ABSTRACT
This study shows the results of Ni replacement with Co in a W-Ni-Co-type tungsten heavy alloy (WHA) in terms of the structure and mechanical properties. Five alloys containing 92 wt. % of tungsten plus Ni and Co changing in the proportions (Co:Ni) of 1:9, 2:8, 3:7, 4:6, and 5:5 were prepared using liquid phase sintering (LPS). The specimens were studied directly after sintering and after solution heat treatment. The tensile strength, yield strength, and elongation were evaluated. The results of tensile tests were supplemented with microhardness measurements of tungsten grains and matrix. Light microscopy was used for the microstructure, and a scanning electron microscope (SEM) equipped with an EDS attachment was applied for the assessment of the fracture mode and chemical microanalysis. It was concluded that the replacement of Ni with Co led to a tensile property increase that was accompanied by a gradual decrease in elongation that started to be critical for a Co:Ni ratio higher than 4:6.
ABSTRACT
The temperature distributions, microstructure, and mechanical properties of tungsten composite with aluminum alloy friction-welded joints are presented in this paper. The effects of welding parameters on flash diameter, shortening, joint efficiency, microhardness, and microstructure were studied. Empirical temperature models for heating and cooling phases are proposed in this study. The predicted maximum temperatures at the periphery and in the axis of aluminum specimens were close to 550 °C and 480 °C at the interface, respectively. Moreover, the peak temperature in the weld zone was studied analytically. A maximum tensile strength of 234 MPa was reached for the following welding parameters: friction time of 3.5 s and friction force of 12.5 kN. The efficiency of the welded samples decreased after reaching the maximum value, with an increase of friction time and force. Maximum hardness at the interface and the half-radius reached 100 HV and 80 HV in the aluminum alloy joints, respectively. Dynamic recrystallisation areas on the aluminum alloy side were observed. Transmission electron microscopy observations of the microstructure in the aluminum alloy revealed the presence of a high dislocation density compared to the parent material.
ABSTRACT
The paper presents the results of studies on the effects of heat treatment and cold-work parameters on the mechanical properties and microstructure of the tungsten heavy alloy (WHA) with the composition W91-6Ni-3Co. Tungsten heavy alloy (WHA) is used in conditions where strength, high density, and weight are required. The material for testing as rod-shaped samples was produced by the method of powder metallurgy and sintering with the participation of the liquid phase and then subjected to heat treatment and cold swaging. The study compares the effect of degree deformation on the strength, hardness, microhardness, and microstructure of WHA rods. The conducted tests showed that heat treatment and cold-work allowed to gradually increase the strength parameters, i.e., tensile strength σuts, yield strength σys, elongation ε, hardness, and microhardness. These processes made it possible to increase the tensile strength by over 800 MPa (from the initial 600 MPa after sintering to the final value of over 1470 MPa after heat treatment with cold swaging deformation with reduction of 30%) and the hardness from 32 to 46 HRC.
ABSTRACT
The results of structure and mechanical property investigations of tungsten heavy alloy (THA) with small additions of rhenium powder are presented. The material for the study was prepared using liquid phase sintering (LPS) of mixed and compacted powders in a hydrogen atmosphere. From the specimens, the samples for mechanical testing and structure investigations were prepared. It follows from the results of the microstructure observations and mechanical studies, that the addition of rhenium led to tungsten grain size decreasing and influencing the mechanical properties of W-Ni-Fe-Co base heavy alloy.
ABSTRACT
Tungsten heavy alloys (THA) are used in the defense industry for subcaliber bullet cores due to their high density and strength. Typically methods of joining tungsten rod elements include: soldering, friction welding or threaded sleeve splicing. The properties of the joints were tested for three types of material containing 90.8, 96.2 and 98.2 wt.%. tungsten, density from 17.3 to 18.4 g/cm3 and strength range 400-1000 MPa. Combination in the liquid phase at the sintering temperature was carried out in a vacuum furnace at a temperature of 1520 °C in a hydrogen atmosphere, and tests used pairs of both identical and dissimilar materials. After that, some of the bars were subjected to additional heat treatment at 1100 °C for 3 h. The tests of the mechanical properties in the static tensile test and the measurement of impact strength showed that the obtained strength of the joints was comparable to that of the parent material. The microstructure analysis showed that the resulting joint area, while maintaining the appropriate roughness of the joined end faces of the bars, is homogeneous without areas of the solidified matrix of the joint line. Research showed that it is possible to bond under sintering conditions with the participation of a solid liquid phase of homonymous and dissimilar THA materials. The strength of joints in dissimilar materials was comparable to a tungsten heavy alloy material with lower strength in the bonded pair while homonymous materials were comparable to the original material. The test results provided a good basis for further research in which the obtained pairs of joints will be subjected to plastic working processes.
ABSTRACT
BACKGROUND: Air pollution triggered diseases have become a leading health problem worldwide. The main adverse effects of air pollutants on human health are related to the cardiovascular system and particularly show an increasing prevalence of myocardial infarct and stroke. The aim of the study was to evaluate the influence of main air pollutants on non-ST-segment elevation myocardial infarction (NSTEMI) and ST-segment elevation myocardial infarction (STEMI) admissions to local interventional cardiology centers. METHODS: Between 2014 and 2015, a multicenter registry of 1957 patients with acute myocardial infarction (STEMI, NSTEMI) admitted to interventional cardiology departments in three Polish cities were under investigation. The air pollution (PM2.5, PM10, NO2, SO2, O3) and weather conditions (temperature, barometric pressure, humidity) data for each city were collected as daily averages. The case-crossover design and conditional logistic regression were used to explore the association between acute myocardial infarctions and short-term air pollution exposure. RESULTS: Occurrence of NSTEMI on the day of air pollution was triggered by PM2.5 (OR = 1.099, p = 0.01) and PM10 (OR = 1.078, p = 0.03). On the following day after the air pollution was recorded, NSTEMI was induced by: PM2.5 (OR = 1.093, p = 0.025), PM10 (OR = 1.077, p = 0.025) and SO2 (OR = 1.522, p = 0.009). For STEMI, events that occurred on the day in which air pollution was triggered by: PM2.5 (OR = 1.197, p < 0.001), PM10 (OR = 1.163, p < 0.001), SO2 (OR = 1.670, p = 0.001) and NO2 (OR = 1.287, p = 0.011). On the following day after air pollution was recorded, STEMI was induced by: PM2.5 (OR = 1.172, p < 0.001), PM10 (OR = 1.131, p = 0.001), SO2 (OR = 1.550, p = 0.005) and NO2 (OR = 1.265, p = 0.02). None of the weather conditions indicated were statistically significant for acute myocardial infarction occurrence. CONCLUSIONS: The most important pollutants triggering acute myocardial infarction occurrence in the population of southern Poland, both on the day of air pollution and the following day are particulate matters (PM2.5, PM10) and gaseous pollutants including NO2 and SO2. These pollutants should be regarded as modifiable risk factors and thus, their reduction is a priority in order to decrease total morbidity and mortality in Poland.
