Determination of onset of plasticity (yielding) and comparison of local mechanical properties of friction stir welded aluminum alloys using the micro‐ and nano‐indentation techniques

Abstract

PurposeThe purpose of this paper is to determine if the nanoindentation technique is a reliable method and whether it can be used to measure the surface hardness (H) in friction stir welded aluminum alloys. In order to test the reliability of nanoindentation technique, nanohardness values for friction stir welded aluminum alloys were compared to microhardness values. Additionally, the onset of plasticity (yielding) is investigated.Design/methodology/approachNanoindentation experiments were performed for the determination of onset on plasticity (yielding) and comparison of local mechanical properties of both welded alloys. In order to test the reliability of nanoindentation technique, nanohardness values for friction stir welded AA6082 were compared to microhardness values. The specimen was tested using two different instruments – a Vickers microhardness tester and a nanoindenter tester for fine scale evaluation of H.FindingsThe results of this study indicate that nanohardness values with a Berkovich indenter reliably correlate with Vickers microhardness values. Nanoindentation technique can provide reliable results for analyzing friction stir welded aluminum alloys. The welding process definitely affects the material mechanical properties.Originality/valueMicrohardness and nanohardness obtained values can be correlated carefully, regarding the similarities and the differences of the two above mentioned techniques.

Multiscale approach of hardness in aluminium alloy: Consideration of rate dependent behaviour

“…44 Rar et al 45 observed that the same material, when allowed to creep for a long time, produced a higher value of pile-up/sink-in, indicating a switch from an initial elastic sink-in to a plastic pile-up. 43 The findings of the present study reveal that creep time has no impact on the pile-up/sink-in of rate sensitive aluminium alloy (Fig. 10), in agreement with similar studies in the literature.…”

“…The contact area is influenced by the formation of pile-ups and sink-ins during the indentation process. 20,43 To accurately measure the indentation contact area, pile-ups/sink-ins should be appropriately accounted for. The presence of creep during nanoindentation has an effect on pile-up, which results in incorrect measurement of the material properties.…”

“…In the present study, no detectable pop-in was found, except for the first yield type pop-in (gradual slope change), attributed to high loading rates (previous work reported the existence of pop-ins at lower rates). 21,23,43 The experimental results showed that a low loading rate favours pop-in. The variation of the locations of pop-ins can be caused by the small perturbation of indentation conditions such as thermal drift, which is more influential on an indentation at a slower loading rate, and surface roughness, which can alter the local stress distribution in the neighbourhood of the indentation zone and in turn affects the process of crystalline growth.…”

Nanomechanical properties and thermal decomposition of Cu-Al2O3composites for FGM applications

“…Cheng and Cheng reported a 22% pile-up for a work hardening exponent [30]. This is consistent with the fact that when h c /h approaches 1 for small H/E*, deformation is intimately dominated by pile-up [31,32]. On the other hand when h c /h approaches 0 for large H/E* it corresponds to purely elastic deformation and is apparently dominated by sink-in in a manner prescribed by Hertzian contact mechanics.…”

Finite element analysis, stress‐strain distribution and size effects rise during nanoindentation of welded aluminum alloy

“…The micro-indentation hardness that predicted from nano-indentation by applying the Nix-Gao model are in good correlation with Vickers hardness measurements for AA 6082-T6. Specifically, as it is reported in the literature for the base metal, Vickers hardness is equal to 0.98 GPa and for the nugget zone, Vickers hardness is equal to 0.69 GPa (Koumoulos et al, 2011a). Also, parent material consists of elongated and with common orientation grains with average grain length ,120 mm and nugget zone consists of much smaller and equiaxed grains with average grain length ,13 mm.…”