Impact of the heat treatments on the microstructure of the aluminum alloys 7075 T6 welded by the friction stir welding process
Khatir Mohamed, Temmar Mustapha, Aissou Hacene, Melzi Nesrine
Department of Mechanical Engineering, University of Blida 1, BP 270, Blida, Algeria
Abstract
The welding process of the aluminum alloys 7075 T6 is increasingly used in the aeronautical industry. The use of the friction stir welding process (FSW) requires a good understanding of the microstructure generated by the rapid temperature rise in the heat affected zone and the thermomechanically affected zone of the used alloy.
By applying the process of welding FSW, the characterization of the microstructure, because of the diversity of its components in terms of size and nature, requires the use of multiple techniques of investigations such as the use of optical microscopy and scanning electron microscopy for local approaches. It also requires the use of the impact strength test to assess the quality of the weld, to characterize the interaction between the material and the welding process, and to provide quantitative data on the welded joints behavior.
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Keywords: Aluminum alloys 7075 T6, Friction stir welding process (FSW), Heat treatment, Microstructure, Impact strength test.
Nomenclature
T°
α
Temperature , °C
Inclination angle of the tool, ° T Time, hour
1. Introduction
The aluminum alloys 7075 T6 are widely used in different sectors of the industry because of their light weight, their mechanical properties and their ecological character. Their mechanical properties are partly related to the nature and proportion of the elements of these alloying elements, and their tendency to favor both the formation and distribution of α and β phases. Any process thermally or mechanically activated globally affects the microstructural morphology, leading to changes in the mechanical properties of the alloys.
Welding is a process that combines two metallic elements by melting and resolidification. It ensures mechanical continuity between the parts to be assembled with or without the help of a filler product achieving a better link between these two metallic elements. Among the various welding processes, we opted for the friction stir welding. This friction welding uses the relative deformation of the two parts to be welded to produce non- contaminated interfaces. At the same time, the energy dissipated by friction heats the interfaces to facilitate their shears.
2. Experimental materials and procedures:
2.1. The material:
The alloy selected for this study is a commercial aluminium alloy 7075 T6. This alloy was provided by the department of maintenance of Air Algeria. It possesses a relatively low content of solute elements compared to high-strength alloy like the Al-Zn-Mg-Cu alloys.
Consequently, it has the advantage of a lower quench sensitivity. Also, it has good mechanical properties that can stem from precipitation strengthening.
The nominal composition of the alloy is shown in the table I:
Table I. Nominal composition of the alloys in weight percent (wt %)
Al Zn Mg Cu Fe Cr
89.72 5.63 2.50 1.53 0.22 0.19
Si Ti Mn V Others
0.10 0.04 0.03 0.01 0.03
The micrograph of the alloy is shown in figure 1:
Fig. 1. Micrograph of the alloy
Laminated industrial Aluminum alloys “7000” are notably used as metal sheets for aircraft wings. These alloys contain the elements Zn, Mg and Cu as the main alloying elements. Their use is due to their higher mechanical properties.
Aluminum alloys undergo during their implementation a complex thermomechanical treatment that includes after melt, a homogenization treatment, and then various rolled (hot then cold). These steps determine the granular structure of these alloys.
In our literature research, we are only interested in the stages succeeding the rolling. These steps involve the precipitation treatment consisting of dissolving, quenching, mechanical treatment (the stress relieving), maturation, and tempering [1].
According to the classical theory of Gibbs extending the work of phase transformations to the solid state by Turnbull and Fischer, the germination is an initial stage of the precipitation. From the matrix, it produces the formation of stable germs which are rich in solute, their composition is very different from that of the solid solution and they have a defined interface with the matrix.
The subsequent growth of germs and precipitates is governed by the diffusion of solute atoms to germs, which is thermally activated at the tempering temperature while the solid solution is supersaturated. The proposed precipitation mechanisms include the consideration of thermodynamic factors and kinetic effects and they are applied to majority of the aluminum alloys hardened by precipitation. [2].
Other research has been done thereafter in order to deepen the sequence of thermomechanical treatments (used in industry) for aluminum alloys “7000”. One can cite the work of Alexis Deschamps and David Dumont.[3],[4].
For Alexis Deschamps, it appears that to obtain the best compromise between different properties to use an alloy, it is necessary to understand the evolution of these properties in an integrated manner throughout the production process. These properties are: mechanical
properties, damage, and corrosion resistance.
The different phases that may appear in the alloys
“7000” can be divided into three categories:[5]
The hardening precipitates that control the mechanical properties of plastic material (yield strength and hardening rate),
The dispersoids that control the phenomena of recrystallization,
The intermetallic particles.
2.2. Friction Stir Welding Process:
In aerospace, fasteners must perform five main functions: [6]
Providing mechanical stress,
Maintaining the integrity of the assembly vis-à-vis corrosion,
Transmitting current metallization lightning,
Ensuring the sealing reservoir area,
Making maintenance as easy as possible.
The main assembly systems used are riveting and bolting. Currently in the field of aeronautics, the welding process joins the technology competitor. This process change has become an important financial issue since it would obviously considerably reduced weight, thus fuel consumption and operating cost.
Welding is an operation to join two metal parts by melting and re-solidification. It ensures mechanical continuity between the parts to be assembled with or without the aid of good filler for a better link between these two metal parts.[7].
Among the different welding process, we used for our work the friction stir welding process (FSW). This process was patented and developed by Thomas Wayne in 1991 at the welding institute in England. Its application differs from the other welding processes by its ability to weld the material in the viscous state without reaching the melting point. It is based on a simple concept and on frictional heat generated between the tool and the alloys. In practice, and while the welding stars, a dowel is rotated between 180 to 300 revolutions per minute, depending on the thickness of the material. The pin tip of the dowel is forced into the material under a force and continues rotating and moves forward. As the pin rotates, friction heats the surrounding material and rapidly produces a softened plasticized area around the pin.
As the pin travels forward, the material behind the pin is forged under pressure from the dowel and consolidates to form a bond. Unlike fusion welding, no actual melting occurs in this process and the weld is left in the same fine-grained condition as the parent metal.
After the welding process, we find the following areas:[8]
The base metal (BM) is the furthest part of the welding process. It that does not undergoes a deformation and microstructural changes. The heating process is not sufficient in order to modify the structure.
Although metallurgically unchanged, the base metal, as the overall solder joint, is a concentration of transverse and longitudinal residual stresses depending on the degree of shrink imposed on the weld.
The Heat affected zone (HAZ) which is close to the center of the welding. In this part, the metal undergoes a heating cycle where the maximum temperature can be above 250 °C. In this case, the microstructure and the mechanical properties are modified. However, no significant deformation (observable across the optical microscopy) occurs in this region. The grain shape is identical to that found in the base metal.
The thermomechanically affected zone (TAZM) is lying around the fusion zone and is specific to the friction stir welding process. It is both deformed plastically and affected thermally. The border between the central welding area and the thermomechanically affected zone is usually very strong, often on the advancing side. A partial recrystallization is also observed along this border. The evolution of the precipitation is resulted in this zone by a partial dissolution of hardening precipitates and heterogeneous precipitation of non-hardening precipitates. In the case
of our alloys, the temperatures reached in this region a range from 300 °C to 400°C.
The fusion zone (FZ)is the region around the center of the welding area. It is heated above the liquidus temperature and corresponds to the maximum deformation and temperature. In the case of our alloys, the maximum temperature reaches a range from 425°C to 500 °C.
FZ TAZM HAZ
Fig 2. Micrograph of the welded and treated sample
Regarding our work, the FSW welding process was conducted at our Mechanical Department.
The following picture shows an example of the experimental set up of this case.
Fig 3. The use of the FSW welding process at our Department
3. Experimental procedure:
After the bibliographical review, we will focus on the experimental part of the studied material. We will proceed to different welding stages, samples preparation, completion of welding and conducting thermal treatments. After welding is carried out we will proceed to a metallographic observation by an optical microscope and then perform some mechanical tests.
Three stages of processing have been hardening procedures:
The dissolution of a relatively high temperature and a maintain allowing a dissolution of a large amount of intermetallic phases in the aluminum matrix.
A quenching whose goal is to maintain at a room temperature a solid solution supersaturated with alloying
elements. This was achieved by performing a formal solution of the elements constituting the precipitates present at room temperature. The after quenching gives supersaturated solid solution element additions. At room temperature, this solution is metastable. The quenching freezes the state of the dissolution and also captures the gap created by the effect of temperature. The elements of additions and gaps are positioned randomly in substitution of aluminum atoms in the crystal lattice nodes of the aluminum matrix.
Tempering is practical after quenching by heating to a temperature lower than that of the dissolving. It improves the mechanical strength of the parts treated by decreasing the hardness and internal thermal stresses obtained during quenching. We heat at a temperature lower than that of austenite, and then we cool more or less rapidly.
In some cases (hardening alloys) allows tempering increase after quenching the mechanical properties.
At the beginning of our experience, the optimum temperature for solution has been selected. Several samples at different temperatures (470, 490, 510, 530°C) were used to allow dissolution of a large amount of phase present in samples and in the same manner to determine the optimal time-keeping at the temperature found. The artificial aging temperature use was 120, 140, 160 and 180 °C for holding time of 30min, 1h, 2h, 4h and 6h. Everything was to get a good hardness. The results for the Vickers Hardness were in succession 178, 174, 176 and 176 with an average of 176.
The table 2 shows the different steps of the heat treatments:
Table 2. The different steps of the heat treatments Solution treat temperature
450-490-510-530°C Soaking time
20min
Water Quenching
Artificial aging temperature 120-140-160-180°C Soaking time
for each aging temperature 30min 1h 4h 6h The parameters used while welding are:
Inclination angle of the tool = 2°,
Rotational speed = 1400 tr/min,
Feed rate = 2 mm/sec.
Samples, having a thickness of 2 mm, were taken thereafter to determine the impact strength test and to check the status of their weld. The samples were performed according to NF EN 10045. They have been executed by using a Hoytom testing machine. Afterwards, optical microscopy and scanning electron microscopy has been used to characterise the structure of the alloys in their initial state and after heat treatment and welding.
4. Results and discussion:
We used the resilience mechanical test in order to determine the energy of rupture (brittle or ductile) and we studied the variation of hardness and micro-hardness in each zone of welded items (BM, HAZ, TAZM and FZ). The hardness characterizes the resistance to penetration of a body by the action of a load. The evaluation of hardness was done according to standard NF A 91 118 and ISO 4516, with Vickers type Indenter. These tests were conducted under a load of 500g. The testing of resilience was intended to measure the required energy to break at once a specimen already notched. The absorbed energy is obtained by comparing the difference of potential
energy with the start of the pendulum and the end of the test. After observation and by the optical microscope and by the scanning electron microscopy, we have determined changes in the microstructure caused by the FSW welding; the latter has positive and negative effects of welding.
Our material is known to have good mechanical properties and once it is submitted to different high temperature, the precipitate microstructure becomes not stable and loses some of the good mechanical properties.
For our alloy, the precipitation sequence is relatively complex because different states may occur when this alloy is solicited.
Fig 4. Respectively microstructures of our alloy at a solution treat temperature and an artificial aging
temperature at 160 °C for 6 hours
The unfavourable influence of the solution treating treatment reduces considerably the hardness of the sample compared with it received state (T6) knowing that à T6 state, the sample has the best mechanical characteristics. It may be due to the presence of a small quantity of η' phase that was not all dissolved during the solution treating. The different micrographs that our alloy contains phases that contain Fe, Si and Cu and the phases Al2CuMg, Mg2Si and (Zn,Cu,Al)2Mg. The particles of intermetallic phases are coarse and this indicates that all used materials were well prepared. The coarse insoluble intermetallic present in our alloy are usually harder than the matrix.
For the case of a sample welded and not treated, the microstructure of the samples shows that there is a nonuniform distribution of the grain size.
While welding, the shape of the grains changes. They begin to grow and the particles begin to diffuse into the TAZM zone and the FZ zone. The change of the microstructure in these areas appears clearly comparing to the base metal.
Fig 5. Microhardness our material welded
We can notice that:
To dissolve the maximum phases present in our material, we must apply hardness for a solution treat temperature of 510 ° C for a time of 10 min.
This treatment must be followed by a water quenching and a solution treat temperature of 160 ° C for 6 hours.
To have a η predominant precipitated, the hardness should be almost identical to the hardness of the alloy in the received state,
For artificial aging temperature greater than the optimum temperature, the hardness is low.
The hardness values of our samples are not affected only by the saturation at higher temperatures of the solution treat temperature but also for the rate of supersaturation during higher temperature. Meanwhile, and in order to obtain a good hardness of our samples, we should after the solution treat temperature cool quickly enough our alloy so that the solution should stay saturated at room temperature.
Therefore, it is necessary that the concentration of the solid solution, at room temperature, should be close to the temperature of the treat ssolution treat.
In the limits of fusion zone and when hardness is changing, the dissolution of precipitates appears when the particles are subjected to temperatures above 430 °C. The dissolution process enriches the solid solution of α phase with Zn, Mg and Cu. It results from the increase of the hardness. Also, the state of the base metal stays the same and where the microstructure stays also the same during the welding process.
The energy varies from one zone to another. Its impact in the melted zone remains the same while in the HAZ and TAZM zones, it varies.
The use of welded 7075 T6 aluminum alloys generates two important applications:
A change of the microstructure and an important variation of the grains sizes.
Emergence and formation of cracks after welding.
From our work, we deduce that the impact test is used to test the resistance in its own conditions but one trial conducted at any temperature has only a very limited significance. In practice, it is the analysis of changes in resilience test results performed at different temperatures that will give its interest at this test type in order to appreciate the resistance to a brittle fracture of our material.
Fig 6. Photographs by the Scanning Optical Microscope of the fracture zone in TAZM of our sample with the
appearance of cracks
5. Conclusion
Our study allowed us to study the influence of the heat treatment of the 7075 T6 aluminum alloys welded by FSW. It allowed us to distinguish the changes characteristics in the structure during the application of this method in the different zones (BM, HAZ, TAZM and FZ) depending on the state initial alloy. We can conclude that:
The application of the FSW welding process has an effect on the reduction of the impact energy in the fusion zone of our material,
An increase or an extension to an isothermal holding, leads to occurrence of phase equilibria incoherent with the matrix. Coalescence of the latter leads to the softening of the alloy,
The thicknesses of the samples have no influence on the microstructure of welded joints in the weld bead. In this section, the conditions of strain and temperature are such that they completely transform the structures exploiting the properties of the joints and the precipitation conditions,
Our alloy shows a certain lightening through the welding process. The experience has highlighted the evolution of the precipitation state through the welding process. As we approach the core, we notice a change in structure represented by a reversion of the GP zones then precipitation η'.
The precipitation phenomena’s are of course influenced by the temperature and by the time, but the influence of the temperature is more important than of the time,
The formation of microstructures during the welding process is mainly due to the low values of the impact strength test and the properties of the energy impact of the welded material.
Acknowledgments
This work was supported by the Laboratory of Structure of the Department of Mechanical Engineering of the University of Blida 1, Algeria.
The authors would like to thank Professor Hadji for his assistance and all the persons that organized the 8th International Conference on Thermal Engineering: Theory and Applications. Also, a special thank to all the participants of this conference.
6. References:
[1]: Embury J. D, Nicholson R.B, The nucleation of precipitates: the system Al-Zn-Mg, Acta Materialia, pp 403- 417, 1965.
[2]: Wilfried Kurz, Introduction to the sciences of materials, second edition, polytechnic and universities Romandes, 1991.
[3]: Alexis Deschamps, thesis of doctorate: Influence of the predeformation and the heat treatments on the microstructure and mechanical properties of the alloys Al-Zn- Mg-Cu, 1997.
[4]: David Dumont, thesis of doctorate: Relation of the microstructure/tenacity in the aeronautical alloys (7000), 2001.
[5]: Hono K, Sano.N, Sakurai.T, Quantitative atom-probe analysis of some aluminum alloys, Surface Science, pp 350- 357, 1992.
[6]: Bertaux, The structural mechanical assemblies in civil aeronautics-Fixations and processes of assemblies, Report of conference, Mecamat, Aussois, January, 2003.
[7]: Blondeau.R, Process and industrials applications of welding, Hermes science publication, Paris, 2001.
[8]: Threadgill P.L, Leonard A.J, Shercliff H.R, Withers P.J, Friction stir welding of aluminum alloys, International Materials Reviews, volume 54, pages 93, 2009.
