Strengthening effects and mechanical properties

In precipitation hardening aluminum alloys, a superposition of several strengthening effects occurs, including grain refinement, precipitation hardening and dislocation strengthening, resulting in complex relations between the initial microstructural condition, the thermomechanical treatment during friction welding and the resulting mechanical properties [90].

Precipitate evolution

The welding process causes profound microstructural changes since the alloys investigated in this study contain precipitates, which undergo transformations at elevated temperature. The precipitate evolution caused by exposure to the weld thermal cycle in different weld zones is a complex function of the peak temperature, exposure time as well as heating and cooling rate and depends highly on the

Process description

41 alloy composition and temper state. As precipitation is the main hardening mechanism in the alloys of interest, the exposure to different thermal cycles in the weld zones causes microstructural changes that lead to local variation in mechanical properties. The precipitation evolution for the alloys of interest is analyzed in Chapter 6.3.2, 6.2.2 and 6.4.2 in each case because the aging response depends on the composition and initial temper and thus cannot be generalized.

Grain size and dislocation density

The grains in the SZ were significantly refined, thereby increasing the number of grain boundaries.

Grain boundaries can either weaken (intercrystalline fracture, stress-corrosion cracking) or strengthen (Hall-Petch effect) polycrystalline metallic materials [91]. For the quasi-static tests conducted in this study, the strengthening impediment of dislocation movement by grain boundaries is assumed to be dominant. Thus, the strength contribution from grain boundary strengthening is higher in the SZ than in the TMAZ and BM.

A different dislocation density in the stirred regions of friction welds from that in the BM caused by the dynamic recrystallization process is commonly reported. The dislocation density and thus the strength contribution were reported to be higher in the BM of various friction welded precipitation hardening aluminum alloys, e.g., of friction stir-welded AA 2198-T8 [90], AA 6061-T6 [92] and AA 7075-T6 [93].

To assess the general influence of grain refinement strengthening and dislocation strengthening in the alloys of interest, welded samples were solution heat treated and aged at RT; see Appendix D.

This treatment equalizes the precipitation morphology in the different weld zones of the welded specimens. Any remaining difference in the hardness of the weld zones can thus be attributed to a difference in grain size and dislocation density. For the alloys of interest, no significant difference in hardness between the weld zones could be determined, whereas significant differences in grain size were apparent. Similar results were reported for solution-treated and artificially aged repair welds using RFSSW of AA 6061-T6 [29]. Since smaller grains cause an increase in strength in aluminum alloys, the weld zones must exhibit a remaining difference in dislocation density to achieve the same hardness. The dislocation density has thus not been equalized in the different weld zones during the solution heat treatment. In AA 2219, the dislocation density might be further reduced in the SZ during the solution treatment due to grain growth. It is thus assumed that the effect of higher grain refinement strengthening is essentially neutralized by the lower dislocation strengthening effect in the SZ than in the HAZ of the welds. This assumption is in reasonable accordance with the results reported by Gao et al. [90] for friction stir-welded AA 2198-T8.

General description of the mechanical properties of keyhole repair welds

The local strength distribution in the different weld zones in the alloys of interest is thus mainly determined by the precipitation morphology. As the alloys were processed in peak-aged conditions, any exposure to elevated temperature will lead to strength reduction by precipitate evolution.

Although the deformation during the weld process will undoubtedly have an influence on the precipitate evolution, it is generally assumed that they are second order compared to effects caused by thermal exposure [94]. The kinetics of dissolution, overaging and re-precipitation are alloy dependent and explained in detail in the Chapters 6.2.2, 6.3.2 and 6.4.3 for the alloys of interest. As a result of the weld thermal cycle, transformation of multiple phases and the tendency towards dissolution and subsequent natural aging occurs, which may continue over long timescales. Since the

Process description

42

thermal cycle differs significantly at different distances from the center of the weld, as described in Chapter 5.2, the precipitate evolution varies in the different weld zones.

Typically, the local strength distribution after friction welding of peak-aged precipitation hardening aluminum alloys follows a W-shape over a cross-section though the weld [14, 29, 95-99]. Normally, the SZ features a relatively constant hardness, whereas the lowest values of hardness occur in the HAZ or TMAZ. The development of a W-shape from a near-U-shape in the as-welded condition mainly results from a strength increase in the weld center due to post-weld natural aging. Still, this does not exclude a strength increase due to natural aging in the TMAZ and HAZ. With increasing distance from the weld center, the temperature exposure decreases. At a certain distance, the thermal cycle will not cause significant precipitate evolution. At this point, the strength equals the BM strength, and the HAZ ends.

The strength reduction in and around the weld spot caused mainly by the thermal cycle exposure during RFSSW leads to a typical response during quasi-static tensile testing. Typical strain maps and the strain distribution at the vertical centerline through the center of the weld acquired via DIC, namely, a 6 mm sheet of AA 7075-T651 with keyhole diameter of 7.5 mm repair-welded using the medium size tool, are presented in Figure 5.11. During tensile testing, strain concentrates at approximately 20 mm around the center of the weld, which corresponds to the HAZ dimensions, resulting in a reduced total elongation compared to the BM values. Yielding begins in the areas of lowest hardness, as shown in Figure 5.11 (a). At higher stress, most of the strain accumulates in the areas of low strength in the HAZ at approximately 10 mm from the center of the weld. Note that the area of lowest strength in AA 2219-T851 typically develops closer to the center of the weld than the distribution shown, which applies to AA 6061-T6 and AA 7075-T651. Additionally, strain concentration peaks occur in the outer regions of the SZ. The strain distribution characteristic is inversely proportional to the hardness distribution, except for the peaks in the SZ that become apparent due to the high resolution of the DIC measurement.

Figure 5.11 Strain maps and strain distribution through the centerline of 6 mm-thick AA 7075-T651 coupons during tensile testing of keyhole closure welded samples (a) at early stages of plastic deformation and (b) close to fracture [31].

Microstructural features, precipitate evolution and mechanical properties

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6 Microstructural features, precipitate evolution and