By performing a dilatometer experiment, measured shrinkage can be used to determine the adjustable parameters in a constitutive model for sintering. While the dilatometer machine is excellent at collecting data, its conditions differ from those in an industrial sintering oven. Therefore, the constitutive model must be robust and in the present study, different sintering time cycles have been used to optimize the adjustable parameters in the model. Investigations on initial density has also been performed to better understand the sintering process. Before sintering of particles begins, during the debinding process, densification can be detected, which is dependent on the initial density. The constitutive model for sintering was improved to include this phenomenon by adding particle rearrangement based on the theoretical packing of spheres.

During sintering, a green body of powder particles is heated to high temperatures, fusing the particles together. In cemented carbide production, the sintering process generally results in substantial densification of the material. By using a dilatometer, shrinkage during the sintering process can be measured. For a green body of lower density, early particle rearrangement has been observed. This is investigated here using different initial densities using the same powder, leading to a suggested addition to the constitutive model. The environment in the dilatometer and the sintering furnace differs, especially with respect to heating and temperature during holding. This effect can be minimized by creating robustness in the model, making it independent of the heating cycle. Here, this is done by optimizing the constitutive parameters towards four heating cycles for a specific powder.

Finite element (FE) simulations are frequently used nowadays in order to analyse powder compaction and sintering, for example, when determining the shape of a cutting blank insert. Such analyses also make it possible to determine in detail the stress state in a powder compact during loading and unloading. This is certainly important as (residual) tensile stresses can lead to cracking, either after unloading or during the subsequent sintering step. The magnitude of plastic deformation is also an issue here. Concerning the stress state in the powder compact, the frictional behaviour (between walls and powder compact) is of great importance. For this reason, in the present study, two frictional models are implemented into a commercial FE software, and numerical results based on the stress state before and during unloading are derived. The two friction models produce quite different results, and it is obvious that the frictional behaviour at powder pressing has to be carefully accounted for in order to achieve results of high accuracy.

The densification of cemented carbides during sintering was studied using an existing constitutive model based on powder particle size and material composition. In the present analysis, we study how well the constitutive model can capture the experimental results of a dilatometer test. Three experiments were performed, where the only difference was the transition between the debinding and sintering process. From magnetic measurements, it is concluded that the carbon level in the specimen is affected by changes to the experimental setup. It is shown, using parameter adjustments, that the constitutive model is more suited for a certain experimental setup and carbon level, which is a limitation of the model. In order to capture the mechanical behaviour under different experimental conditions, further constitutive modelling relevant to the carbon level is recommended.

During unloading, pressed hard metal powder compacts expand (spring back), leading to unwarranted tensile stresses and, subsequently, crack initiation in the green body. Here, the elastic spring back and the green strength are analyzed for different types and amounts of the pressing agent PEG (polyethylene glycol). The results show that the plastic behavior, but not the elastic behavior, is influenced by a change of the pressing agent PEG. In this context, it should be stated that the risk for the initiation of cracks is influenced by both the elastic and plastic behavior after compaction. The spring back after compaction and the green strength defines to a large extent the risk for cracks. In addition, it is concluded that a standard three-point bending test is sufficient to analyze the risk for the initiation of cracks when comparing different spray-dried powders.

The correlation between granule strength and green strength of hard metal powders is examined. The approach is based on experiments and numerics. In the latter case, a Design of Experiment software is used. The granule strength of the powder (particle) is determined by GFP-measurements (“Granularfestigkeits-Prüfsystem”). During this test, a single particle is pressed from one side until breakage. The corresponding measurements of the green strength are done using three-point bend (3PB) testing. The experimental results show that the pressing agent has a strong influence on the behavior of both quantities. The statistical evaluation shows that the relation between the two strength properties is very close to linear with coefficient of determination R2 taking on the value 0.97. This of course indicates that it is possible to get information about one of the properties for a similar set of materials by experimentally determining the other one. This is of substantial practical importance as for one thing it can limit the amount of testing required. Even though the present investigation is pertinent to hard metal powders, the results could be of value for many other types of powder materials. 

It is known that the shape of a cutting insert blank, after pressing and sintering, can be predicted with finite element (FE) simulations. It is also known that such simulations have the potential to save costs and time when used for press tool compensation. However, in such simulations (and in a real situation), the frictional behaviour has a great influence on the results. Therefore, in this study, two frictional models are discussed and implemented into a commercial FE-program. The results from the different frictional descriptions, when for instance analysing density after compaction, shows a clear difference. It can be concluded that the frictional behaviour at powder pressing has to be modelled in detail, also at low forces.

Material parameter curves in an advanced material model describing compaction of spraydried cemented carbide powder are determined successfully based on a general approach formaterial characterization of powder materials. Pressing forces from a production machineand equivalent finite element (FE) calculations are used in inverse modelling. A pressingmethod that includes multiple unloading steps is used. The material model is of DruckerPrager CAP kind and friction between powder and pressing tool is modelled as a function ofnormal pressure. The results are verified with density gradient measurements using aneutron source. The method is proven to be robust and the results show good agreementbetween experiment and simulation. Effects that have not been captured numericallypreviously are captured due to the high accuracy of material characterization. The presentapproach is detailed for tungsten carbide powders but is valid for other powder materialswhen properly calibrated for constitutive and frictional effects in the same manner asoutlined here.

Cutting tool inserts, for instance used in steel machining, have the requirement to be toughand are therefore most often manufactured out of cemented carbides, using powdermetallurgy. Manufacturing components with powder metallurgy has its advantages in highproductivity and good net shape. The powder is spray dried and compacted to half itssintered volume. Because of friction between the powder and the pressing tool, the densityafter compaction is uneven, leading to uneven shrinkage during sintering. To get the rightshape after pressing and sintering, the pressing tool must often be compensated, which isboth expensive and time consuming. By doing computer simulations of the manufacturingprocess, the shape after sintering can be predicted and used to compensate the pressing toolbefore it is manufactured, thus saving both time and money. Also cracks and porosity in thepowder blank can be predicted with such simulations.

This thesis studies mechanical modelling of powder compaction in general and compactionof cemented tungsten carbide powders in particular. Because of the amount of powdergranules in a typical geometry, the mechanical behavior is modelled with a continuumapproach, using the finite element method (FEM). Accuracy is important in the presentapplication and therefore a detailed elastic-plastic material model with a density dependentyield surface of Drucker-Prager CAP kind is used.

For accurate material modelling it is important to include relevant features and to excludeunimportant physical effects. In Paper A sensitivity studies are therefore performed inorder to conclude which properties in the material model that have a significant influence onthe result. The studies show that anisotropy can be disregarded in the current application.

In Paper B the effects from creep and compaction speed are studied. It is concluded thatcreep has no influence on the density after compaction, which also is confirmed by densitymeasurements using a neutron source in Paper D. The compaction speed on the other handinfluences the friction coefficient between powder and pressing tool, lower at increasedspeed. In Paper C frictional behavior is scrutinized experimentally with the aid of aninstrumented die. The friction coefficient is determined and analyzed, and it is shown that itdepends on the normal pressure.

The sensitivity studies in Paper A show that measurements of the local density are neededin order to determine and verify material properties. Since the analyzed powder containstungsten (W), which has a high atomic number, a polychromatic beam of thermal neutronsis needed. In Paper D it is shown that the local density can be measured with 3D imagingand a thermal neutron source.

From the results and conclusions in the above-mentioned papers, a material description forpowder compaction is suggested. This description is implemented in FEM in Paper E andapplied to reverse engineering in order to determine important material parameters.Experiments in a pressing machine with a pressing method that includes multiple unloadingsteps is used. The material description with the determined parameters is verified withdensity measurements using a neutron source.

Inom skärande bearbetning av exempelvis stål, där skärspetsen måste vara hård ochhållfast, används oftast hårdmetallskär, tillverkade med pulvermetallurgi. Att tillverkakomponenter med pulvermetallurgi har fördelen att hög produktivitet nära den slutgiltigaformen kan uppnås. Pulvret sprejtorkas och kompakteras till halva den sintrade volymen.Eftersom det uppstår friktion mellan pulver och pressverktyg är densiteten i det pressadeämnet ojämn och därmed krymper ämnet ojämnt under sintring. För att få rätt form efterpressning och sintring måste därför pressverktyget ofta kompenseras, vilket är både dyrtoch tidskrävande. Genom att göra datorsimuleringar av framställningsprocessen kanformen efter pressning och sintring istället predikteras, och pressverktyget kankompenseras före tillverkning, vilket sparar både tid och pengar. Även sprickor ochporositet efter pressning kan förutsägas med sådana simuleringar.

I denna avhandling studeras mekanisk modellering av pulverkompaktering, generellt ochspecifikt för hårdmetall. På grund av mängden granuler i en typisk skärgeometri modellerasde mekaniska egenskaperna med en kontinuumansats och finita elementmetoden (FEM).Eftersom noggrannhet är viktig i denna applikation, används en detaljerad elastisk-plastiskmaterialmodell med en densitetsberoende Drucker-Prager CAP flytyta.

Grundläggande för relevant materialmodellering är att inkludera viktiga egenskaper och attutelämna oviktiga fysikaliska effekter. I Artikel A görs därför känslighetsanalyser för attundersöka vilka delar i materialmodellen som har en signifikant påverkan på resultatet.Slutsatsen är att anisotropi inte behöver modelleras för denna applikation.

I Artikel B studeras effekten av kompakteringshastighet och kryp. Slutsatsen är att krypinte har någon inverkan på densiteten efter pressning, vilket också valideras medneutronmätningar i Artikel D. Presshastigheten påverkar däremot friktionskoefficientenmellan pulver och pressverktyg, lägre vid högre hastighet. I Artikel C analyserasfriktionsbeteendet experimentellt med hjälp av en instrumenterad dyna.Friktionskoefficienten bestäms och analyseras, och slutsatsen är att den beror pånormaltrycket.

Känslighetsanalysen i Artikel A visar att mätningar av den lokala densiteten är nödvändigaför att bestämma och verifiera materialegenskaper. Eftersom det analyserade pulvretinnehåller wolfram (W), som har ett högt atomnummer, krävs en polykromatisk stråle avtermiska neutroner. I Artikel D visas att den lokala densiteten kan mätas med 3D-bildanalysoch termiska neutroner.

Utifrån resultaten och slutsatserna i ovannämnda artiklar föreslås en materialbeskrivningför pulverkompaktering. Beskrivningen är implementerad i FEM i Artikel E och användsmed baklängesoptimering för att bestämma viktiga materialparametrar. Experiment i enpressmaskin och en pressmetod som inkluderar flera avlastningar används.Materialbeskrivningen verifieras med densitetsmätningar där en neutronkälla används.