Issue34

A. Tajiri et alii, Frattura ed Integrità Strutturale, 34 (2015) 347-354; DOI: 10.3221/IGF-ESIS.34.38 348 Recently, friction stir processing (FSP) technique draws attention as a microstructural modification technique. In the FSP process, rotating tool with shoulder and probe is plunged into the material and travels leaving severely deformed area behind the tool. FSP was applied to cast Al and magnesium (Mg) alloys, and it has been reported that casting defects, such as porosities and large grains, are successfully removed [4-6]. Consequently, it has been revealed that fatigue strengths of cast Al or Mg alloys could be improved by the application of FSP [5, 7, 8]. It is well know that the fatigue properties are sensitive to the microstructures. It is believed that FSP would leave strong texture behind the tool, and the texture would be dependent on the FSP conditions. However, the effects of texture on fatigue properties, such as crack initiation and propagation resistances, are not clear. In this study, FSP was applied to A356-T6 under two different processing conditions, where the tool rotational speed was changed at the fixed tool traveling speed. Subsequently, fully reversed plane bending fatigue tests had been performed to investigate the effect of processing conditions on the crack initiation and growth behavior. E XPERIMENTAL PROCEDURE he material used is T6-treated A356 cast Al alloy. The chemical composition (wt. %) is as follows, Si: 6.66, Mg: 0.383, Fe: 0.153, Ni: 0.009, Cr: 0.002, Sn: 0.002, Al: balance. The microstructure of the as-received material is shown in Fig.1. Typical dendrite structures are seen. It should be noted that large casting defects are recognized in the microstructure as shown by the arrows in the figure. Specimen blanks with the thickness of 5 mm were cut from the ingots and subsequently friction stir processed (FSPed), from which plane bending fatigue specimens were machined. The FSP direction corresponds to the longitudinal direction of the specimen. To remove stress concentration sites, both the upper and lower surfaces were removed 0.3 and 0.7 mm in depth, respectively. Before fatigue test, the surface of the gauge section was mechanically polished using #2000 grade emery paper followed by buff-finishing. Figure 1 : Microstructure of as-received material. Arrows indicate casting defects. The FSP tool consists of concave shoulder with a diameter of 14 mm and M6-threaded probe with a length of 4.7 mm. The tool travelling speed was fixed at 150 mm/min. It is considered that the development of texture will be affected by the strain rate. Accordingly, the tool rotational speed was set to be 500 and 1000 rpm to investigate fatigue properties in the FSPed specimens fabricated under low and high strain rates. Hereafter, the specimens are designated as L (Low strain rate) and H (High strain rate) samples. Fatigue tests had been conducted using resonance-type plane bending fatigue testing machine, SIMADZU TB-10, at a test frequency f =33.3Hz and load ratio R =-1 (fully reversed bending). The hardness was measured by micro-Vickers hardness tester at a load of 2.49 N and dwell time of 30 s. R ESULTS Microstructure he cross section of FSP line in L and H specimens are revealed in Fig.2. Dendrite structures were broken-up by the stirring action during FSP in the stir zone (SZ), and porosities shown in Fig.1 were not recognized in the SZ. The so-called onion ring patterns are clearly seen in H specimen while they were partially formed in L specimen. It T T 300μm