A novel surface modifying technique, friction stir processing¡]FSP¡^, has been developed for fabrication of surface composite. Al-Al2O3 surface composites with different volume fractions of particles were successfully fabricated. The Al2O3 particles were uniformly distributed in the aluminum matrix. The surface composites have excellent bonding with the aluminum alloy substrate. The microhardness of the surface composite reinforced with 40 vol% Al2O3 of ~50nm, average particle size was ~150 HV, almost doubt that of the 5083 Al alloy substrate¡]86HV¡^. The distribution curves showed that the SD was increased steeply when the volume fractions of Al2O3 particles of SZ attained to about above 30 vol%. In addition, it is difficult to reduce the grain size of SZ stirring with powder by increasing traveling speed or adding more volume fractions of Al2O3 particles because the processing temperature is higher than 0.5 Tm.

We applied the Friction-Stir Process (FSP) to make the ZrO2 /5083 Al alloy composite material, and analyzed its physical properties in different aspects. Different weight percents of nanometer composite materials, ZrO2/Al, with well distributed strengthening grains were manufactured with the FSP which was used for five runs on ZrO2 along with the matrix material, aluminum, at 505¢XC, and created reactants of Al3Zr, tetragonal D023 structure, and Al2O3, identified with X-ray diffraction analysis. The grain size of 5083 Al-alloy could be finer, around 2.6£gm, by the FSP. This study suggests that increasing the addition of ZrO2 into the Al matrix could make the grain size of aluminum finer. We found that the Al grain size would be able to down to 0.66£gm, as 15.3 wt% of ZrO2 powder was reached. The mechanical properties of the Al-matrix material could be also modified by adding ZrO2 that reduces the ductility but boosts the strength of the matrix material. When we put 15.3 wt% of ZrO2 powder, 5083 Al-alloy attained the hardness of 158Hv, almost twice of hardness of the original alloy material, and its yield strength also increased from 125MPa to 400MPa as well.

Nanoscaled-Al2O3 particles reinforced 1050 Aluminum composites by FSP were successfully fabricated in this study. The grain size of 1050 aluminum was obviously refined to about 0.5£gm by friction stir process(FSP), and there was a tendency that grain size decreased with increasing of Al2O3 content, where grain size of 0.84£gm was achieved with 24.7vol% of Al2O3. Nanoscaled-Al2O3 particles reinforced 1050 Al alloy by FSP revealed an excellent strengthening effect and excellent ductileity, Where hardness and UTS of the composite with 24.7vol% nanoscaled-Al2O3 particles were increased up to Hv113 and 310MPa respectively. The tensile result showed a 400% of increase in UTS comparing to the pure Al after FSP.

Friction stir process (FSP) was employed to investigate the grain evolution of AA1050 aluminum alloy in this study. The rotation speeds for the tool were set from 500 to 2000 rpm with a constant traverse speed of 0.5mm/s. The temperature under pin was measured by K-type thermocouple imbedded under the pin. Grain sizes were determined by scanning electron microscopy. The maximum temperature at the bottom of pin increased with the increasing of rotation speed but not exceeding 0.8Tm. Grain size at center and bottom of stirred zone was in linear increase at low rotation speed, but increased a little at high rotation speed (>1000rpm). The grain size grew rapidly into a stable size in a 2mm distance measured from the passing of pin. When rotation speed is above 1000rpm, average grain growth rate is 1£gm/s. When rotation speed is lower than 700rpm, average grain growth rate is slower than 0.2£gm/s. BEI/ECCI observations revealed that grains in SZ became equaxied.

Additive friction stir deposition (AFSD) is an emerging solid-state metal additive manufacturing technology that uses deformation bonding to create near-net shape 3D components. As a developing technology, a deeper understanding of the processing science is necessary to establish the process-structure relationships and enable improved control of the as-printed microstructure and material properties. AFSD provides a unique opportunity to explore the friction stir fundamentals via direct observation of the material during processing. This work explores the relationship between the processing parameters (e.g., tool rotation rate Ω, tool velocity V, and material feed rate F) and the thermomechanical history of the material by process monitoring of i) the temperature evolution, ii) the force evolution, and iii) the interfacial contact state between the tool and deposited material. Empirical trends are established for the peak temperature with respect to the processing conditions for Cu and Al-Mg-Si, but a key difference is noted in the form of the power law relationship: Ω/V for Cu and Ω2/V for Al-Mg-Si. Similarly, the normal force Fz for both materials correlates to V and inversely with Ω. For Cu both parameters show comparable influence on the normal force, whereas Ω is more impactful than V for Al-Mg-Si. On the other hand, the torque Mz trends for Al-Mg-Si are consistent with the normal force trends, however for Cu there is no direct correlation between the processing parameters and the torque. These distinct relationships and thermomechanical histories are directly linked to the contact states observed during deformation monitoring of the two material systems. In Cu, the interfacial contact between the material and tool head is characterized by a full slipping condition (δ=1). In this case, interfacial friction is the dominant heat generation mechanism and compression is the primary deformation mechanism. In Al-Mg-Si, the interfacial contact is characterized by a partial slipping/sticking condition (0<δ<1), so both interfacial friction and plastic energy dissipation are important mechanisms for heat generation and material deformation. Finally, an investigation into the contact evolution at different processing parameters shows that the fraction of sticking is critically dependent on the processing parameters which has many implications on the thermomechanical processing history. / Doctor of Philosophy / Additive manufacturing or three-dimensional (3D) printing technologies have been lauded for their ability to fabricate complex geometries and multi-material parts with reduced material waste. Of particular interest is the use of metal additive manufacturing for repair and fabrication of industrial and structural components. This work focuses on characterizing the thermomechanical processing history for a developing technology Additive Friction Stir Deposition (AFSD). AFSD is solid-state additive manufacturing technology that uses frictional heat and mechanical mixing to fabricate 3D metal components. From a fundamental materials science perspective, it is imperative to understand the processing history of a material to be able to predict the performance and properties of a manufactured part. Through the use of infrared imaging, thermocouples, force sensors, and video monitoring this work is able to establish quantitative relationships between the equipment processing parameters and the processing history for Cu and Al. This work shows that there is a fundamental difference in how these two materials are processed during AFSD. In the future, these quantitative relationships can be used to validate modeling efforts and improve manufacturing quality of parts produced via friction stir techniques.

Magnesium and its alloys have been considered for load-bearing implant materials due to their similar mechanical properties to the natural bone, excellent biocompatibility, good bioactivity, and biodegradation. Nevertheless, the uncontrollable corrosion rate in biological environment restrains their application. Hydroxyapatite (HA, Ca10(PO4)6(OH)2) is a widely used bio-ceramic which has bone-like mineral structure for bone fixation. Poor fracture toughness of HA makes it not suitable for load-bearing application as a bulk. Thus, HA is introduced into metallic surface in various forms for improving biocompatibility. Recently friction stir processing (FSP) has emerged as a surface modification tool for surface/substrate grain refinement and homogenization of microstructure in biomaterial. In the pressent efforts, Mg-nHA composite surface on with 5-20 wt% HA on Mg substrate were fabricated by FSP for biodegradation and bioactivity study. The results of electrochemical measurement indicated that lower amount (~5% wt%) of Ca in Mg matrix can enhance surface localized corrosion resistance. The effects of microstructure,the presence of HA particle and Mg-Ca intermetallic phase precipitates on in vitro behavior of Mg alloy were investigated by TEM, SEM, EDX,XRD ,and XPS. The detailed observations will be discussed during presentation.

A Lagrangian finite-element model of the plunge phase of the friction stir welding process was developed to better understand the plunge. The effects of both modeling and experimental parameters were explored. Experimental friction stir plunges were made in AA 7075-T6 at a plunge rate of 0.724 mm/s with spindle speeds ranging from 400 to 800 rpm. Comparable plunges were modeled in Forge2005. Various simulation parameters were explored to assess the effect on temperature prediction. These included the heat transfer coefficient between the tool and workpiece (from 0 to 2000 W/m-K), mesh size (node counts from 1,200 to 8,000), and material model (five different constitutive relationships). Simulated and measured workpiece temperatures were compared to evaluate model quality. As spindle speed increases, there is a statistically significant increase in measured temperature. However, over the range of spindle speeds studied, this difference is only about 10% of the measured temperature increase. Both the model and the simulation show a similar influence of spindle speed on temperature. The tool-workpiece heat transfer coefficient has a minor influence (<25% temperature change) on simulated peak temperature. Mesh size has a moderate influence (<40% temperature change) on simulated peak temperature, but a mesh size of 3000 nodes is sufficient. The material model has a high influence (>60% temperature change) on simulated peak temperature. Overall, the simulated temperature rise error was reduced from 300% to 50%. It is believed that this can be best improved in the future by developing improved material models.