Densification and microstructural evaluation during laser sintering of M2 high speed steel powder

Abstract

In the present work, the densification and microstructure of M2 high speed steel powder processed by direct laser sintering method was studied. Test specimens were produced using a 200 W continuous wave CO₂ laser beam at different scan rates ranging from 50 to 175 mm s^?1. The building process was performed under argon and nitrogen atmospheres in order to evaluate the role of sintering atmosphere. It was found that the sintered density strongly depends on the laser scan rate and thus on the duration time of the laser beam on the surface of the powder particles. Generally, with a decrease in the scan rate higher densification was obtained. However, formation of large cracks and delamination of the sintered layers is feasible at low scan rates. The results also demonstrated that sintering under argon atmosphere yields better densification compared to a nitrogen atmosphere, in particular at higher scan rates. The microstructure of laser sintered parts consisted of large and elongated pores parallel to the building direction. The metal matrix structure was found to be heterogeneous, i.e. carbon rich austenite was formed due to carbon segregation. This structure consisted of fine cellulars or dendrites of martensite and retained austenite. This article describes the influence of manufacturing parameters on the densification of laser sintered M2 high speed steel powder. The microstructural features of the processed parts are also addressed.     

High strain rate superplasticity of ultrafined Ti–10V–2Fe–3Al compact prepared by sintering and isothermal forging

Abstract

High strain rate superplasticity was obtained for powder Ti–10V–2Fe–3Al (Ti-1023) alloy prepared by powder sintering and isothermal forging technology. The selected powder was cold isostatic pressed, sintered and isothermal forged to prepare this powder alloy. Tensile testing was conducted at optimum superplastic temperaure of 1023 K with different initial strain rate, and the elongation to failure, the flow stress and the microstructure were analysed. The experiment results exhibited that the microstructure of this powder alloy is extraordinary uniform and fine, resulted in considerable enhancement of optimum initial strain rate increased from 3·3×10^?4 s^?1 of conventional cast and wrought Ti-1023 alloy to 3·3×10^?3 s^?1 of this powder alloy. The elongation to failure increased first and then decreased with initial strain rate from 3·3×10^?4 to 3·3×10^?2 s^?1. The strain rate sensitivity m is about 0·46 near initial strain rate of 3·3×10^?3 s^?1, larger than conventional cast and wrought Ti-1023 alloy. Microstructure observations showed that dynamic recrystallisation and grain growth were present during superplastic deforming.     

Production of Ti–13Nb–13Zr Alloy by Powder Metallurgy (P/M) via Sintering Hydrides

The blended elemental method was selected for the manufacture of Ti–13Nb–13Zr alloy by a cold isostatic pressing process and sintering densification under high vacuum. The samples were sintered at the different temperatures from 1250°C to 1450°C with a pressure of 10^?3 ～ 10^?5 Pa. The decomposition of titanium, niobium, and zirconium hydride powders was discussed by thermal gravimetric analyses and differential scanning calorimetry. The phase composition, microstructure and fracture morphology of Sintered Ti–13Nb–13Zr samples were determined by X-ray diffraction and scanning electron microscopy. The results indicate that the hydrogen can be removed effectively. Chemical analysis shows that the Nb, Zr alloying element and hydrogen contents accord with the standard of the ASTM-1713. The final density of sintered Ti–13Nb–13Zr specimens is 4.99 g cm^?3 after sintering at 1450°C for 4 h, representing 99.69% of the theoretical density. The microstructure of sintered Ti–13Nb–13Zr alloys by powder metallurgy is a typical Widmannstätten (α + β).     

Measurement of activation energy values for sintering of active urania powder compacts

Abstract

Wide variations in sintering behaviour have been observed for uranium dioxide powder compacts prepared from ammonium diuranate (ADU) powder precursor and calcined at various temperatures. In general, it is observed using X-ray diffraction that powders calcined below 800°C are non-crystalline and they are also highly sinterable (active), whereas powders calcined above 800°C have poor sinterability and are designated inactive. A model has been proposed for measurement of the activation energy values for sintering of compacts of urania powder based upon a modification derived from Frenkel's sintering model as applicable to non-crystalline powders, from which the shrinkage rate at zero time (t → 0) can be obtained. Moreover, the associated effect on sintering due to the non-crystalline to crystalline transition with progressive thermal treatment is eliminated using this model. The activation energy values for sintering of the non-crystalline powders at various calcination temperatures are in good agreement and, below 800°C, values of ~167·5 kJ mol^?1 are obtained. For powders calcined above 800#x00B0;C, values of ~314·1 kJ mol^?1 are obtained.

MST/812     

Structural investigation of nanohydroxyapatite doped with Y3+ and F– ions

Abstract

Pure and 2·5 mol.-%Y³⁺ and F^– doped nanohydroxyapatites were synthesised by a precipitation method and sintered at 900 and 1100°C to investigate their densification and structure. X-ray diffraction and Fourier transform infrared spectroscopy methods were performed to identify the presence of phases and bonds after the sintering respectively. Densification of hydroxyapatite was improved by the addition of these ions. No second phases were observed by X-ray diffraction method. Structure of the materials was hexagonal. Fourier transform infrared spectroscopy investigations showed that the Y³⁺ and F^– doped hydroxyapatites had F^– bonds in addition to OH^– bonds after the sintering.     

Integrating copper at the nanometer length scale with Sn–3·5Ag solder to develop high performance nanocomposites

Abstract

In the present study, copper at the nanometer length scale is integrated with Sn–3·5Ag using the technique of powder metallurgy incorporating energy efficient microwave sintering. Superior mechanical characteristics were realised for the formulations containing nanometer length scale copper in excess of 1 vol.-%. Sn–3·5Ag reinforced with 2·5 vol.% nanosize copper particulates exhibited the best overall mechanical characteristics. Particular emphasis is placed on studying the effect of the increasing presence of nanosize copper particulates on the microstructure and property evolution of the Sn–3·5Ag matrix.     

Sintering and electromagnetic properties of BaFe12O19 ferrite prepared by co-precipitation method

BaFe₁₂O₁₉ (BaM) was synthesized through the co-precipitation route. Pure phase BaM was formed after calcination of precipitated powder at 900 °C. BaM was sintered at three different temperatures; 1100, 1200, and 1300 °C to study the sintering kinetics by varying the sintering time from 1 to 4 h. Apparent porosity decreased, and bulk density increased with increasing sintering temperature and period. A bulk density of about 4.6 g/cm³ was achieved after sintering at 1300 °C/4 h. The rate-controlling mechanism of BaM densification was the diffusion of oxygen, and the activation energy for the sintering process was 274 kJ/mol. The grain size of BaM increased with rising sintering temperatures. Permittivity increased from about 11 to 17 and the permeability increased from about 10 to 16 with the increase in sintering temperature from 1100 to 1300 °C. Saturation magnetization was also enhanced to about 69 emu/g after sintering at 1300 °C/4 h. Therefore, BaM ferrite synthesized through the co-precipitation route can be effectively used for high-frequency applications after sintering at 1300 °C.

     

Sintering of diamond with cobalt

Diamond powder was sintered by aid of a cobalt at 1600–1800°C and 80–100 kbar. The microstructural observation of the resultant compacts showed that the masses were held together by diamond-to-diamond bonding. The compacts had Knoop microhardness of 5000–8000 kg/mm². The sintering process of the specimen is described.     

Synthesis of submicron sized silver powder for metal deposition via laser sintered inkjet printing

Submicron sized silver powder was prepared from AgNO₃ using a chemical-reduction method. A spherical silver power exhibiting an average particle size distribution of 0.2–0.4 μm and an excellent dispersibility was achieved and applied to the inkjet printing process. A drop-on-demand (DOD) inkjetting system was used to print the silver particles suspended in a terpineol solvent. Through sintering at 300 °C, the size of the particles adjacent to the borderline of droplets were gradually increased and necking was observed between the droplets. Alternatively, the substrate for the particles could be heated to a lower temperature, and the sintering process of the conducting line was completed by the application of a laser beam. Increase in the laser power reduces the resistivity of the line. Through microstructure analysis, the necks between droplets were sintered at a specific energy density (ψ = 0.0398 J mm⁻³). The conducting lines were soldered and of a larger aggregation, between which a discontinuous micro-crack was observed. This was attributed to the surface tension effect and shrinkage during solidification. Influence of the densification parameters on resistivity was significant.     

Preparation of Fe3AI based iron aluminides from elemental and prealloyed povvders

Abstract

Iron aluminides were prepared by a powder metallurgy process from elemental powders, mixtures of prealloyed and elemental powders, and prealloyed powder. The sintering behaviour of various powders was studied using scanning electron microscopy, optical microscopy, and density measurement. It was found that sintering of elemental powder involved two distinct processes, i.e. alloying and densification, but sintering of prealloyed powder involved densification alone. The addition of prealloyed powder to elemental powders was helpful in restraining the swelling of sintered samples, the degree of swelling of sintered samples being reduced as the amount of prealloyed powder increased. For samples made from Fe-25 at.-%Al prealloyed powder, remarkable shrinkage was measured after sintering at 1250°C for 1 h. Within the correct range, their density increased with sintering temperature and time, but prolonged sintering at high temperature resulted in the loss of aluminium and a two phase microstructure. The difference in sintering behaviour between the various powders was discussed on the basis of thermodynamics.     

In situ fabrication and wettability of Ca2SiO4/CaTiO3 biocoating by laser cladding technology on Ti–6Al–4V alloy

Abstract

Wetting is the first and foremost event when a biomaterial is implanted into the biological system. Hence, it is very essential to investigate the wettability of a biomaterial before further biological studies. A textured coating with different relative amounts of Ca₂SiO₄ and CaTiO₃ was in situ fabricated by varying laser scan speed. The wettability of different coatings was investigated. The results indicate that the relative amount of Ca₂SiO₄ phase increased with decreasing laser scan speeds and reached the highest value of 48·17±2·10 mJ mm^?2, and the geometrically textured topography with a surface roughness of 9·17 μm was obtained at a laser scan speed of 2 mm s^?1. The microstructure in the coating can be characterised as fine dendrites. Surface energy values varied with the relative amount of Ca₂SiO₄ phase. The coating obtained at the laser scan speed of 2 mm s^?1, which contains more relative amount of Ca₂SiO₄, presents the highest surface energy, indicating most desirable wettability. This resulted in an increase in contact angle in simulated body fluid solution for improved wettability. The microhardness presented a gradient distribution from the coating surface (1072 HV) to the substrate (260 HV).     

Synthesis of magnetite by coprecipitation and sintering and its characterization

Magnetite powder was synthesized via chemical co-precipitation method using FeCl₃?·?6H₂O and FeCl₂?·?4H₂O as salts and ammonium hydroxide (25% NH₄OH) solution as precipitating agent. The phases, composition and morphology of the magnetite was characterized by X-ray diffraction (XRD), micro laser Raman spectroscopy and scanning electron microscopy–energy dispersive X-ray spectroscopy. The XRD and Raman studies confirmed the formation of magnetite phase only. The results showed that the particles were properly crystallized with no other impurity. Very fine and non-uniform powder sizes were observed even after sieving operation. The magnetite particles were also characterized after sintering the powder at 1100°C, grinding and sieving. The resultant powder size in the range of 105–125?µm was obtained. After sintering, magnetite peaks got sharpened due to increase in the crystallite size. Raman peaks even at a higher laser power were observed for magnetite that were absent before sintering. Hematite peaks observed for the un-sintered powder at higher laser power was attributed to oxidation by laser radiation/heat. Efficiency of the co-precipitation process (before sintering) was established to be ～85%.     

Processing of alumina-based composites via conventional sintering and their characterization

The present work relates to the processing of dense alumina-based composites, their microstructural characterization and study of mechanical properties. Alumina ceramic material and alumina-based composites with m-Zirconia and Ceria addition are sintered at 1600°C, 1650°C and 1700°C temperatures via conventional sintering. Solid-state diffusion during sintering led to volume diffusion in alumina, and volume and grain boundary diffusion in alumina composite. In the present sintering conditions alumina is found to be the least dense as improper solid-state diffusion resulted in porosity, whereas alumina–zirconia composite achieved the highest density of 97%. Scanning electron microscope (SEM) micrograph shows homogeneous distribution of fine zirconia particles inside the alumina matrix, filling the voids of the alumina skeletal structure. Zirconia connects to alumina particles, restricting its abnormal grain growth. It results in strong bonding and grain refinement. Alumina–zirconia composite exhibits the highest hardness and fracture toughness of 14.37?GPa and 4.6?MPa?·?m^1/2 at 1700°C. Alumina suppresses the transformation of m-t zirconia, resulting in high toughness of alumina composites. Alumina–zirconia–ceria composite revealed the presence of porosity, which led to less densification and low mechanical properties.     

Production of injection moulded 316L stainless steels reinforced with TiC(N) particles

Abstract

Metal matrix composites, based on 316L stainless steel and reinforced with TiC and TiCN particles, were manufactured following a powder injection moulding route: mixing, preparation of feedstock, moulding, debinding and sintering. The 316L stainless steel and carbide powders were dry mixed and moulded with wax based binder. The critical powder loading for injection moulding was 62·5 vol.-% for all samples. Binder debinding was performed by solvent and thermal method. After debinding, the samples were sintered at 1250 and 1385°C for 1 h in pure H₂. Metallographic studies were conducted to extend densification and the corresponding microstructural changes. The sintered samples were characterised by measuring tensile strength, hardness and wear behaviour. Wear loss was determined for all samples after wear tests. All powder, fracture surfaces of moulded and sintered samples, and worn surfaces of all the samples, were examined using scanning electron microscope. The sintered density of injection moulded 316L stainless steel samples, reinforced and unreinforced, increases with increasing sintering temperature. The addition of TiC and TiCN improves the hardness and wear resistance with increasing sintering temperature.     

Development of high performance Mg–Al2O3 composites containing Al2O3 in submicron length scale using microwave assisted rapid sintering

Abstract

In the present study, magnesium composites reinforced with different volume fraction of submicron size Al₂O₃ particulates were synthesised using powder metallurgy technique incorporating an innovative microwave assisted rapid sintering technique. The sintered materials were subsequently hot extruded for characterisation in terms of microstructural, physical and mechanical properties. Microstructural characterisation results revealed a reasonably uniform distribution of Al₂O₃ particulates, minimal porosity and good matrix reinforcement interfacial integrity. The average coefficient of thermal expansion (CTE) value for Mg–Al₂O₃ composites was found to decrease with increasing amount of submicron Al₂O₃ particulates. Mechanical characterisation of the composites revealed an increase in hardness, elastic modulus, 0·2% YS and ultimate tensile strength (UTS) with the increase in amount of alumina particulates. Ductility exhibited the reverse trend. An attempt is made in the present study to correlate the effect of the presence of submicron alumina and its increasing amount with the microstructural, physical and mechanical properties of magnesium.     

Fabrication of controlled expansion Al-Si composites by pressureless and spark plasma sintering

A combination of low coefficient of thermal expansion (CTE) and decent thermal conductivity (TC) is the reason for the Al-high vol% Si system to become popular for electronic packaging material. In the present work, two process routes, firstly conventional powder metallurgy and then spark plasma sintering (SPS) were utilized for the fabrication of Al-20-60 wt.% Si composites. In addition, effect of small fraction of CNT addition on the CTE of Al-20?wt% Si was studied. Effect of process parameters on the consolidation of the composites in terms of densification, microstructure evolution along with fractographic analysis and strength was studied. CTE and TC of the sintered composites were measured and correlated with the densification, percentage of Si and morphologies of the sintered products. Overall, better densification could be achieved in SPS and the Al-30%Si and Al-40%Si composites SPSed at 550?°C showed average CTE values of 14.52?×?10^?6/K and 13.36?×?10^?6/K, respectively, in the temperature range of 30–200?°C, which were better than some of the existing alloys with higher Si content. Simultaneously, TC values were 114.4?W/mK and 107.12?W/mK, respectively, for the above two SPSed composites.     

Effect of C and Cu addition on the densification and microstructure of iron powder in direct laser sintering process

Laser sintering of Fe–C–Cu steel powder for rapid manufacturing of sintered components for functional testing was studied. The effects of C and Cu addition on the densification and the attendant microstructural features were investigated. The influence of iron particle size on the sintering kinetics was also examined. It is shown that the alloying elements significantly improve the densification rate when fine iron particles and high laser intensity are used. The mechanism of particle bonding and the effect of the alloying elements are presented.     

Damping properties of open cell microcellular pure Al foams

Abstract

Damping behaviours of the open cell microcellular pure Al foams fabricated by sintering and dissolution process with the relative density of 0·31–0·42 and the pore size of 112–325 μm were investigated. The damping characterisation was conducted on a multifunction internal friction apparatus. The internal friction (IF) was measured at frequencies of 1·0, 3·0 and 6·0 Hz over the temperature range of 298–725 K. The measured IF shows that the open cell pure Al foam has a damping capacity that is enhanced in comparison with pure Al. At a lower temperature (～400 K), the IF of the open cell pure Al foams increases with decreasing relative density, with decreasing pore size and with increasing frequency. The IF peak was found at the temperature range of 433–593 K in the IF curves. It is clear that the IF peak is relaxational type and the activation energy associated with the IF peak is about 1·60 ± 0·02 eV. Defect effects can be used to interpret the damping mechanisms.     

Sintering studies on submicrometre-sized Y-Ba-Cu-oxide powder

The isothermal sintering behaviour of submicrometre-sized (<50 nm) powders of single-phase YBa₂Cu₃O _x (123) and unreacted stoichiometric mixture of submicrometre-sized (<50 nm) powders of BaCO₃, Y₂O₃ and CuO (which on calcination at 1173 K gives YBa₂Cu₃O _x ) was investigated through dilatometry under different sintering atmospheres. The sintering rate of the powder compacts was impeded by the presence of oxygen. The activation energies,Q, of sintering were determined to be 1218 kJ mol^–1 in argon, 1593 kJ mor^–1 in air and 2142 kJ mol^–1 in oxygen. A decrease in the apparent sintered density with increasing oxygen partial pressure was also observed. X-ray diffraction and thermal analyses (thermogravimetry and differential thermal analysis) showed no reaction during sintering of the single-phase product. Pellets fabricated from uncalcined powder exhibit two stages of sintering, one between 1073 and 1173 K having an activation energyQ=627kJ mol^–1, and a second one above 1173 K withQ=383.7 kJ mol^–1. A.c. susceptibility, resistivity and critical current density were determined as a function of the temperature of the sintered samples.     

Microstructure and mechanical properties of BAS/SiC composites sintered by spark plasma sintering

High-density BAS/SiC composites were obtained from β-SiC starting powder by the spark plasma sintering technique. Various physical properties of the BAS/SiC composites were investigated in detail, such as densification, phase analysis, microstructures and mechanical properties. The results demonstrated that the relative density of the BAS/SiC composites reached over 99.4% at 1900 °C. The SiC grains were uniformly distributed in the continuous BAS matrix which is probably because of complete infiltration of the SiC particles in BAS liquid-phase formed during sintering. The pull-out of SiC particles, crack deflection and bridging were observed as the major toughening mechanism. The flexural strength and fracture toughness of the BAS/SiC composites sintered at 1900 °C were up to 560 MPa and 7.0 MPa·m^1/2, respectively.