A micro-ultrasonic powder moulding method to fabricate Sn–Bi alloy micro parts

The Sn–Bi eutectic alloy powder in the micro-cavity was melted by ultrasonic vibration and rapidly cooled to form micro parts; no subsequent sintering process was required. Three kinds of Sn–Bi alloy micro gears were successfully fabricated, which verified the feasibility of the micro-ultrasonic powder moulding method. The effect of ultrasonic exposure time on the microstructure and mechanical properties of the micro parts was investigated respectively by metallographic analysis, XRD and tensile testing. Reasonable process parameters for this method were determined, which were sonotrode pressure of 63.5 MPa, ultrasonic output power of 2475 W and ultrasonic exposure time of 1.5 s.     

Thermal de-binding kinetics of injection molded W-8%Ni-2%Cu

The pre-mixed W-8%Ni-2%Cu with 50 vol% AIT binder specimens were fabricated by powder injection molding machine. The injected specimens were solvent de-waxed by dipping in n-heptane. Solvent de-waxed specimens were used to investigate thermal degradation of backbone at different heating rates by the mean of nano-balance coupled with mass spectrometry. The evolved products were captured and analyzed online by TG/MS. The heavy alloy shows catalytic effect on thermal decomposition of backbone binder. To investigate this catalytic effect, different metals feedstock were also analyzed and different mass loss behaviors were obtained. Studies show that tungsten metal has pronounced catalytic effect on backbone decomposition. By the help of degradation mechanism, the kinetic analysis was accomplished. Smooth thermal de-binding temperature–time profile was calculated under rate controlled de-binding conditions. The comparison was made between experiment and prediction in Elnik production furnace.     

Thermal de-binding kinetics of injection molded W-8%Ni-2%Cu

The pre-mixed W-8%Ni-2%Cu with 50 vol% AIT binder specimens were fabricated by powder injection molding machine. The injected specimens were solvent de-waxed by dipping in n-heptane. Solvent de-waxed specimens were used to investigate thermal degradation of backbone at different heating rates by the mean of nano-balance coupled with mass spectrometry. The evolved products were captured and analyzed online by TG/MS. The heavy alloy shows catalytic effect on thermal decomposition of backbone binder. To investigate this catalytic effect, different metals feedstock were also analyzed and different mass loss behaviors were obtained. Studies show that tungsten metal has pronounced catalytic effect on backbone decomposition. By the help of degradation mechanism, the kinetic analysis was accomplished. Smooth thermal de-binding temperature–time profile was calculated under rate controlled de-binding conditions. The comparison was made between experiment and prediction in Elnik production furnace.     

Microstructure and mechanical properties of magnesium composites prepared by spark plasma sintering technology

Spark plasma sintering (SPS) technology was used to determine the appropriate conditions for SPS sintering of commercially pure magnesium as well as the magnesium alloy AZ31. It was found that the sintering temperatures of 585 °C and 552 °C were the most suitable sintering temperatures for the magnesium and the AZ31 alloy, respectively. Magnesium matrix and AZ31 alloy matrix composites reinforced with SiC particles were then successfully fabricated by the SPS method at sintering temperatures of 585 °C and 552 °C, respectively. A uniform distribution of SiC particles was observed along the boundary between matrix particles. The mechanical properties, i.e. hardness and tensile strength increased with increasing SiC content up to 10 wt%. However, when the SiC content was larger than 10 wt%, the tensile strength decreased due to the agglomeration of SiC particles. The agglomeration of SiC particles was found to lead to the degradation of the interfacial bonding strength between matrix and reinforcement.     

Preparation of nanostructured high-temperature TZM alloy by mechanical alloying and sintering

Mechanical milling proceeded by sintering was used to synthesize nanostructured temperature-resistant TZM alloy. Milling under Ar for different times (1, 2, 3, 5, 10, 15, 20, 25, and 30 h) and sintering at 1500, 1600 and 1700 °C for 30, 45, 60 and 90 min resulted in increasing of low-energy grain boundaries (LEGBs) and dispersion of TiC and ZrC with a size of ~ 65 nm in the matrix near LEGBs. Morphology and grain size of the products were determined from scanning electron microscope (SEM) images and X-ray diffraction (XRD) patterns, almost precisely. Optimum density of nanostructured TZM alloy ~ 9.95 ± 0.01 g/cm³ was achieved by sintering at 1700 °C for 90 min.     

Effects of ThO2 on the solid-state sintering behavior of W-Ni-Fe alloy

Dilatometric analyses were employed to investigate the influence of ThO₂ particles on the sintering behavior of W-Ni-Fe alloys up to 1450 °C which is below the liquid formation temperature. The activation energies were obtained by analyzing by the shift of the iso-density points as a function of the linear-heating rate. It is well known that the dominant sintering mechanism of tungsten particles is grain-boundary diffusion in the solid-state. The activation energies were 304.00-467.39 kJ/mol for W-Ni-Fe and 519.10-2949.60 kJ/mol for the thoria doped alloy, respectively. The results for the non-added alloy are in agreement with the values reported in the literature. The possibility that the much higher energies for the doped alloy reflect a retardation of sintering due to the presence of thoria particles at the surface of the tungsten is discussed.     

Effects of cooling rate on mechanical properties and corrosion resistance of vacuum sintered powder injection molded 316L stainless steel

This study presents the results of corrosion behavior of powder injection molded 316L stainless steel parts sintered in vacuum. The feedstocks of metal powder and plastic binder were prepared and their viscosity was measured. Green samples were injection molded and binder was removed from the green parts. Brown test parts were sintered at 1325 °C with heating rate of 5 °C/min and 10 °C/min for 2 h followed by the same cooling rate. Corrosion response of the sintered test samples was measured by weight loss method in Ringer's Solution of pH 7.4 for 15 days. The test samples using cooling rate 10 °C/min showed higher mechanical properties and improved corrosion resistance compared to those sintered at low heating and cooling rate. High cooling rate reduced the evaporation of Cr and developed passive chromium oxide layer on the test samples resulting improved corrosion resistance.     

Investigations on kinetics of formation of fcc-free surface layers on cemented carbides with Fe-Ni-Co binders

The formation of surface layers free of face centered cubic (Ti,Ta,Nb,W)(C,N) carbonitrides and enriched in ductile binder phase (fcc-free surface layers) was investigated on cemented carbides containing Fe-Ni-Co binders. Cemented carbide alloys with varying Fe-Ni-Co binders were sintered in vacuum atmospheres at 1450 °C for 2, 3 and 5 h. Independent of the binder composition the growth of fcc-free surface layers obeys a parabolic law. For same sintering conditions, fcc-free layer growth kinetics is enhanced by the addition of Fe to Co and Ni binders. Thermodynamic calculations showed that adding Fe to Co and Ni binders increases the solubility of the element nitrogen in the liquid binder phase. The higher solubility of N in Fe-containing binder phases promotes the formation of larger fcc-free surface layers, so that the width of fcc-free surface layers can be modified by controlling the Fe content in the binder phase.     

Synthesis of ultrafine/nanocrystalline W-(30-50)Cu composite powders and microstructure characteristics of the sintered alloys

Ultrafine/Nanocrystalline W-Cu composite powders with various copper contents (30, 40 and 50 wt.%) have been synthesized by sol-spray drying and a subsequent hydrogen reduction process. The powders were consolidated by direct sintering at temperatures between 1150 and 1260 °C for 90 min. The powder characteristics and sintering behavior, as well as thermal conductivity of the sintered alloys were investigated. The results show that the synthesized powders exist in ultrafine composite particles containing numerous nanosized particles, and the composition distributed very homogeneously. As the copper contents increase, the grain size of the powders decreases. The subsequent sintered parts show nearly full density with the relative density more than 99% at the temperature of 1250 °C. The sintered parts have very fine tungsten grains embedded in a bulk matrix. With increased copper contents, the tungsten grain size decreases and the microstructural homogeneity of the sintered alloys improves further. The thermal conductivity properties, while a little lower than that of the theoretical value, depend on the copper contents.     

Sol-gel silica coatings on ZE41 magnesium alloy for corrosion protection

Silica coatings have been applied on the surface of ZE41 magnesium alloy following the organic sol-gel route and the dip-coating technique. Three different concentrations of sol solution and two densification temperatures of the coating (400 °C and 500 °C) were used to optimize the compaction of the coatings and as a result reach the corrosion protection of the metallic substrate tests in 3.5 wt.% NaCl aqueous solution. Crack-free coatings with thickness in the 2-3 μm were obtained on the ZE41 magnesium alloy. The combination of high alkoxide concentration in the sol-gel formulation, and the high sintering temperature (500 °C) leads to coating (D500) with the optimal physical barrier against the corrosion process. This coating was capable of resisting more than 7 days in contact with the aggressive electrolyte suffering minor corrosion degradation. A corrosion mechanism for each of the tested specimens has been proposed.     

Studies on the development of TZM alloy by aluminothermic coreduction process and formation of protective coating over the alloy by plasma spray technique

TZM alloy is a potential candidate for high temperature structural applications. However, in the preparation of this alloy by conventional melt-casting route, difficulties are encountered in achieving homogenized alloy composition in view of high melting temperature of the alloy and presence of minor alloying components. Therefore, an alternative technique of aluminothermic co-reduction was adopted to prepare TZM alloy of composition, Mo-0.5Ti-0.1Zr-0.02 °C, wt.% by simultaneous reduction of uniformly premixed oxides of MoO₂, TiO₂ and ZrO2 by aluminium in presence of requisite amount of carbon. The as-reduced alloy was further arc melted for consolidation. Since, TZM alloy is by nature highly susceptible to oxidation at elevated temperature in air or oxygen, therefore feasibility of development of silicide type of coating over the synthesized alloy by plasma coating technique was also examined. Silicon powder coated on TZM alloy surface by plasma spray technique was finally converted into MoSi₂ coating by sintering at 1350 ^°C for 2-4 h duration under argon. A double layer coating structure was formed with two distinct phases. The inner thin layer was consisted of Mo₂Si₅ phase (~ 10 μm) followed by thick outer layer of MoSi₂ (~ 150 μm). The coating showed good adhesion strength and stable oxidation with negligible mass gain (10 g/m²) at 1000 ^°C in air.     

Reactive sintering study of a novel cemented carbide hard alloy (W0.5Al0.5)C0.5-Ni

Cemented carbides hard alloy (W_0.5Al_0.5)C_0.5-13.3 vol% Ni was successfully prepared by reactive sintering of carbon, nickel powder and W_0.5Al_0.5 alloy powder. The novel cemented carbide hard alloy has superior mechanical properties. The influence of sintering time and temperature on the microstructure, mechanical properties and density of the specimens are well described. Interestingly, both sintering time and temperature have amazing influence on the mechanical properties, density and microstructure of the specimen. During the reactive sintering process, Ni was the binder phase for sintering (W_0.5Al_0.5)C_0.5-Ni cemented carbide, and it also accelerated the reaction rate of synthesizing (W_0.5Al_0.5)C_0.5. The reactive sintering is a good method for preparing cemented carbide hard alloy (W_0.5Al_0.5)C_0.5-Ni. Another phenomenon is that no WNi/W₃Ni₃C/NiC_x type phases are found in the bulk specimens, although it was prepared by reactive sintering the carbon, nickel powder and W_0.5Al_0.5 alloy powder directly and the carbon vacancy reach to the astonished 50% value.     

Effect of SPS process sintering on the microstructure and mechanical properties of nanocrystalline TiC for tools application

The most important spark plasma sintering (SPS) parameters (Temperature, holding time and pressure), have been reviewed to assess their effect on the densification, grain growth kinetics and mechanical properties of nanocrystalline TiC synthesized by mechanical alloying. Experiments were performed in the 1350-1800 °C temperature range with holding time from 1 to 10 min under various pressure values of 50, 80 and 100 MPa. The results of experiments revealed that the mechanical properties of the material were improved with raising the sintering temperature and extending the sintering time. However, a hardness decrease was observed as a result of abnormal grain growth under higher sintering temperatures. The optimized process parameters for SPS process are identified as a sintering temperature of 1650 °C, a pressure 100 MPa and a sintering time of 5 min. The resulting mechanical properties are: a relative density of 97.9%, a micro-hardness of 2570 Hv, a nano-hardness of 28 GPa, a fracture toughness of 4.9 MPa·m^1/2 and a compressive strength of about 2210 MPa.     

Friction and wear behavior of laser-sintered iron–silicon carbide composites

Laser sintering is currently one of the most popular techniques to develop innovative materials for many of the high tech industrial applications owing to its ability to build complex parts in a short time. As such, material researchers are focusing on developing advanced metal matrix composites through selective laser sintering method to develop an intricate component eliminating delay in production time. In the light of the above, the present work focuses on developing iron–silicon carbide (nickel coated) composites using direct metal laser sintering technology. A laser speed of 50, 75, 100 and 125 mm/s were adopted. Metallographic studies, friction and wear test using pin-on-disc have been carried out on both the matrix metal and its composites. Load was varied from 10 to 80 N while sliding velocity was varied from 0.42 to 3.36 m/s for a duration of 30 min. A maximum of 7 wt.% of silicon carbide has been successfully dispersed in iron matrix by laser sintering. Increased content of SiC in iron matrix has resulted in significant improvement of both hardness and wear resistance. Lower the sintering speed, higher is the hardness and wear resistance of both the matrix metal and its composites. However, coefficient of friction of composites increased with increased SiC under identical test conditions. SEM observations of the worn surfaces have revealed extensive damage to the iron pins, when compared with that of the composites.     

Enhanced sintering, microstructure evolution and mechanical properties of 316L stainless steel with MoSi2 addition

Sintering 316L stainless steel to near full density with an appropriate sintering additive can ensure high mechanical properties and corrosion resistance. We present here a sintering approach which exploits the dissociation of ceramics in steels at high temperatures to activate sintering densification to achieve near full dense 316L stainless steel materials. MoSi₂ ceramic powder was used as a sintering additive for pre-alloyed 316L stainless steel powder. Sintering behavior and microstructure evolution were investigated at various sintering temperatures and content of MoSi₂ as sintering additive. The results showed that the sintering densification was enhanced with temperature and MoSi₂ content. The distribution of MoSi₂ was characterized by XMAPs. It was found that MoSi₂ dissociated during sintering and Mo and Si segregated at the grain boundaries. Excess Mo and Si were appeared as separate phases in the microstructure. Above 98% of theoretical density was achieved when the specimens were sintered at 1300 °C for 60 min with 5 wt.% MoSi₂ content. The stainless steel sintered with 5 wt.% MoSi₂ exhibited very attractive mechanical properties.     

Single track formation in selective laser melting of metal powders

Selective laser melting (SLM) is a powder-based additive manufacturing capable to produce parts layer-by-layer from a 3D CAD model. Currently there is a growing interest in industry for applying this technology for generating objects with high geometrical complexity. To introduce SLM process into industry for manufacturing real components, high mechanical properties of final product must be achieved. Properties of manufactured parts depend strongly on each single laser-melted track and each single layer. In this study, effects of the processing parameters such as scanning speed and laser power on single tracks formation are explored. Experiments are carried out at laser power densities (0.3–1.3) × 10⁶ W/cm² by cw Yb-fiber laser. Optimal ratio between laser power and scanning speed (technological processing map) for 50 μm layer thickness is determined for stainless steels (SS) grade 316L (−25 μm) and 904L (−16 μm), tool steel H13 (−25 μm), copper alloy CuNi10 (−25 μm) and superalloy Inconel 625 (−16 μm) powders. A considerable negative correlation is found between the thermal conductivity of bulk material and the range of optimal scanning speed for the continuous single track sintering.     

Rapid discharge sintering of nickel-diamond metal matrix composites

In this study rapid discharge sintering (RDS) and furnace sintering of nickel-diamond metal matrix composites (MMCs) is compared. Nickel-diamond powder composites (80-20% by weight respectively) were uniaxially pressed into 20 mm discs at compaction pressures of 100, 200 and 300 MPa. Discharge sintering was carried out using a microwave plasma formed with hydrogen and hydrogen/nitrogen as the discharge gases and tube furnace sintering carried out in a argon or a hydrogen/nitrogen (3:1) atmosphere. Discs pressed to 300 MPa were treated at both 850 and 1000 °C. The properties of the sintered nickel-diamond composites were characterized using density, approximate flexural strength, hardness, wear resistance, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The RDS samples sintered at 1000 °C achieved the maximum approximate disc flexural strength of 473 MPa within a 20 min treatment time compared with 6 h for furnace sintered samples. Samples sintered using the RDS technique exhibited increased hardness values and a finer nickel matrix over furnace sintered samples. Using the RDS technique it has been possible to process nickel-diamond MMCs without oxidation or graphitisation at temperatures above 900 °C. Minimal diamond destruction was observed during abrasive wear testing of the RDS samples compared with damage and pull-out observed for furnace sintering.     

Pressureless sintering of ZrC-based ceramics by enhancing powder sinterability

The sinterability of ZrC was enhanced by high-energy ball milling as well as introduction of graphite and SiC as sintering additives. Densification process and microstructure development were investigated for ZrC-based ceramics densified by pressureless sintering. As-received ZrC powder showed poor sinterability. After high-energy ball milling, ZrC powder can be sintered to 98.4% theoretical density at 2100 °C. The obtained ceramic had fine microstructure and fewer entrapped pores. Introduction of 2 wt.% graphite combined with high-energy ball milling lowered the densification temperature of ZrC. The relative density of obtained ceramic reached up to 95% at 1900 °C. Introduced SiC inhibited ZrC grain growth during sintering and consequently avoided the entrapped pores within the grains. The relative density of ZrC-SiC reached up to 96.7% at 2100 °C. ZrC-SiC composite formed an interesting intragranular structure and had high fracture strength at room temperature.     

In-situ metal binder-phase formation to make WC-FeNi Cermets with spark plasma sintering from WC,Fe, Ni,and carbon powders

High-density WC-FeNi ceramic-metal (cermet) composites were fabricated using liquid-phase spark plasma sintering/field-assisted sintering technology (SPS/FAST) with in-situ formation of metal binder phase. The precursor materials were micron-sized powders of WC, Fe, Ni, and C. A low melting point from a eutectic reaction of the powders enabled the in-situ formation of FeNi alloy and facilitates liquid-phase sintering of the WC. The carbon powder was added to stabilize the formation of the binder phase. Electron backscatter diffraction (EBSD) was performed to measure grain size and orientation. The composite exhibited a 99% theoretical density and a microstructure consisting of rounded and contiguous WC grains. The average grain size is 10.5 μm. The composite has a maximum hardness of 16.1 GPa. This research provides a fast and cost-effective approach to fabricate hard metals.     

A feasibility study of W-Cu composites production by high pressure compression of tungsten powder

For manufacturing a heavy duty W-Cu composite, a porous tungsten skeleton is required; which later can be filled by molten copper via infiltration technique. The compression force usually up to 200 MPa can be provided by cold isostatic press (CIP) and the temperatures used for sintering the green compacts are more 2000 °C. However, in this research, high pressure within the range of 200 to 663 MPa was used to produce high density green specimens (60-80%) by CIP while sintering was carried out at a moderate temperature of 1550 °C. The tungsten skeletons were infiltrated with molten copper at 1300 °C.The reduction of sintering temperature from over 2000 °C to 1550 °C for a highly densified W-skeleton not only resulted into a successful production of W-Cu composites but also the obtained physical and mechanical properties of these composites are comparable to those obtained for lower compaction pressures and sintering temperature higher than 2000 °C.