Experimental investigations on welding behaviour of sintered and forged Fe–0.3%C–3%Mo low alloy steel

Tungsten Inert Gas (TIG) welding is considered as one of the cleanest welding methods. It is generally adopted for thinner materials with moderate weld joint strengths. Welding of sintered porous materials continues to be a challenge due to the inherent porosity of the parent metals. The present research work attempts to address some of the issues relating to the welding behaviour of sintered and forged Fe–0.3%C–3%Mo low alloy steels under TIG welding. Rectangular strips of size 70 mm × 15 mm × 5 mm, obtained by blending, compacting and sintering of elemental powders of iron, graphite and molybdenum, were upset forged – both hot and cold in order to obtain alloy steel strips of various porosities. Two identical alloy steel strips of equal density were then welded both along longitudinal and transverse directions, by TIG welding, employing filler metal of suitable composition. The welded strips were then subjected to tensile test, hardness test, microstructural and Scanning Electron Microscope (SEM) fractography studies. Cold/hot upsetting of the sintered alloy preforms has led to enhanced density. As a result of improved density, their tensile strength and hardness values were also found to be enhanced. The welded alloy exhibited higher tensile strength compared to the un-welded base metal, due to strengthening by residual stress. Similarly, the strength and hardness of the welded alloy strips were found to be enhanced with increase in density. The tensile strength of welded joint is found to be higher compared to that of the base metal due to alloy metals segregation, rapid cooling and formation of acicular ferrite at the weldment of welded joint. No porosity was observed in the weld metal or Heat Affected Zone (HAZ) of the weld joint. However, the base metal had numerous micro pores, though pore migration towards weldment has not been observed.     

Phase,microstructure and properties evolution of fine-grained W–Mo–Ni–Fe alloy during spark plasma sintering

Fine-grained tungsten (W) heavy alloy containing molybdenum (Mo) with W particle sizes of less than 5 μm were fabricated by spark plasma sintering (SPS) pre-milling W–2Mo–7Ni–3Fe powder at a lower temperature of 1000–1250 °C. Phase, microstructure and mechanical properties evolution of W–Mo–Ni–Fe alloy during spark plasma sintering were studied in detail. As increasing sintering temperature, the hardness of the alloy decreased rapidly. However, bending strength of the alloy demonstrated a fall–rise–fall trend, and the maximum strength was obtained at 1150 °C. The W–2Mo–7Ni–3Fe alloy microstructure was composed of white W-grain, gray W-rich structure, black γ-(Ni, Fe, W, Mo) binding phase, and deep-gray W-rich structure. The intergranular fracture along the W/W grain boundary is the main fracture modes of W–2Mo–7Ni–3Fe alloy.     

Experimental investigation on workability and strain hardening behaviour of Fe–C–Mn sintered composites with different percentage of carbon and manganese content

An experimental investigation on the workability and strain hardening behaviour of iron–carbon–manganese (Fe–C–0.50Mn) Powder metallurgy (P/M) steel composite sintered preforms under triaxial stress state has been carried out. Cold upset forming of the above composite preforms was conducted with variation in carbon content (0.10% and 0.25%) and manganese content (0.50% and 1.05%) with an aspect ratio 0.45. The powders were collected on weight basis and then gradually compacted up to 1.2 GPa pressure and then sintered at 950 ± 5 °C to form composite preforms. Sintered preforms were cold deformed with uniform incremental loading. The effect of different percentage of carbon and manganese on the iron-based P/M composite was investigated thoroughly in the cold deformation experiment. Comparison between the effect of carbon and manganese on the workability and strain hardening behaviour of the composites was analysed and presented. The analysis of the experimental results has shown that the P/M steel which contains 0.10% carbon and 0.50% manganese exhibited greater values of stresses, initial relative density, strain hardening and workability parameters.     

Dry sliding wear behaviour of hypereutectic Al–Si piston alloys containing iron-rich intermetallics

The effect of iron-rich intermetallics on the wear behaviour of Al–Si hypereutectic alloys has been studied. Dry sliding wear tests have been conducted using a pin-on-disk machine under different normal loads of 18, 51, 74 and 100 N and at a constant sliding speed of 0.3 m/s. The addition of 1.2 wt.% Fe to the LM28 alloy increased the wear rate due to the formation of needle beta intermetallics. Introducing 0.6 wt.% Mn to the iron-rich alloy changed the beta intermetallics into the modified alpha phases, and therefore reduced the detrimental effect of iron. TIG welding method as a surface melting process was applied on the iron and manganese containing alloy and led to a fine microstructure and increased the wear resistance.     

Effects of Mo on high-temperature strength of fire-resistant steel

A series of Fe–Mo–C steels with Mo addition from 0.1 to 0.8 wt.% have been prepared to study the effects of Mo on high-temperature strength of fire-resistant steel. The high-temperature hardness tests were carried out to investigate the strengthening mechanisms of Mo in fire-resistant steel. The results show that the hardness of Fe–Mo–C steels increases with the increase of Mo content at a given temperature, and the strengthening effect of Mo becomes remarkable when the temperature is on the rise. Theoretical analysis indicates that the solid-solution strengthening of Mo is the dominant high-temperature strengthening mechanism in fire-resistant steel, but this strengthening effect becomes relatively weak when Mo content is more than or equal to 0.5 wt.%. Moreover, the bainite strengthening plays an important role in improving the high-temperature strength of fire-resistant steel. Furthermore, the analysis indicates that the ferrite grain size has less effect on high-temperature strength of fire-resistant steel. The present results also provide fundamentals to design low-cost fire-resistant steels with excellent high-temperature properties and the most reasonable range of Mo addition is 0.2–0.3 wt.%.     

Martensitic transformation behaviors of rapidly solidified Ti–Ni–Mo powders

For the fabrication of bulk near-net-shape shape memory alloys and porous metallic biomaterials, consolidation of Ti–Ni–Mo alloy powders is more useful than that of elemental powders of Ti, Ni and Mo. Ti₅₀Ni_49.9Mo_0.1 shape memory alloy powders were prepared by gas atomization, and transformation temperatures and microstructures of those powders were investigated as a function of powder size. XRD analysis showed that the B2–R–B19 martensitic transformation occurred in powders smaller than 150 μm. According to DSC analysis of the as-atomized powders, the B2–R transformation temperature (T_R) of the 25–50 μm powders was 18.4 °C. The T_R decreased with increasing powder size, however, the difference in T_R between 25–50 μm powders and 100–150 μm powders is only 1 °C. Evaluation of powder microstructures was based on SEM examination of the surface and the polished and etched powder cross sections and the typical images of the rapidly solidified powders showed cellular morphology. Porous cylindrical foams of 10 mm diameter and 1.5 mm length were fabricated by spark plasma sintering (SPS) at 800 °C and 5 MPa. Finally these porous TiNi alloy samples are heat-treated for 1 h at 850 °C, and then quenched in ice water. The bulk samples have 23% porosity and 4.6 g/cm³ density and their T_R is 17.8 °C.     

Fabrication and characterization of Ni–W solid solution alloys via mechanical alloying and pressureless sintering

Ni–W solid solution alloy powders and sintered compacts were fabricated via mechanically alloying and pressureless sintering of powder batches with the compositions of Ni–xW (x = 20, 30, 40 wt.%). The crystallite size of the powders were between 11 nm and 17 nm, which decreased with increasing W contents, where a microhardness value of 6.88 GPa for the Ni powders MM’d for 48 h increased to 9.37 GPa for the Ni40W powders MA’d for 48 h. The MM’d/MA’d powders were sintered at 1300 °C for 1 h under Ar and H₂ gas flowing conditions. X-ray diffraction (XRD) patterns of the sintered Ni, Ni20W and Ni30W samples revealed the presence of only the solid solution phase, whereas the presence of elemental W and Ni₄W intermetallic phase were observed in the XRD patterns of the sintered Ni40W sample. Among all sintered samples, the sintered Ni sample had the highest relative density value of 96.36% and the lowest microhardness value of 1.59 GPa. The relative densities of the sintered samples decreased with increasing W amounts, contrary to microhardness values which increased with W contents. Moreover, microstructural characterizations via scanning electron microscope and electron backscatter diffraction, room temperature compression tests and sliding wear experiments were conducted in order to reveal the effects of W contents on the properties of the sintered Ni–W alloys.     

Influence of alloying elements and density on aqueous corrosion behaviour of some sintered low alloy steels

Low alloy steels produced through powder metallurgy route of sintering followed by forging are promising candidate materials for high strength small components. Porosity in such steels poses a real challenge during acid pickling treatment, which is one of the processing steps during manufacturing. The present research work attempts to investigate the mechanism underlying the acid corrosion behaviour of some sintered low alloy steels under induced acid pickling conditions. Sintered-forged low alloy steel samples containing molybdenum (Mo), copper (Cu) and titanium (Ti) were subjected to aqueous corrosion attack by immersing the samples in 18% HCl (Hydrochloric acid) solution for 25 h. Sample weight loss and Fe (Iron) loss were estimated for the corroded samples. The morphology of the corroded surfaces was studied through metallography and scanning electron microscopy. Higher porosity alloys underwent enhanced corrosion rates. Both corrosion rate and iron loss are found to decrease linearly with reduction in porosity in all cases of the alloys. The alloying elements Mo, Ti and Cu, when added in combination, have played a complementary role in the reduction of corrosion rate by almost one order of magnitude compared to unalloyed steel. Presence of carbides of the carbide forming elements Mo and Ti played a positive role on the corrosion behaviour of the low alloy steels.     

Experimental and prediction of sintered Cu–W composite by using artificial neural networks

High-energy mechanical milling was used to mix Cu and W powders. Cylindrical preforms with initial preform density of 85% were prepared using a die and punch assembly. The preforms were sintered in an electric muffle furnace at 750 °C, 800 °C, 850 °C, and subsequently furnace cooled and then the specimens are hot extruded to get 92% preform density. Scanning Electron Microscope and X-ray diffraction observations used to evaluate the characteristics. The pore size reduction during extrusion was studied using Auto CAD 2010. Neural networks are employed to study the tribological behavior of sintered Cu–W composites. The proposed neural network model has used the measured parameters namely the weight percentage of tungsten, sintering temperature, load and sliding distance to predict multiple material characteristics, hardness, specific wear rate, and coefficient of friction. The predicted values from the proposed networks coincide with the experimental values. In addition, a relative study between the regression analysis and the networks revealed that the artificial neural networks can predict the tribological characteristics of sintered Cu and W composites better than regression polynomials within a very few percent error.     

Effect of heat treatment on mechanical properties of low alloy steel foams

Effect of heat treatment on compressive properties of low alloy steel foams (Fe–1.75 Ni–1.5 Cu–0.5 Mo–0.6 C) having porosities in the range of 47.4–71.5% with irregular pore shape, produced by the space holder-water leaching technique in powder metallurgy, was investigated. Low alloy steel powders were mixed with different amounts of space holder (carbamide), and then compacted at 200 MPa. Carbamide in the green compacts was removed by water leaching at room temperature. The green specimens were sintered at 1200 °C for 60 min in hydrogen atmosphere. Sintered compacts were heat treated by austenitizing at 850 °C for 30 min and then quenched at 70 °C in oil and tempered at 210 °C for 60 min. In this porosity range, compressive yield strengths of as-sintered and heat treated specimens were 28–122 MPa and 18–168 MPa, respectively. The resultant Young’s moduli of the as-sintered and heat treated specimens were 0.68–3.12 GPa and 0.47–3.47 GPa, respectively. The heat treatment enhanced the Young’s modulus and compressive yield strength of the foams having porosities in the range of 47.4–62.3%, as a consequence of matrix strengthening. However, the compressive yield stress and Young’s modulus of the heat treated foam having 71.5% porosity were lower than that of the as-sintered foam’s, as a result of cracks in the structure. The results were discussed in light of the structural findings.     

Preparation and properties of porous Ti–10Mo alloy by selective laser sintering

In this study, porous Ti–10Mo alloy was prepared from a mixture of titanium, molybdenum and epoxy resin powders by selective laser sintering preforming, debinding and sintering at 1200 °C under a pure argon atmosphere. The influence of sintering process on the porous, microstructural and mechanical properties of the porous alloy was discussed. The results indicate that the pore characteristic parameters and mechanical properties mainly depend on the holding time at 1200 °C, except that the maximum strain keeps at about 45%. The matrix microstructure is dominated by α phase with a small quantity of β phase at room temperature. As the holding time lengthens from 2 to 6 h, the average pore size and the porosity decrease from 180 to 50 μm and from 70 to 40%, respectively. Meanwhile, the Young's modulus and the compressive yield strength increase in the ranges of 10–20 GPa and 180–260 MPa, respectively. Both the porous structure and the mechanical properties of the porous Ti–10Mo alloy can be adjusted to match with those of natural bone.     

Processing and mechanical properties of porous Ti–7.5Mo alloy

Titanium (Ti) and its alloys continue to be utilized extensively for skeletal repair and dental implants. Most metallic implant materials including pure Ti and Ti alloys used today are in their solid forms and are often much stiffer than human bone. However, the elastic modulus of Ti and Ti alloys can be reduced through the introduction of a porous structure, which may also provide new bone tissue integration and vascularization abilities. In the present study, porous Ti–7.5Mo alloy scaffolds made from ball-milled alloy particles and sintered at 1100 °C for 10, 15 and 20 h respectively were successfully prepared through a space-holder sintering method. In the sintered Ti–7.5Mo, no obvious diffraction peaks of elemental Mo remained after the sintering, and a duplex α + β microstructure was confirmed from the XRD pattern. The samples made from BM15 (the alloy particles ball-milled for 15 h) had higher relative density, compressive strength and elastic modulus performance than those from BM3 and BM30 (the alloy particles ball-milled for 3 and 30 h, respectively) when they were sintered under the same conditions. Moreover, the longer sintering time lead to the higher relative density and the greater compressive strength and modulus of the sample. In this work, the strength and modulus of the sintered porous Ti–7.5Mo conforms to the basic mechanical property requirement of cancellous bones.     

A simple process for magnetic nanocrystalline porous Co–Fe alloy hollow microfibers

A simple process has been developed to fabricate the magnetic nanocrystalline porous Co–Fe alloy microfibers with a hollow structure by the sol–gel and phase transformation at a low temperature. The alloy microfibers consisting of nanoparticles about 30 nm are characterized with a fiber diameter around 0.5 μm, a ratio of the hollow diameter to the fiber diameter about 1/2 and pore sizes of 50 to 300 nm. These nanocrystalline porous Co–Fe alloy hollow microfibers have a good magnetic property, with the specific saturation magnetization of 212.8 A m² kg^?1and coercivity of 15.6 kA m^?1 at room temperature.     

Tribological behavior of NiTi alloy against 52100 steel and WC at elevated temperatures

The dry tribological behavior of a Ti–50.3 at.% Ni alloy at temperatures of 25 °C, 50 °C and 200 °C was studied. The wear tests were performed on a high temperature pin-on-disk tribometer using 52100 steel and tungsten carbide pins. The worn surfaces of the NiTi alloy were examined by scanning electron microscope. The results showed that in the wear tests involving steel pins, the wear rate of the NiTi decreased as the wear testing temperature was increased. However, for the NiTi/WC contact, a reverse trend was observed. There was also a large decrease in the coefficient of friction for the NiTi/steel contact with increasing wear testing temperature. The formation of compact tribological layers could be the main reason for the reduction of the wear rate and coefficient of friction of the NiTi/steel contact at higher wear testing temperatures.     

The liquid phase sintering of molybdenum with Ni and Cu additions

To increase the sintered density of Mo compacts, liquid phase sintering was employed by adding sintering aids, Ni and Cu. With 1.5 wt.% Ni sintered at 1370°C, the sintered density of Mo increased from 86.0 to 97.5%. When one-third of the Ni was replaced by Cu, the density was improved from 82.1 to 99.1% when sintered at 1300°C. Although copper has little solubility in molybdenum, and vice versa, it aids nickel in enhancing the sintering of molybdenum compact during heating in the solid state and during the liquid phase sintering. The dilatometer run indicated that the liquid formation temperature was lowered by 90°C. Better wetting on molybdenum particles was also observed. However, the Mo–Ni–Cu compact was still brittle due to the presence of brittle Mo–Ni compounds at the grain boundaries.     

Dry sliding tribological behavior and mechanical properties of Al2024–5 wt.%B4C nanocomposite produced by mechanical milling and hot extrusion

In this paper, tribological behavior and mechanical properties of nanostructured Al2024 alloy produced by mechanical milling and hot extrusion were investigated before and after adding B₄C particles. Mechanical milling was used to synthesize the nanostructured Al2024 in attrition mill under argon atmosphere up to 50 h. A similar process was used to produce Al2024–5 wt.%B₄C composite powder. The milled powders were formed by hot pressing and then were exposed to hot extrusion in 750 °C with extrusion ratio of 10:1. To study the microstructure of milled powders and hot extruded samples, optical microscopy, transmission electron microscopy and scanning electron microscopy (SEM) equipped with an energy dispersive X-ray spectrometer (EDS) were used. The mechanical properties of samples were also compared together using tension, compression and hardness tests. The wear properties of samples were studied using pin-on-disk apparatus under a 20 N load. The results show that mechanical milling decreases the size of aluminum matrix grains to less than 100 nm. The results of mechanical and wear tests also indicate that mechanical milling and adding B₄C particles increase strength, hardness and wear resistance of Al2024 and decrease its ductility remarkably.     

Microstructure and mechanical property of sintered Fe-Cr-Mo steels due to phase transformations with fast cooling rates

The influence of carbon content in the range of 0.01–0.3 wt.% on microstructure, hardness and tensile property of sintered Fe-Cr-Mo steels was investigated. The sintered Fe–3.0 wt.%Cr–0.5 wt.%Mo–(0.1, 0.2, 0.3) wt.% C steels were prepared by using powder metallurgical process. After sintering, the specimens were rapidly cooled by nitrogen at the rate of 5.4 °C/s. It was found that in the sintered steels with a lower carbon content of 0.01 and 0.1 wt.%, the allotriomorphic ferrite and Widmanstӓtten ferrite formed at austenite grain boundaries and grew to occupy the whole prior austenite grains. With higher carbon contents of 0.2 and 0.3 wt.%, the microstructures consist of bainite, martensite and some retained austenite. These steels exhibited increases of hardness, tensile strength and elongation at break with increasing carbon content. Increase of strength is due to the transformations from austenite, formed during sintering, to hard bainite and martensite structures.     

Combustion characteristics of the heat pellet prepared from the Fe powders obtained by spray pyrolysis

Fe powders for thermal batteries were prepared by reduction of iron oxide powders obtained by spray pyrolysis. The iron oxide powders prepared by spray pyrolysis had fine size, spherical shape and high surface area. The morphologies of the Fe powders were affected by the preparation temperatures of the iron oxide powders. The Fe powders obtained from the iron oxide powders prepared by spray pyrolysis at 900 and 1000 °C had slightly aggregated structure of the primary powders with several microns sizes. The powders had pure Fe phases at reducing temperatures between 600 and 800 °C. The heat pellets with diameter of 18.2 mm were prepared using Fe powders and potassium perchlorate (KClO₄). The porosity of the prepared heat pellet was about 40%. The break strength of the heat pellet was 0.9 kgf. The ignition sensitivity of the heat pellet was 4 W. The maximum burn rate of the heat pellet obtained from the Fe powders were 8.6 cm s^?1.     

Experimental study on the plastic deformation and densification characteristics of some sintered and heat treated low alloy powder metallurgy steels

Sintered low alloy steels containing the alloying elements molybdenum, copper and titanium were synthesised through powder metallurgy route from mixed elemental powders to yield the compositions: Fe–0.5% C, Fe–0.5% C–2% Cu, Fe–0.5% C–2% Mo and Fe–0.5% C–2% Cu–2% Mo–2% Ti. Green cylindrical compacts were made using a 1000 kN hydraulic press using suitable cylindrical die-punch combination. The ceramic coated cylindrical preforms were sintered at 1000 ± 10 °C in a muffle furnace for a period of 120 min. After sintering, the preforms were subjected to different heat treatment processes, namely, heating to 900 °C, soaking for 60 min and quenching in air or oil or cooled inside the furnace. The heat treated preforms were subject to axial upsetting deformations, at various applied loads and their densification behaviours were compared. The influence of various heat treatment processes on deformation and densification of the alloys was studied and correlated with their microstructures. The plain carbon steel preforms were observed to respond well to the three heat treatment cycles by way of exhibiting the highest levels of densification and plastic deformation. However, both alloy addition and heat treatment have led to a reduction in densification and deformation of the alloy steel preforms. Presence of titanium carbide particulates in the microstructure of the Ti-alloyed steel has played a significant role in reducing the densification as well as deformation. The basic ferritic–pearlitic microstructure of Fe–0.5% C steel has essentially promoted the largest deformation levels coupled with higher densification.     

Mechanical properties of Ti–4.5Al–3V–2Mo–2Fe and possibility for healthcare applications

The mechanical properties of Ti–4.5Al–3V–2Mo–2Fe, a relatively low cost titanium alloy originally designed for structural applications (especially for aerospace applications), were investigated. The alloy was subjected to heat treatments with various solution treatment temperatures (annealing temperature) and cooling rates. The mechanical properties of the heat-treated alloys were then used in order to judge the prospects of practical usage of the alloy for healthcare equipment such as wheelchairs.The mechanical properties of Ti–4.5Al–3V–2Mo–2Fe are highly affected by either solution treatment or cooling rate, and they change as a result of the change in the microstructure. The alloy single annealed at temperature in the α + β field has very high fatigue ratio (0.80–0.85) and high specific strength (210–260 MPa/g cm^− 3) with a modest fracture toughness (J_IC = 25–35 kN/m). This balance of fatigue ratio and specific strength is better than that of the existing wheelchair materials.Thus, from the point of view of mechanical properties, Ti–4.5Al–3V–2Mo–2Fe has high potential to be used for healthcare applications.