Al-Al2O3 precipitated alloys and Al-Zn solid solution alloys fabricated by friction stir process are investigated in this study. The mechanical specimen cutting from stir zone were tested by Instron machine. Micro-structure was observed by Scanning Electron Microscopy and Transmission Electron Microscopy. Phase composition was measured by X-ray diffraction. Different Grain sizes sample were obtained at condition with constant traverse speed of 1.0mm/s, different RPM(500rpm, 550rpm, 700rpm, 1500rpm and 1500rpm with subsequent annealing treatment) and pin shape. Mechanical properties and ductility improvement on grain size effect are discussed in this research. In Al/Al2O3 composite materials, mechanical strength is enhanced by Al2O3 precipitation distributed homogeneously in Al matrix and ductility is improved simultaneously by increment of work hardening rate due to interaction between obstacles and dislocations. In Al-Zn solid solution alloys, ductility enhancement takes place not only in refining grain sizes but also occurs obviously with different weight fraction of Zn addition.

Abstract Friction stir processing (FSP) is modified to produce various grain sizes of aluminum matrix composites (Al-Al2O3, Al-Zn-Al2O3 and Al-Zn) ranging from 300 nm to 3£gm. The microstructures of the composites were characterized using SEM and TEM. Tensile tests were performed to evaluate the mechanical properties of these composites. In the Al-Al2O3 system, it was found that the nano-scale alumina made it very effective to accumulate dislocations within grains during deformation, and resulted in increasing working hardening rate which is very critical to extend uniform elongation for materials with submicron grain sizes. In the Al-Zn-Al2O3 system, addition of Zn which dissolved into Al matrix to form solid solution and subsequently uniformly distributed G-P zones can improve strength and uniform ductility to some extent, comparing to those without addition of Zn. In addition, the relaxed and dislocation-free boundaries were observed regardless the existence of Al2O3 particles on boundaries. As a result, ko derived from Hall-Petch equation from various strain region decrease as Zn increases. In the Al-Zn system, experimental evidence suggests that increasing Zn content from 0 to 15wt% can enhance the total elongation but not uniform elongation as a result of the uniform spreading of the fine slip bands all over the gauge length and the contribution of grain boundary sliding (GBS) at RT. The relaxed and dislocation-free boundaries and GBS are attributed to the combination of high fraction of high-angle GBs and high GB diffusion to help fast dislocations annihilation at boundaries.

In this research, one solid state processing technique, friction stir processing, is applied to modify the AZ61 magnesium alloy billet and to incorporate 5-10 vol% nano-sized ceramic particles SiO2 into the AZ61 magnesium alloy matrix to form bulk composites, using the characteristic rotating downward and circular material flow around the stir pin. The microstructure and mechanical properties of the modified alloy and composite samples are examined and compared. The FSP modified AZ61 alloy could be refined to 3-8um via the dynamic recrystallization during processing. However, the one-pass FSP modified alloy appeared the inhomogeneous grain structures to influence the tensile ductility along the welding direction at elevated temperatures due to the onion splitting. In contrast, the multi-pass FSP could improve the inhomogeneous grain structures to reduce the influence of the onion-splitting to the deformation at elevated temperatures. The FSP modified alloys show the lower yielding stress due to the unique texture of (0002) basal planes, with roughly surround the pin column surface of the pin tool in the stirred zone. In addition, it is suggested that the second processing of the subsequent compression along the normal direction might be necessary to alter the texture and to improve the lower yielding stress after modifying the grain size by FSP. Friction stir processing could successfully fabricate bulk AZ61 Mg based composites with 5 to 10 vol% of nano-sized SiO2 particles. The nano-sized SiO2 particles added into magnesium matrix could be uniformly dispersed after four FSP passes. The average grain sizes of the composites varied within 0.5-2um, and the composites nearly double the hardness as compared with the as-received AZ61 cast billet. This composite exhibited high strain rate superplasticity, with a maximum ductility of 470% at 1x10-2 s-1 and 300oC or 454% at 3x10-1 s-1 and 400oC while maintaining fine grains less than 2um in size.

Friction stir processing (FSP) has recently become an effective microstructural modifications technique. Reported results showed that for different alloys, FSP produces very fine equiaxed and homogeneous grain structure. FSP is considered to be a new processing technique and more experimental and analytical investigations are needed to advance the industrial utilization of FSP. Most of the work that has been done in the friction stir processing field is experimental and limited modeling activities have been conducted. Attempts to develop a predictive model to correlate the resulting microstructure with process parameters are scarce. In this work, commercial 5052 Aluminum alloy sheets are friction stir processed at different rotational and translational speeds. The effects of process parameters on the resulting microstructure and mechanical properties are investigated. The results show that FSP produces very fine and homogenous grain structure, and it is observed that smaller grain size structure is obtained at lower rotational speeds. It is also observed that the hardness of the processed sheet depends strongly on the rotational and translational speeds and varies widely within the processed region. The results suggest that the temperature achieved during processing plays an important role in determining the microstructure and properties of the processed sheet. In addition, a new modeling approach based on experiments and theory is proposed to predict the grain size of the friction stir processed material as a function of process parameters. The proposed approach involves determination of the strain rate distribution in the processed (deformation) zone based on the velocity fields of the material and correlating the strain rate distribution with the average grain size of the resulting microstructure using Zener-Holloman parameter.

In this study, a novel technique was used to produce aluminum based in situ composites from powder mixtures of Al and CeO2. This technique has combined hot working nature of friction stir processing (FSP) and exothermic reaction between Al and oxide. Billet of powder mixtures was prepared by the use of conventional pressing and sintering route. The sintered billet was then subjected to multiple passages of friction stir processing (FSP). The microstructure was characterized by the use of TEM, SEM and XRD. The reinforcing phases were identified as Al11Ce3 and £_*-Al2O3. The Al2O3 particles with an average size of ~10 nm are uniformly distributed in the aluminum matrix, which has an average grain size about 390-500 nm. The analysis of TEM indicated that these Al2O3 particles exhibit crystallographic orientation relationship with the aluminum matrix, i.e., (223)£_*-Al2O3//(111)Al and [1-10]£_*-Al2O3 roughly parallel to [1-10]Al. The precipitates of Al2O3 exhibiting crystallographic orientation relationship with the aluminum clearly indicates that they were formed from solid state precipitation. Apparently, significant supersaturation of oxygen in aluminum had been created in FSP, and nanometric Al2O3 particles were then precipitated uniformly in the aluminum matrix. This study shows that both sintering temperature and the tool traversing speed used in FSP have significant influence on the microstructure and mechanical properties of the composite. The composites produced exhibit high strength both at ambient and elevated temperatures. For example, the composite produced by 833K sintering followed by FSP with tool traversing speed of 30 mm/min possesses enhanced modulus (E = 109 GPa) and strength (UTS = 488 MPa) as well as a tensile ductility of ~3%. The major contributions to the high strength of the composite are the submicrometer grain structure of aluminum matrix and the Orowan strengthening caused by the fine dispersion of nanometer size Al2O3 particles inside aluminum grains. In addition, the composite also exhibits high strength at elevated temperatures up to 773 K. The good thermal stability and high temperature strength of the composite may be attributed to the uniform dispersion of nanometric Al2O3 particles, which are very stable at elevated temperatures.

Al-Mg solid solution alloys of various grain sizes were prepared by friction stir processing (FSP). The mechanical properties and micro-structure evolution were studied. The results show that the mechanical properties including tensile strength and ductility are improved by increasing Mg weight fraction. The homogeneous deformation is enhanced by fined slip bands within the grains. On the other hand, Dynamic strain aging or serrated flow stress has been wildly investigated in Al-Mg alloys. Effects of strain rate and magnesium content on dynamic strain aging are also discussed.

This book has the introduction of the friction stir welding and friction stir processing, and introduces the newest development in FSW.Finding out the appropriate paraments of the grain size refinement in AZ31 Mg. The relationship between the resulting grain size and the applied working strain rate and temperature for the friction stir processing in AZ31 Mg is systemically examined. The Zener-Holloman parameter is utilized in rationalizing the relationship. The grain orientation distribution is also studied using the X-ray diffraction.

The friction stir processing is applied in mixing elemental thin sheets of Mg, Al, and Zn in various portions to result in hard intermetallic alloys with Vicker¡¦s hardness in excess of 350. The Mg3Al2Zn3

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In this study, billet of a binary Al-10at%Fe alloy was prepared from pure Al and Fe powders by the use of conventional press and sinter route. The sintered billet was then subjected to multiple passages of friction stir processing (FSP). After FSP, the structure of a binary Al-10at%Fe alloy can be refined to sub-micrometer scale. Transmission electron microscopy (TEM) showed that particles of Fe-containing phase were distributed uniformly in the aluminum matrix, and the mean size of these second phase particles was about 100nm. From the results of X-ray diffraction and energy dispersive spectroscopy (EDS), the Al-Fe second phase was identified as Al13Fe4. We also observed obvious reaction zone around iron particles in the friction-stirred zone. Apparently, a rapid in-situ reaction between Al and Fe had occurred in FSP. In order to reduce the reaction time and the heat input, the higher traversing speed was used. In addition, a higher sintering temperature was used to promote Al-Fe reaction. Furthermore, micro-hardness, tensile and compressive tests were performed to evaluate the mechanical properties of the Al-10at%Fe alloy fabricated by FSP.