Processing of ultrafine-size particulate metal matrix composites by advanced shear technology

Copyright @ 2009 ASM International. This paper was published in Metallurgical & Materials Transactions A 40A(3) and is made available as an electronic reprint with the permission of ASM International. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplications of any material in this paper for a fee or for commercial purposes, or modification of the content of this paper are prohibited.Lack of efficient mixing technology to achieve a uniform distribution of fine-size reinforcement within the matrix and the high cost of producing components have hindered the widespread adaptation of particulate metal matrix composites (PMMCs) for engineering applications. A new rheo-processing method, the melt-conditioning high-pressure die-cast (MC-HPDC) process, has been developed for manufacturing near-net-shape components of high integrity. The MC-HPDC process adapts the well-established high shear dispersive mixing action of a twin-screw mechanism to the task of overcoming the cohesive force of the agglomerates under a high shear rate and high intensity of turbulence. This is followed by direct shaping of the slurry into near-net-shape components using an existing cold-chamber die-casting process. The results indicate that the MC-HPDC samples have a uniform distribution of ultrafine-sized SiC particles throughout the entire sample in the as-cast condition. Compared to those produced by conventional high-pressure die casting (HPDC), MC-HPDC samples have a much improved tensile strength and ductility.EP-SR

Outcomes of gynecologic cancer surgery during the COVID-19 pandemic: an international, multicenter, prospective CovidSurg-Gynecologic Oncology Cancer study

Background The CovidSurg-Cancer Consortium aimed to explore the impact of COVID-19 in surgical patients and services for solid cancers at the start of the pandemic. The CovidSurg-Gynecologic Oncology Cancer subgroup was particularly concerned about the magnitude of adverse outcomes caused by the disrupted surgical gynecologic cancer care during the COVID-19 pandemic, which are currently unclear. Objective This study aimed to evaluate the changes in care and short-term outcomes of surgical patients with gynecologic cancers during the COVID-19 pandemic. We hypothesized that the COVID-19 pandemic had led to a delay in surgical cancer care, especially in patients who required more extensive surgery, and such delay had an impact on cancer outcomes. Study Design This was a multicenter, international, prospective cohort study. Consecutive patients with gynecologic cancers who were initially planned for nonpalliative surgery, were recruited from the date of first COVID-19-related admission in each participating center for 3 months. The follow-up period was 3 months from the time of the multidisciplinary tumor board decision to operate. The primary outcome of this analysis is the incidence of pandemic-related changes in care. The secondary outcomes included 30-day perioperative mortality and morbidity and a composite outcome of unresectable disease or disease progression, emergency surgery, and death. Results We included 3973 patients (3784 operated and 189 nonoperated) from 227 centers in 52 countries and 7 world regions who were initially planned to have cancer surgery. In 20.7% (823/3973) of the patients, the standard of care was adjusted. A significant delay (>8 weeks) was observed in 11.2% (424/3784) of patients, particularly in those with ovarian cancer (213/1355; 15.7%; P<.0001). This delay was associated with a composite of adverse outcomes, including disease progression and death (95/424; 22.4% vs 601/3360; 17.9%; P=.024) compared with those who had operations within 8 weeks of tumor board decisions. One in 13 (189/2430; 7.9%) did not receive their planned operations, in whom 1 in 20 (5/189; 2.7%) died and 1 in 5 (34/189; 18%) experienced disease progression or death within 3 months of multidisciplinary team board decision for surgery. Only 22 of the 3778 surgical patients (0.6%) acquired perioperative SARS-CoV-2 infections; they had a longer postoperative stay (median 8.5 vs 4 days; P<.0001), higher predefined surgical morbidity (14/22; 63.6% vs 717/3762; 19.1%; P<.0001) and mortality (4/22; 18.2% vs 26/3762; 0.7%; P<.0001) rates than the uninfected cohort. Conclusion One in 5 surgical patients with gynecologic cancer worldwide experienced management modifications during the COVID-19 pandemic. Significant adverse outcomes were observed in those with delayed or cancelled operations, and coordinated mitigating strategies are urgently needed

A direct method of preparing cast aluminium alloy-graphite particle composites.

A direct method of preparing cast aluminium alloy-graphite particle composites using uncoated graphite particles is reported. The method consists of introducing and dispersing uncoated but suitably pretreated graphite particles in aluminium alloy melts, and casting the resulting composite melts in suitable permanent moulds. The optical pretreatment required for the dispersion of the uncoated graphite particles in aluminium alloy melts consists of heating the graphite particles to 400° C in air for 1 h just prior to their dispersion in the melts. The effects of alloying elements such as Si, Cu and Mg on the dispersability of pretreated graphite in molten aluminium have also been reported. It was found that additions of about 0.5% Mg or 5% Si significantly improve the dispersability of graphite particles in aluminium alloy melts as indicated by the high recoveries of graphite in the castings of these composites. It was also possible to disperse upto 3% graphite in LM 13 alloy melts and retain the graphite particles in a well distributed fashion in the castings using the pre-heat-treated graphite particles. The observations in this study have been related to the information presently available on wetting between graphite and molten aluminium in the presence of different elements and our own thermogravimetric analysis studies on graphite particles. Physical and mechanical properties of LM 13-3% graphite composite made using pre-heat-treated graphite powder, were found to be adequate for many applications, including pistons which have been successfully used in internal combustion engine

Optimization of hot workability of an Al-Mg-Si alloy using processing maps

The hot workability of an Al-Mg-Si alloy has been studied by conducting constant strain-rate compression tests. The temperature range and strain-rate regime selected for the present study were 300–550°C and 0.001–1 s–1, respectively. On the basis of true stress data, the strain-rate sensitivity values were calculated and used for establishing processing maps following the dynamic materials model. These maps delineate characteristic domains of different dissipative mechanisms. Two domains of dynamic recrystallization (DRX) have been identified which are associated with the peak efficiency of power dissipation (34%) and complete reconstitution of as-cast microstructure. As a result, optimum hot ductility is achieved in the DRX domains. The strain rates at which DRX domains occur are determined by the second-phase particles such as Mg2Si precipitates and intermetallic compounds. The alloy also exhibits microstructural instability in the form of localized plastic deformation in the temperature range 300–350°C and at strain rate 1 s–1

Deformation mechanisms and texture evolution of in-situ magnesium matrix composites containing polymer derived SiCNO dispersoids during hot compression

In-situ magnesium matrix composite was fabricated by injecting a liquid polymer directly into, and having it converted into 2.5 vol% SiCNO ceramic dispersoids, within molten Mg using a stir-casting method. The deformation mechanisms and texture evolution for these as-cast composites were investigated in a strain rate range of 10(-3) - 1 s(-1) within a temperature range of 150-350 degrees C under uniaxial compression. It was observed that the deformed composites follow a power-law creep having a stress exponent, n = 8 and activation energy, Q = 149 kJ mol(-1) which suggest that deformation mechanism is controlled by lattice self-diffusion for constant structure creep. It was found that in the range of 150-250 degrees C, with a ratio of rate of work-softening to rate of work-hardening of about 0.80, twinning induced shear bands nucleate and propagate along the direction of maximum shear stress. When the temperature approaches 350 degrees C, the plastic flow is dominated by dislocation assisted slip. Analysis of Zener-Hollomon parameter (Z) revealed that the transition from twinning into dislocation slip dominated deformation progresses at 10(13) s(-1) < Z < 10(13) s(-1). Macro-textural studies confirm that while basal plane assists deformation by twinning mechanism, the non-basal prismatic planes favor significant plastic deformation by dislocation assisted slip for the in-situ composites

Effect of melt temperature on microstructural and strength properties of in-situ aluminum metal matrix composites containing SiCNO particles

Polymer Injection Pyrolysis (PIPs) can be adopted to synthesize in-situ ceramic particles within molten metal by stir-casting process. This paper investigated the effect of pyrolysis temperatures on microstructural and strength properties of in-situ aluminum matrix composites containing 2.5 vol% of SiCNO particles. In-situ composites were synthesized by stir-mixing of cross-linked polysilazane at four different pyrolysis temperatures (675–850°C) at which in-situ pyrolysis occurred and then followed by ultrasonic agitation and squeeze casting process. Microstructural data reveals that grain size and the particle size of SiCNO particles decreases with increasing the temperature of the melt at which polymer was introduced into the melt. The increase in the strength properties of the fabricated composites as compared to pure aluminum is almost 210% for the composites fabricated at 850°C while it is marginally 17% for the composites fabricated at 720°C. Fractography studies suggest that composite fabricated at 675°C exhibits a better combination of yield strength and ductility