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Microstructural characterization and tensile properties of hot pressed Al–SiC composites prepared from pure Al and Cu powders

Abstract A conventional hot pressing method was used to produce Al–Cu–SiC particulate metal matrix composites. The matrix alloy was prepared from elemental powders of Al and 5 wt.% Cu. The composite comprises 13.3 or 27.2 vol.% SiC p with an average size of 10 μm. The powder mixtures were hot pressed uniaxially under nitrogen atmosphere. It is shown that Cu aids in the formation of a liquid phase. Practically pore free structures are obtained after pressing at 600°C. Elastic modulus, tensile strength and yield strength values were improved with incorporation of the reinforcement in composites prepared by fine Al powder. The ductility, on the other hand, decreased sharply with increasing amount of strengthening phase.

The effect of coarse second phase particles on fatigue crack propagation of an Al-Zn-Mg-Cu alloy

The objective of this study is to determine the role of the most commonly observed coarse second phase particles; Al[sub 7]Cu[sub 2]Fe, Mg[sub 2]Si and CuAl[sub 2]Mg on the Stage 2 fatigue crack propagation of a 7050 aluminum alloy. The differences in the composition of this alloy when compared to the conventional 7075 alloy are: (1) increased Cu content for additional strengthening during aging and for increasing the temperature range of GP zone stability; (2) replacement of Cr by Zr to reduce quench sensitivity; (3) reduced Fe and Si contents to improve fracture toughness which, however, decreases fatigue crack growth threshold, [Delta]K[sub th], slightly; and (4) increased Zn content for strengthening.

A pilot study of joint stability at the zirconium or titanium abutment/titanium implant interface.

PURPOSE To compare the interfaces of loaded and unloaded zirconium and titanium abutments with titanium implants using scanning electron microscopy (SEM). MATERIALS AND METHODS Zirconium and titanium abutments (n = 5 per group; four test and one control) were torque-tightened into titanium implants secured into metal blocks, and computer-aided design/computer-assisted manufacture-based zirconium oxide copings were fabricated and cemented to the abutments with temporary resin-based cement. Specimens of each restoration were subjected to cyclic axial and lateral loading of 30 N at 2 Hz for 500,000 cycles using a servohydraulic test system; control specimens were left unloaded. Then, the abutment/implant assemblies were embedded in acrylic resin, sectioned longitudinally along the midline, and inspected under SEM with x-ray microanalysis. RESULTS Loosening or fracture of the copings and implant components was not observed after dynamic loading in both groups. SEM and x-ray microanalysis revealed unexpected microleakage of acrylic resin at the interface. Acrylic resin in the implants tightened to the titanium abutments was limited to the cervical part, and the components displayed scratched and smashed regions, suggesting slight deformation of the implant neck. Microleakage and pooling of acrylic resin were observed approaching the screw joint in loaded implants tightened to zirconia abutments, and the amount of microleakage was greater than in the unloaded control specimens, which had a larger microgap than the titanium abutment/titanium implant interface. Loaded zirconia abutments were associated with wear, scratches, and, in one sample, chipping. CONCLUSIONS Zirconium abutment/titanium implant interface may be susceptible to wear of the abutment coupled with deformation of the implant neck greater than that associated with the conventional titanium abutment/titanium implant interface under dynamic loading.

Fatigue crack growth behaviour in aluminium alloy 7475 under different aging conditions

Abstract The effects of aging temperature and aging time on fatigue crack growth resistance have been studied for a 7475 Al-Zn-Mg based aluminium alloy. The alloy was tested in the underaged, peak aged, and overaged conditions after aging at 120 and 160C. Fatigue crack propagation tests were conducted in laboratory air using compact tension specimens in L-S orientation, under constant amplitude sinusoidal loading with an R ratio of zero. Results are discussed on the basis of resultant microstructures, fatigue crack growth rate diagrams and fractographic analysis. At 120C, a considerable effect of aging time on crack velocities at high stress intensities was seen. However, at 160C no significant dependency of crack growth rate on aging time was observed. The fatigue performance of overaged specimens was better for both aging temperatures. Also, lower aging temperature resulted in a more resistant structure against fatigue crack growth. Fractographic inspection showed that intermetallic particles play an important role in the crack growth behaviour of the Al-Zn-Mg alloy.

Characterization of duplex stainless steel weld metals obtained by hybrid plasma-gas metal arc welding

Despite its high efficiency, autogenous keyhole welding is not well-accepted for duplex stainless steels because it causes excessive ferrite in as-welded duplex microstructure, which leads to a degradation in toughness and corrosion properties of the material. Combining the deep penetration characteristics of plasma arc welding in keyhole mode and metal deposition capability of gas metal arc welding, hybrid plasma - gas metal arc welding process has considered for providing a proper duplex microstructure without compromising the welding efficiency. 11.1 mm-thick standard duplex stainless steel plates were joined in a single-pass using this novel technique. Same plates were also subjected to conventional gas metal arc and plasma arc welding processes, providing benchmarks for the investigation of the weldability of the material. In the first place, the hybrid welding process enabled us to achieve less heat input compared to gas metal arc welding. Consequently, the precipitation of secondary phases, which are known to be detrimental to the toughness and corrosion resistance of duplex stainless steels, was significantly suppressed in both fusion and heat affected zones. Secondly, contrary to other keyhole techniques, proper cooling time and weld metal chemistry were achieved during the process, facilitating sufficient reconstructive transformation of austenite in the ferrite phase.

Fatigue resistance of 2 different CAD/CAM glass-ceramic materials used for single-tooth implant crowns.

Purpose:To evaluate the fatigue resistance of 2 different CAD/CAM in-office monoceramic materials with single-tooth implant-supported crowns in functional area. Materials and Methods:A metal experimental model with a dental implant was designed to receive in-office CAD/CAM-generated monoceramic crowns. Laterally positioned axial dynamic loading of 300 N at 2 Hz was applied to implant-supported crowns machined from 2 different glass materials for 100,000 cycle. Failures in terms of fracture, crack formation, and chipping were macroscopically recorded and microscopically evaluated. Results:Four of 10 aluminasilicate glass-ceramic crowns fractured at early loading cycles, the rest completed loading with a visible crack formation. Crack formation was recorded for 2 of 10 leucite glass-ceramic crowns. Others completed test without visible damage but fractured upon removal. Discussion:Lack in chemical adhesion between titanium abutment and dental cement likely reduces the fatigue resistance of machinable glass-ceramic materials. However, relatively better fractural strength of leucite glass-ceramics could be taken into consideration. Accordingly, progress on developmental changes in filler composition of glass-ceramics may be promising. Conclusion:Machinable glass-ceramics do not possess sufficient fatigue resistance for single-tooth implant crowns in functional area.

Effect of aging on the fatigue crack growth kinetics of an Al-Zn-Mg-Cu alloy in two directions

There have been investigations discussing the effect of aging condition, and thereby the microstructure, on the fatigue crack growth characteristics of precipitation hardening alloys. Lindigkeit et al.., testing an Al-Zn-Mg-Cu alloy of composition corresponding to the commercial alloy 7075 concluded that the crack propagation resistance of underaged microstructures with shearable precipitates is significantly higher than overaged samples of same strength containing non-shearable particles. They reported that this behavior cannot be explained on the basis of slip reversibility alone. A similar conclusion is drawn by Zaiken and Ritchie from investigations on the effect of microstructure on the fatigue crack growth rate of an 7150 aluminum alloy, which is a somewhat high-purity version of the alloy 7050, with lower Fe and Si contents. It is also interesting that aging conditions showing high resistance to fatigue crack growth at low [Delta]K regimes, do not necessarily retain their superiority at medium and high stress intensity ranges.

Effect of salt-water fog on fatigue crack growth behaviour of 7050 aluminium alloy in different orientations

The fatigue crack growth behaviour of 7050 T73651 high strength aluminium alloy that was originally developed for the aircraft industry was investigated in this study. The tests were conducted by using C-T specimens machined in six orientations under the action of constant amplitude sinusoidal load cycles. The tests were first carried out in laboratory air and then repeated in salt-water fog of a 5% NaCl solution to observe the effect of the environment on the fatigue crack growth behaviour. The experimental results showed that the fatigue life, maximum stress intensity range and the fatigue crack growth rate of the specimens were seriously affected by the environment. The severity of the effect, on the other hand, was observed to be dependent on the orientation. The strongest orientation was determined to be L-S, while the weakest was S-L.

Biomechanical Evaluation of a Novel Apatite-Wollastonite Ceramic Cage Design for Lumbar Interbody Fusion: A Finite Element Model Study

Objectives Cage design and material properties play a crucial role in the long-term results, since interbody fusions using intervertebral cages have become one of the basic procedures in spinal surgery. Our aim is to design a novel Apatite-Wollastonite interbody fusion cage and evaluate its biomechanical behavior in silico in a segmental spinal model. Materials and Methods Mechanical properties for the Apatite-Wollastonite bioceramic cages were obtained by fitting finite element results to the experimental compression behavior of a cage prototype. The prototype was made from hydroxyapatite, pseudowollastonite, and frit by sintering. The elastic modulus of the material was found to be 32 GPa. Three intact lumbar vertebral segments were modelled with the ANSYS 12.0.1 software and this model was modified to simulate a Posterior Lumbar Interbody Fusion. Four cage designs in different geometries were analyzed in silico under axial loading, flexion, extension, and lateral bending. Results The K2 design had the best overall biomechanical performance for the loads considered. Maximum cage stress recorded was 36.7 MPa in compression after a flexion load, which was within the biomechanical limits of the cage. Conclusion Biomechanical analyses suggest that K2 bioceramic cage is an optimal design and reveals essential material properties for a stable interbody fusion.

Effect of Fillet Rolling Load on the Fatigue Performance of a Micro-Alloy Steel Diesel Engine Crankshaft

Fillet rolling process is an effective method used to improve the fatigue performance of crankshafts by hardening the fillet region and inducing compressive residual stresses. This paper summarizes the work conducted to investigate the effect of rolling load on fatigue behaviour of a micro-alloy steel crankshaft used in diesel engine applications. Based on the staircase test methodology, component-scale resonant bending fatigue tests were conducted to obtain stress versus number of cycles curves and to evaluate the fatigue endurance limits of the crankshaft at un-rolled condition and fillet-rolled conditions at three different loads. Test data was analysed by Dixon-Mood method to calculate the endurance limits. Results showed that the endurance limit increased significantly with fillet rolling process in comparison to un-rolled condition. Endurance limit further increased with the increasing rolling load however with a limited extent above which excessive hardening deteriorates the fillet region; that is the workability limit. The outcomes of this study has shed light on the fillet rolling process to select the optimum rolling load for the used design and material conditions.