Development of ultrasonic sensor for non-intrusive measurement of temparture in powder materials

Abstract

Fine and ultrafine powders are often used as starting materials for fabrication of advanced materials; they are first formed into desired shapes and then consolidated into high density parts by application of pressure and heat. Electroconsolidation is a densification method for rapid pressure assisted densification of complex shaped parts made from powder preforms. The part to be densified is immersed in a bed of free flowing, electrically conducting, graphite powder medium within a cylindrical die chamber. Pressure is applied externally and heat is generated internally by resistively heating the graphite powder. Because of rapid heating and the attendant wide temperature fluctuations possible with resistive heating, it is of paramount importance to measure and control the temperature in the die to achieve reproducible densification properties. This paper describes an ultrasonic pitch-catch sensor that can be used to non-intrusively measure the temperature of a graphite powder bed. Results of ultrasonic data for heating trials conducted at up to ≈3000°C indicate that the average temperature in a graphite powder bed can be predicted to within 2-3%. The ultrasonic sensor can be used to precisely control the heating trajectory and densification of parts with reproducible properties.     

Investigation of Fe2O3–Al and Cr2O3–Al reactions using a high speed video camera

Abstract

Self propagating high temperature synthesis is a simple, fast and energy efficient process with a wide range of applications, one of which is the coating of the internal surfaces of steel pipes using a centrifugal thermit process. The process involves a highly exothermic reaction between powder reactants distributed around a steel tube rotating at high speed. Although the process has been widely studied, important features, particularly how the reaction propagates, have not been completely revealed due to extremely high reaction rates and temperatures. In the present work, Fe₂O₃–Al and, to a lesser degree, Cr₂O₃–Al reactions were studied under stationary (non-rotating) and rotating conditions using a high speed video camera by which the centrifugal thermit process was, for the first time, recorded optically. Video recordings clearly demonstrate that, in contradiction to current belief, the reaction does not always propagate in a well ordered (spiral) pattern, but involves multiple, randomly distributed ignition sites.     

Reaction sintering and joining nickel aluminide to AISI 316 stainless steel by self-propagating high temperature synthesis

Abstract

Self-propagating high temperature synthesis (SHS) is a process whereby reactants are ignited to spontaneously transform to products in an exothermic reaction. The aim of this study is to propose a method to join nickel aluminide with AISI 316 stainless steel by SHS and to study the combustion synthesis of nickel aluminide. From the heat of combustion synthesis junctions were formed between annular AISI 316 stainless steel and a powder metallurgy compact of Ni and Al blends. The Al mole ratio for testing the joining grade in the initial powder mixture varied from 25 at.-% to 40 at.-%. In order to check the sufficiency of the SHS reaction, the test temperature was compared with the thermodynamic calculation values. The metallographic analysis indicated that NiAl and Ni₃Al were formed in the joint layer.     

Metal flow behaviour of wide flat bar in the spreading extrusion process

Abstract

To examine metal flow behaviour experimentally in the spreading extrusion process, a round billet is spread by means of a spread ring and extruded through a die opening wider than the internal diameter of the container. In spreading extrusion, the extrusion load is reduced by about 30% in comparison with basic extrusion. However, the metal flow balance in spreading extrusion worsens, because of the high frictional force generated at the interface of the billet and spread ring during the filling process. When the profile is wide or when spread ring height is low, the geometry of the plastic deformation zone on the die face changes from an ellipse into a figure of eight at the wide zone at both ends of the die opening. As a result, the metal flow balance markedly worsens, due to an increase in the size of the plastic deformation zone and frictional resistance acting on the spread ring wall.     

Aluminium powder particles produced by SAMD technique: typical characteristics and correlations between processing conditions and powder size

Abstract

The present paper consists of two parts. In the first part the principles of a new method of metal powder production, termed 'solid assisted melt disintegration (SAMD)' are discussed and the typical characteristics of the produced powder are outlined. In the second part the effects of some processing parameters on the size distribution and mean diameter of the powder are reported. The SAMD method involves mixing solid particles (i.e. alumina) with the liquid aluminium alloy aided by mechanical agitation. The shear force induced by the impeller is transferred to the metal via the non-wetting solid medium and results in melt disintegration. The resulting mixture of aluminium droplets and alumina particles are subsequently cooled in air and screened through 300 μm sieve to separate alumina from solidified aluminium powder particles. The SAMD technique has demonstrated the capability to produce a wide particle size distribution. The small sized particles (i.e. <53 μm) exhibited irregular shapes, but larger ones were mostly spherical. These powder particles were dense (pore free) without attached satellite particles and exhibited a relatively coarse microstructure. The processing parameters investigated include the size of Al₂O₃ particles, Al₂O₃/Al weight ratio, stirring speed and stirring time. It was concluded that there exists an optimum value for each of the aforementioned parameters corresponding to a minimum in the mean particle size.     

Variability of powder characteristics and their effect on dimensional variability of powder injection moulded components

Abstract

In this study, the effect of powder characteristics and their variability on the dimensional variability of green and sintered PIM components has been examined for 316L stainless steel. Three lots of gas atomised and three lots of water atomised powders were characterised and used to make six batches of PIM compound. These compound lots were injection moulded using a cavity pressure transducer and screw position regulation controls. The moulded geometry was measured in the green state and sintered state for dimensional variability. The general findings are that gas atomised powder produce less dimensional variability than the water atomised powder from lot to lot, however, the water atomised powders produce less in lot dimensional variability and are generally less susceptible to distortion of cantilevered members during sintering. Also, the lot to lot variation in the powder characteristics, such as particle size and pycnometer density, have an effect on dimensional stability whereas variations in powder characteristics such as surface area, tap and apparent density, and chemistry have little effect on dimensional stability.     

Investigation of deformation and heat treatment effects on properties of PM 92.5W–5Ni–2.5Fe heavy alloys

Abstract

This paper presents the effects of vacuum heat treatment under different cooling conditions on mechanical and structural properties of forged heavy alloys, such as 92.5W–5Ni–2.5Fe and 92.5W–5Ni–25Fe microalloyed with Co. The tungsten composition in the c phase has proved to be higher and more homogenous in the rapidly cooled alloys than in the slowly cooled ones. The effects of chemical composition inhomogeneity on mechanical and structural properties of alloys were also analysed and discussed. The results of tensile and toughness testing have shown an increase in ductility and toughness, while the strength of heat treated alloys decreased in comparison with the strength of forged alloys. The fracture analysis has shown that in the sintered and rotary forged alloys, intergranular fracture of the tungsten phase and transgranular fracture of the γ phase occurred, respectively. The fracture of these phases after heat treatment was characterised by transgranular morphology.     

Microstructural evolution an dmechanical properties of Rapidsteel 2.0

Abstract

At present, the research on rapid tooling by the selective laser sintering (SLS) method is mainly focused on the production of parts with high accuracy and definition. Very little effort has been devoted to the microstructural evolution and mechanical properties of this material. This paper gives detailed information about microstructural development and mechanical behaviour of Rapidsteel material after subsequent heat treatment cycles for binder removal, partial sintering, and liquid phase infiltration.

The microstructure of SLS samples heated to 1120°C for 3 h in 30H₂-70N₂ atmosphere at 2 K min^-1 consisted of a mixture of austenite, M₂₃C₆, and Cr₂N phases. Subsequent infiltration of the above sample with bronze at 1050°C for 2 h in 30H₂-70N₂ atmosphere at 2 K min^-1 produced similar phases together with an additional α(Cu-Sn) phase. The mechanical tensile fracture strength of the partially sintered part increased ten times after infiltration with bronze. However, the fracture behaviour is found to be different between the samples subjected to various heat treatment cycles. The sample heated after the first cycle showed fracture along the necks between stainless steel particles, whereas the infiltrated sample showed fracture along the bronze infiltrant.     

Evaluation of drilling parameters effects on machinability of PM materials using ANOVA

Abstract

The aim of this paper is the machinability analysis, in different drilling conditions, of sintered metals filled with a machinability enhancer. The cutting parameters involved in the study are: rotating velocity, feedrate and coating of the tool. To reduce the number of experimental tests required, a 2^k–1 fractional design of experiment with three two level factors and two replications for each run was employed. The experimental design was repeated for three sintered metals with different hardnesses or densities. The obtained results show new insight into the machining mechanism of sintered material and into the influence of the cutting parameters on the machining forces.     

Microstructural modelling of electrical conductivity and mechanical properties of sintered ferrous materials

Abstract

The role of microstructure on mechanical properties of sintered ferrous materials was studied using a method based on electrical conductivity measurement. The method was accompanied by quantitative fractography to evaluate the dewaxing and sintering process in iron compacts. The effects of manufacturing parameters, such as compacting pressure in the range of 150–800 MPa, sintering temperature from 400 to 1300°C, sintering time up to 8 h, and lubrication mode were investigated. Several mathematical models were checked to obtain the best one for prediction of electrical conductivity changes as a function of manufacturing parameters. The mechanical properties of the sintered compacts were also evaluated to establish a relationship between conductivity, total porosity, pore morphology, and mechanical behaviour. The results show that the electrical conductivity/resistivity of sintered materials is closely related to its microstructure, so that measuring these properties can replace destructive test methods for prediction of mechanical strength of sintered materials with homogeneous matrix microstructure. The application of the method is shown for sintered Fe, Fe–0·8%C, and Fe–1·5%Mo–0·7%C compacts.