Effect of the length of the tool pin on the hardening of 2024 aluminum alloy under friction stir processing

The paper reports the results of studies on the effect of the tool pin length on the microstructure and hardness of 2024 aluminum alloy sheets with a thickness of 3.0 mm during friction stir processing (FSP), as well as computer simulation data. The alloy in the initial state contains particles of phases theta (Al2Cu), S (Al2CuMg), and phases of complex composition (AlCuMnSiFe) with sizes ranging from 0.5 - 3 to several tens of microns. There is also a small amount of silicon-rich particles with sizes of 2- 3 mu m. The phases (AlCuMnSiFe) in their initial state often form skeletal precipitates up to 30 - 40 mu m in size. Such precipitates consist of three phases differing in the ratio of elements. FSP of the material led to an increase in microhardness in the center of the treated zone and the advancing side from 53 +/- 4 IIV to 110 +/- 8 IIV, while the highest microhardness values practically do not depend on the pin length. The distribution of microhardness and microstructure in the processing area is uneven and depends on the length of the tool pin. The highest hardness of the central FSP zone is provided by a pin 2.0 mm long. It has been established that during the FSP of the alloy, coarse particles of intermetallic phases are refined and the products of refining in the FSP zone are distributed. The particles of the theta and S phases are refined weakly, while the others are refined to submicron sizes. To substantiate the choice of the effective pin length, a three-dimensional finite element modeling of the FSP in the DEFORM-3D software environment was performed. Distributions of effective deformation, temperature fields, and displacement of material points in the zone of thermomechanical action are analyzed. The simulation results agree with data of the physical experiment in that the most preferable pin under the considered processing conditions is a pin with a length of 2.0 mm since it allows one to obtain the widest and most symmetrical regions of intense heating and deformation.