the weld zones of this specimen are shown at two differentcross sections.

In Fig. 14a the dimensions of the weld zone are shown at a cross section located18.6mm from the edge from which solar treatment had started, while in Fig. 14b thecross section is located at a distance of 36.5mm from the same edge. The dimensionsof the weld zone are summarized in Table 4. The big difference in weld zone sizebetween the two sections is due to the higher total energy input the second sectionhas absorbed during welding. As the solar spot is moving on the specimen surface,the area in front of the solar spot is preheated to a higher temperature level than theinitial temperature of the first sections where the treatment started. As a result, thesection shown in Fig. 14b has absorbed more energy that of Fig. 14a.

As observed from Fig. 14b, the microstructure of the weld zone consists of grainsof different size. At position No. 1 of Fig. 14b, coarse basaltic grains containingprecipitates are observed with a dimension of about 250 mm (Fig. 15a), while atposition No. 2 of Fig. 14b equiaxial grains are present with a mean diameter of50 mm (Fig. 15b, upper part). Both zones 1 and 2 that contain coarse basaltic andequiaxial grains form the melted zone (MZ). In partially melted zones, longitudinally

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Table 4

Dimensions of weld zone of the welded specimen at different sections

Sectioning position, x (mm) Depth of weld zone, z (mm) Width of weld zone, w (mm)

0 0 0

18.6 3.59 0.76

36.5 16.46 3.84

Fig. 14. Treated zone of the specimen of Fig. 5b at two different cross sections. (a) x ¼ 18:6mm and (b)

x ¼ 36:5mm:

Fig. 15. Optical micrographs of the weld: (a) Zoom on point 1 of Fig. 14b. (b) Zoom on point 2 of Fig.

14b.


oriented grains with a mean dimension of 200 mm are also observed (Fig. 15b, lowerpart). Heat affected zone is also extended until the lower bottom edge of specimen.

In Fig. 16 the cross-section area of the plate and the block is shown for the sectionat x ¼ 18:6mm across the intermediate surface of the plate and the block. Fig. 16crefers to the plane of symmetry of the solar treatment, while Figs. 16a and b refer to

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Fig. 16. Weld area at x ¼ 18:6mm: (a) position w ¼ 3mm (b) position w ¼ 1mm and (c) axis of

symmetry, w ¼ 0:


a small distance from it. From Figs. 16a–c it is obvious that welding has not beenfully achieved, as there are several pores along the intermediate surface. Welding wasachieved mainly in areas where the solar heat flux input reached its maximum value,namely at the vertical plane of symmetry of the specimen, shown in Fig. 16c.

Micro-hardness measurements, presented in Fig. 17, were taken on this specimenalong the vertical axis of the section at x ¼ 18:6mm: This section was chosen sincewelding was achieved along its vertical axis, while at the same time the plateexhibited small geometrical deformation as compared to its initial shape. Fig. 17shows that the block has a mean hardness value of 157HV0.3, which is lower than theinitial level of 200HV0.3. Micro-hardness appears to be uniform along the depth ofthe specimen. The mean micro-hardness value of the aluminum plates was 132HV0.3,which is smaller than that in the as-received condition (167HV0.3).

As far as the microstructure is concerned, several similarities are observed betweenplate-on-block and bead-on-block treatments. The melted zone in both cases consistsof coarse basaltic grains containing precipitates and equiaxial grains of smallerdiameter than those of the basaltic ones. In the partially melted zone longitudinallyoriented grains are observed in both cases. Grain sizes appear to be of the same meandiameter, except those of the coarse basaltic area in which bead-on-blockexperiments resulted in bigger grain size. In the heat affected zones in bothspecimens, extensive solution of precipitates has taken place.

Theoretically, plate-on-block specimens absorb less thermal energy compared tothe bead-on-block ones, even though the experimental parameters have similarvalues. This may happen due to several reasons. First of all, the existence of a smallgap between the plate and the block prevents the heat flux dissipation through theblock. As a result, the temperature history may by altered, resulting in a differentmicro-hardness spectrum in the area of interest. Furthermore, the steel bolts used tofasten the plate on the block have a lower thermal conduction coefficient comparedto aluminum and thus absorb a lot of energy, especially at temperatures close to themelting temperature of aluminum.

On the basis of what was presented, it can be said that, in general, the higher theheat input, the larger the affected zones observed. As a result, both the width anddepth of the affected zones of the material grow with increasing heat input of thetreatment. On the other hand, specimens that were treated with different velocities

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00

5 10 15 20 25 30

Distance (mm)

20

40

60

80

100

120

140

160

180

Mic

roha

rdne

ss (

HV

)

Bl

o

c

k

P

a

t

e

l

Fig. 17. Micro-hardness measurements along the vertical axis (x ¼ 18:6mm; w ¼ 0mm).


and almost constant heat input showed different size of the treated zones. They werewider in both directions for relatively low velocities, as compared to other specimensthat were treated with higher speed values. These conclusions are in agreement withthe theory and the experimental results obtained from similar treatments using highdensity energy sources, e.g. laser beams [5,15].

4. Conclusions

The application of concentrated solar energy for the welding of 7075 aluminumalloys was attempted in the present work. Bead-on-block and plate-on-blockexperiments were carried out aimed at investigating the effects of this heat treatmenton the welding metallurgy of aluminum alloy specimens. The main observations arethat:

�
the microstructure of the treated zone appears to be of three different types,namely coarse basaltic and equiaxial in the melted zone, with longitudinal grainsin the partially melted zone and finally grains without precipitates due toprecipitation solution in the heat affected zone, and
�
the micro-hardness values were smaller in all regions as compared to the values inthe as-received condition, a fact owing to solar treatment. The micro-hardnessreduction observed was about 25% for the heat affected zone, 20% for thepartially melted zone and 10% for the melted zone.
Welding of aluminum plates on aluminum blocks was partially achieved usingconcentrated solar energy; further experimental study and investigation should be

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undertaken, however, in order to achieve, if possible, welds with satisfactorypenetration and strength. The most important observations are that:

�
The produced weld is considered to have satisfying penetration only in thelongitudinal axis of the solar treatment;
�
The microstructure is changed compared to the initial microstructure of thematerial, more specific, it appears to be of various morphology, with non-uniformgrain size in the welded area (similar to bead on plate experiments);
�
Both plate and block in the welded area showed lower hardness values (78% incase of block, 79% in case of plate) compared to specimens in the as-receivedcondition;
�
Metallurgical transformations that have been involved during the solar treatmentof a 7075-T6 aluminum alloy, regard to melting or partial melting, solidification,solution or partial solution treatment and retrogration.
Acknowledgments

The authors gratefully acknowledge that the solar experiments were made possiblethrough the financial assistance of the ‘‘Training Mobility Research—Large ScaleEuropean Research Facilities’’ European program, with contract number ERBFMGE CT980113.

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