Influences of post weld heat treatment on tensile properties of friction stir welded AA2519-T87 aluminium alloy joints

3.1 Tensile properties

The super imposed stress strain diagram of AW, AA and STA joints are shown in the Figure 1. The tensile properties like yield strength, ultimate tensile strength and elongation are derived from the stress-strain diagram and presented in the Table 2. The tensile strength is recorded as 248 MPa for AW, 326 MPa for AA and 395 MPa for STA condition. The notch tensile strength is recorded as 240 MPa for AW, 263 MPa for AA and 316 MPa for STA. The AW joint exhibits lower tensile strength than the parent metal. The STA treatment improves the tensile strength of FSW joints. Thus a maximum joint efficiency of 85% is observed in the STA joint. The notch strength ratio (NSR) of the parent metal is calculated as 1.064. The NSR of the welded joints is less than unity. The NSR values for the AW and PWHT joints are more or less similar, but lower than the parent material. Elongation of 9.02% was observed in AW joint which is lower than the parent metal but higher than AA and STA joints.

Figure 1:

Stress-strain curves.

Figure 2 shows the fractographs of the parent metal and welded joints. The SEM fractographs of both smooth and notched tensile specimens of the parent metal show fine populated dimples oriented towards the loading direction (Figure 2A and B). The AW joint fractographs show large sized dimples than the parent metal for both notched and unnotched conditions (Figure 2C and D). The fractographs of the smooth tensile specimen of AA joints shows populated dimples and a few flat featureless faces (Figure 2E). The fractographs are characterised by the presence of fine dimples along with a few tear ridges (Figure 2E and F). Populated dimples are observed in all the joint conditions, but the sizes of dimples are comparatively higher than the parent metal.

Figure 2:

Fractograph of tensile specimens of the parent metal and FSW joints.

3.2 Microhardness

The microhardness measured across the welded joint at mid-thickness region is shown in Figure 3. The AW joint shows lower hardness values in the TMAZ than the other region. The stir zone results show moderate hardness of 116 HV. The hardness values decreased towards the TMAZ and then increased towards the parent metal. The stir zone hardness of the AA joint is 148 HV, but the other regions of the AA joint show a similar trend as seen in the AW joint. In the STA joint, a maximum hardness of 151 HV was attained in the stir zone, which is marginally lower than parent metal hardness of 161 HV. The inset figures shown on each microhardness plot display the cross sectional tensile fracture locations (Figure 3A–C). In the AW joint, tensile fracture was observed at the interface of SZ and the retreating side of TMAZ. In the AA joint, the tensile fracture was shifted to the stir zone in the shoulder influenced region, however, the facture fall in the TMAZ region from the mid thickness region to pin influenced region (Figure 3B). In the STA joint, the tensile fracture location was shifted to the advancing side of the stir zone.

Figure 3:

Microhardness distribution (LHDR: lowest hardness distribution region).

3.3 Microstructure analysis

Figure 4 shows the optical micrograph of the parent metal and stir zone of AW, AA, STA. The grains of the parent metal are coarse and elongated towards the rolling direction (Figure 4A). The stir zone of AW joint shows recrystallised grains of finer size than the parent metal. Fine grains are observed in the stir zone of AA and STA joints. Even later the ageing treatment, no considerable grain growth is noticed in the stir zone of AA and STA joints (Figure 4C and D).

Figure 4:

Optical micrograph of the parent metal and stir zone.

Figure 5 shows the optical micrograph of the thermo mechanical affected zone (TMAZ) of all the joints. The interface microstructure shows transition grain size towards the stir zone. In the TMAZ of the retreating side (RS), the coarse deformed grains are pulled towards the stir region. The advancing side (AS) TMAZ shows the abrupt transformation in the grain size and orientation towards the stir zone. There are no significant changes observed in the AW and PWHT joints under OM.

Figure 5:

Optical micrographs of interface regions of welded joints.

Figure 6 shows the TEM images of the parent metal and stir zone of AW, AA and STA joints. AA 2519-T87 is an age hardenable aluminium alloy strengthened by the θ′-CuAl₂ phase, which is semi-coherent with the Al matrix [22]. Figure 6A reveals the presence of θ′ precipitates in the parent metal microstructure. Here the precipitates are of needle like morphology oriented normally to each other. The precipitate distribution is even and denser in the Al matrix. Figure 6B shows the TEM image of AW joints. This microstructure is characterised by the presence of precipitate free zones (PFZ) and dense dislocations. In the AA joint, the reprecipitation along the grain boundary is observed. The region adjacent to the grain boundary is identified as PFZ. However, fine θ′ precipitates with denser distribution are seen in the other region of the Al matrix (Figure 6C). Figure 6D shows the TEM image of STA joint. Dense distribution of θ′ precipitates are observed in the Al matrix.

Figure 6:

TEM micrograph of the parent metal and stir zone.

Table 3 shows the image analysis results of the TEM image of stir zone. The area fraction, average precipitate size, approximate particle spacing and grain size of stir zone microstructure were analysed and estimated. The parent arent material shows a dense distribution of θ′ precipitate of 22% and no stable θ precipitate is noted. The size of the precipitate is 43 nm in diameter and 3 nm in thickness. The precipitates are dense and evenly distributed and thus results in lower particle spacing of 48 nm. In AW joints, the stir zone microstructure reveals the minimum amount of θ′ precipitate of 1% and θ precipitate of 6%. The precipitates are coarse and have a diameter of 93 nm and thickness of 36 nm. The AA joint shows 32% of θ′ precipitate and 2% of θ precipitate. The diameter and the thickness of the precipitates are 62 nm and 17 nm, respectively. The dense precipitate distribution of AA joint shows particle spacing of 28 nm. In an STA joint, the θ′ precipitate contributes 42%, which is higher than the other joints. The average precipitate diameter is 41 nm and thickness is 21 nm. The inter particle spacing between the precipitate is 18 nm in distance. The average grain size is measured using the line intercept method.

Similarly, the grain size of the parent metal was measured and compared with the grain size of stir zone of AW, AA and STA joints. The average grain diameter of the parent metal is 49 μm. The grain diameter is greatly reduced to 3.95 μm, 4.2 μm and 4 μm in the stir zone of AW, AA and STA joints, respectively.

4.1 Influences of PWHT on mechanical properties

The strength or hardness of the aluminium alloys is mainly dependent on the interaction between the dislocation motions and its hindrance [23]. In an age hardenable aluminium alloy, the hindrance of dislocations is mainly achieved by the grain boundary and the precipitate availability. Using the Hall-Petch relation, it was found that grain size is inversely proportional to the hardness [24, 25]. Finer grain size results in higher hardness or strength due to the higher grain boundary energy. However, the stir zone of AW and AA joints results in lower hardness values despite the finer grain size. It can be inferred that, for heat treatable aluminium material, the effect of grain boundary on hardness is less significant.

During indentation, the precipitates act as the obstacles for the dislocation motion. The plastic deformation was resisted by the precipitates which in turn increase the hardness or strength. In the AW condition, the area fraction of θ′ precipitates is too low to resist the indentation (Table 2). The stir zone exhibits PFZ which is identified as the softest region. However, the hardness value in the stir zone is higher than the TMAZ. This is because, the PFZ identified by TEM analysis cannot show the tiny cluster of solute particle called the Guinier-Preston (GP) zone which is few angstroms in size. The presence of this GP zone increases the hardness or strength of the stir zone. Similar strengthening by GP zone is observed by Fonda et al. [26].

Few researchers have studied the mechanical properties of FSW joints of AA2519 alloy. Xiao et al. found that TMAZ is the weakest region and similarly Fonda et al. also reported that TMAZ as the softer region in the joint [26, 27]. They attribute the softening of TMAZ to the dissolution of precipitates. During FSW, the precipitates in the stir zone and TMAZ region are dissolved. The stir zone exhibit higher temperature than the TMAZ and thus the lowest cooling rate is observed in the stir zone. The slow cooling in the stir zone enabled the diffusion to form clusters of solute particle (GP zone). But the higher cooling rate prevailed in the TMAZ restricts the reprecipitation. And so the TMAZ of AW condition exhibited lower hardness values (Figure 3A). In the AA condition, the hardness in the stir zone and TMAZ-stir zone boundary region are recovered to a maximum value of 148 HV (Figure 3B). This region is regarded as the pre-solutionise region created by FSW. Thus, it is able to re-precipitate on ageing. The high area fraction of θ′ precipitates in the stir zone are the reason for the higher hardness. However, the fluctuating hardness is observed due to the presence of PFZ and the precipitate zone (Figure 3C).

During FSW, the heat conducted to HAZ has coarsened the precipitates. During ageing, the coarsened precipitates grow in size to become stable θ precipitates. Thus the hardness values are lower at TMAZ-HAZ boundary and HAZ. In the STA condition, the loss of hardness values at the stir zone, TMAZ and HAZ are recovered to a greater extent. The ST followed by ageing treatment increase the area fraction of θ′ precipitates in the all the zone. This is the only reason for the improvement of hardness in the whole joint (Figure 3C). However, due to the accelerated nucleation enabled by the dislocations and grain boundaries, few coarsened stable θ precipitates were formed which results in fewer lower hardness values in the stir zone.

In addition to the precipitate availability, the strengthening of material is governed by the interparticle spacing between the precipitates. If the interparticle spacing is high, the hindrance of dislocation movement is prominently reduced. The dislocation movement cuts through or bypasses the precipitate during tensile loading. The precipitate required shear energy to break up which determines the strength of the joint. On the other hand, the dislocations bypass the precipitates leaving dislocation loops called the Orowan loop. The finer precipitates undergo break ups and on the increase of the precipitate above a critical radius, the dislocation is moved by bypassing. Thus the precipitate size has an influence on deciding the mode of the dislocation movement. If the particles are fine, closely distributed and dense in number, then the movement of the dislocation is hindered greatly. Thus based on these factors, the strength of the joint is decided. Among the PWHT condition, STA joints have dense and closely distributed precipitates. This is the reason for enhancing the tensile strength of STA joints. However, due to the high dislocation density in the stir zone, the precipitate growth becomes accelerated to 21 nm thickness which is much higher than the parent material. Thus during tensile loading, the fracture falls in the stir zone which can also be correlate with the softer region of the microhardness plot (Figure 3C).

As discussed in the microhardness plot, for the AW condition the fracture falls in the retreating TMAZ which is the softest region among the various regions. Due to less area fraction of θ′ precipitates and occurrence of wide PFZ, the AW joint results lower tensile strength of 248 MPa. In the AA joint, a high degree of heterogeneous microstructure was evolved due to the partial precipitation and coarsening of precipitates. Despite the TMAZ being softer, the tensile fracture occured in the weld centre. This is because during ageing the copper depletion creates nano level PFZ near to the grain boundaries of the stir zone. From this investigation, it is found that ageing has a greater effect on the enhancement of hardness and strength. But the ductility properties are not significantly altered after ageing which can be confirmed by the fracture surface analyses. This is because the presence or absences of precipitates does not considerably influence the ductility due to the inherent Al matrix exhibitting a high degree of ductility.

4.2 Influences of PWHT on microstructure

The microstructure of FSW joint shows variation in the grain size from the stir zone to the parent metal region. The stir zone exhibits recrystallised grains of fine size (3.95 μm). As aluminium has high stacking fault energy, the grains are expected to be dynamically recrystallised during the plastic deformation at elevated temperatures created by tool stirring. A few researchers like Liu et al. and Hassan et al. investigated the ageing behaviour of AA2219 and AA7010 FSW joints, respectively [28, 29]. From these investigations they reported that on ageing, the stir zone of AA2219 and AA7010 joints undergo abnormal grain coarsening due to the thermal activation energy supplied. However, on AA and STA, the grain size is very similar to the AW condition (Table 2), owing to the precipitation along the grain boundary pins the grain growth further [30].

AA2519-T87 aluminium was alloyed with copper for the enhancement of the strength of the material [31]. The solubility of the Cu element in the Al system is varied with respect to the temperature. In addition the Cu induces a significant misfit strain in the aluminium. By satisfying the above two conditions, the AA2519 T87 aluminium alloy can be substantially strengthened by the precipitation process. In the age hardenable aluminium alloys, the precipitates searched for the existence of favourable sites for nucleation. The sites like the grain boundary, dislocations or dislocation loops are the favourable sites which accelerate and assist the precipitate nucleation [32–34]. The tempering condition T87 stands for solution treated and cold worked followed by artificial ageing. During cold working of the parent material, dislocations were evenly formed in the entire matrix. Dislocations are the favourable site and so the precipitates nucleate at the dislocations during artificial ageing. This is the reason for the evenly distribution of θ′ precipitate in the parent metal (Figure 6A).

During FSW, the high strain induced plastic deformation result in dense dislocations in the weld region. The dislocation favours the precipitation because dislocations can lower the energy barrier and motivate the formation of θ′ precipitates [35]. The degree of straining is varied from the shoulder influenced region to pin influenced region and from weld centre to the periphery. Thus the heterogeneous formation of dislocation is observed in the stir zone of the AW condition (Figure 6B). During FSW, the stir zone attains nearly 80% of melting temperature of the parent material. This heat is sufficient for solutionising the precipitates in the stir zone region. During the cooling cycle, the time and heat is not sufficient in the stir zone to diffuse Cu atoms from the Al matrix. The temperature distribution is varied from the shoulder influenced region to pin influenced region and from the weld centre to the periphery. Thus the weld joint attains further heterogeneity due to the difference in the thermal condition. Because of this, both the precipitate zone and PFZ are observed in the AW condition (Figure 6B).

However, the diverse precipitation can be thermally stabilised to a great extent by the ST. During ST, the precipitates in the entire joint are dissolved and a stable microstructure is observed. On quenching, it results in the super-saturated solid solution. During quenching, the time and temperature are insufficient for the system to diffuse Cu atoms from the Al matrix. In the STA process, the precipitates nucleate more evenly from the matrix (Figure 6D). In AA joints, precipitates of different size and distribution were observed. During FSW, the temperature distribution and cooling rates differ from the shoulder influenced region to the pin influenced region and from the stir zone to the parent metal region. Thus, in the AW condition, the geometry and availability of the precipitates are different. It is composed of PFZ, fine precipitates and coarsened precipitates. On further ageing of this heterogeneous structure, a varied degree of precipitate nucleation and growth resulted (Figure 6C).