Numerical simulation of metal powder die compaction with special consideration of cracking

Abstract

Powder die compaction is modelled using the finite element method and a phenomenological material model. The Drucker–Prager cap model is modified with the goal to describe the formation of cracks during powder transfer, compaction, unloading, and ejection of the parts from the die. This is achieved by considering the cohesive strength and the cohesion slope, which characterise the current strength of the powder compact in the Drucker–Prager model, as state dependent variables. Evolution equations are formulated for these variables, so that the strength increases by densification and decreases by forced shear deformation. Some of the parameters appearing in the evolution equations are determined from measured green strength values. An iron based powder (Distaloy AE) is used for the experiments. Examples are shown to demonstrate that the density distribution can be calculated accurately as compared with an experiment, that cracking can be modelled at least qualitatively correctly, and that the compaction of complex 3D parts can be simulated.     

Effect of carbon content,sintering temperature,density, and cooling rate upon properties of prealloyed Fe–1·5Mo powder

Abstract

Mixtures of prealloyed Fe–1·5Mo (Astaloy Mo) with and without additions of 0·5–1·2 wt-%C were prepared and their sintering, as well as their mechanical, properties investigated under different process conditions. It was found that carbon content, sintering temperature, and cooling rate had marked effect on physical and mechanical properties. Sintered density decreased with increase in carbon content and sintering temperature. On the other hand, UTS, TRS, and hardness values improved with up to 0·8 wt-%C addition, sintering temperature, and cooling rate. Percentage elongation decreased with increase in carbon content and cooling rate, but was higher for specimens sintered at higher temperatures. The as sintered microstructures consisted of either fine or coarse pearlite, upper or lower bainite, and their mixture depending on the carbon content and cooling rate. The heat treated mechanical properties showed some improvement for the specimens containing 0·5 and 0·8 wt-%C. It became evident that a variety of ternary low alloy steels consisting of Fe + 1·5Mo + 0·5–0·8 wt-%C can be produced and used in the as sintered or heat treated conditions for PM structural parts having good physical and mechanical properties as well as high dimensional accuracy with acceptable microstructures.     

On phosphorus as additive in iron based soft PM magnets

Abstract

The effect of a phosphorus addition to iron powder for production of soft magnetic materials is investigated. The phase diagrams calculated by ThermoCalc¹ and experiments show the beneficial effect of the phosphorus addition especially when sintering in a carburising atmosphere. The carbon uptake is greatly reduced and is explained by the presence of ferrite in the material during sintering. The phosphorus addition also reduces the magnetic aging both in samples sintered in a carbon containing atmosphere but also in a hydrogen atmosphere. No magnetic aging from precipitation of iron phosphide is seen.     

Impact properties of sintered and wrought 17–4 PH stainless steel

Abstract

Charpy V notch (CVN) impact testing was conducted on full size and subsize specimens of sintered and wrought 17–4 PH stainless steel (17–4 PH SS) in the as sintered and H900 heat treated conditions. Test geometries correspond to the American Society for Testing and Materials (ASTM) and Metal Powder Industries Federation (MPIF) impact testing standards. Merits of a notched specimen compared with an unnotched specimen were analysed for both the wrought and sintered materials. The notched ASTM standard bars had a lower coefficient of variance for impact energy than the unnotched MPIF standard bars and displayed greater toughness. Porosity and grain size have a detrimental synergistic effect on impact toughness for the sintered material. Following a discussion about the differences in the wrought and sintered microstructures, it is recommended that impact testing of the injection moulded and sintered specimens should be evaluated according to the ASTM test specifications.     

Compaction and sintering behaviour of A356–fly ash composites: a preliminary investigation

Abstract

In the present study, A356–fly ash metal matrix composites were developed through powder metallurgy route. The composites were mixed by using the ball milling technique, shaped through uniaxial and cold isostatic compaction, and then sintered at 520°C. Scanning electron microscopy and X-ray diffraction were used for microstructure and phase characterisation. The density and microhardness of the composites were evaluated as a function of fly ash content, compaction pressure, sintering time and age hardening time. Uniaxial cold compaction of the composites increased their green density and cold isostatic compaction of the compacts led to a further increase in the density. At a constant compaction pressure, the density decreased with increasing fly ash content, resulting in light weight composites. The microhardness of the composites increased with the addition of 10 wt-% fly ash while it decreased with the addition of 20 and 30 wt-% fly ash. Sintering at 520°C increased the density of the composites and the grain size of the α-Al phase of the matrix. The matrix alloy and the composite containing 10 wt-% fly ash showed some response to age hardening at 160°C. However, no response to age hardening was observed at 200°C.     

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.     

Hipping after sintering of titanium diboride cermets

Abstract

A new fabrication route, an alternative to glass encapsulated hipping (GEHIP), has been developed to produce dense TiB₂ cermets. Key points of this technique, based on hipping after vacuum sintering (VS + HIP), are the use of Ni₃ (Al,Ti) as binder phase and the selection of the proper amount of additions. The main advantage of VS + HIP with respect to GEHIP is the simplification of the sintering procedure which avoids the glass encapsulation step that makes it more adaptable for industrial use. Successful application of VS + HIP requires a minimum binder content about 10 vol.-% below which a significant hardness reduction is observed owing to the presence of residual porosity as compared with GEHIP. The materials produced by this technique combine low density and high stiffness with high hardness and toughness values, thus giving a set of properties especially attractive for applications where inertial loads are responsible for failure.     

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.     

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.     

Experimental investigation of yield behaviour of metal powder compacts

Abstract

In this paper the compaction and yield response of two steel and two copper powders are examined. These were chosen to determine how the material response depends on the type of material and the morphology of the powder particles. Experiments were conducted in a computer controlled triaxial cell. Here, concentration is on the response during simulated, frictionless closed die compaction, whereby the radial stress is controlled so as to keep the radius of the sample constant. The compaction process was stopped at regular intervals and a series of probing paths were followed in stress space to construct the yield surface for the compact.

The experimentally determined yield surfaces are compared with yield surfaces predicted by empirical models and micromechanical models of the Fleck type, which assume that the compact consists of monosized spherical particles. During the early stages of compaction the form of the yield surfaces for spherical powders are consistent with Fleck's micromechanical model, but the surfaces become less elongated in the direction of loading at high densities. The yield surfaces for irregular shaped powders are significantly different from the predictions of the Fleck micromechanical model. A modified anisotropic Cam-Clay model is proposed, which is able to predict yield surfaces for the four powders at all densification levels.