The mechanical properties of components made by PM steels are normally inferior to those made by alternative processes. One of the main reasons is that a large amount of pores are present in sintered components. The other main reason is that the alloying elements, particularly Ni, are not uniformly distributed after conventional sintering procedures.  This work is aimed at a better understanding of the influence of alloying additions on mechanical properties and homogeneity of the microstructure. The experimental work has been carried out in two trials. Trial 1 was performed to investigate mechanical properties of Distaloy powders (commercial grades) and second trial to examine influence of alloying additions on homogeneity of microstructure.  For trial 1, as-sintered and heat treated specimens were produced by mixing commercial powders with two different carbon levels. Whereas, alloying elements were admixed to base iron powder for producing  sintered specimens for trial 2. Mechanical properties including dimensional changes, micro-hardness, tensile strength and impact resistance were measured. Distribution of alloying elements was studied using LOM and SEM-EDS analysis. The results obtained show that additions of alloying elements enhance the mechanical properties. Moreover, interaction of C with Cu and Ni as well as interaction between Cu and Ni have a deceive role in determining final properties of the components. The metallographic investigation indicated that major reasons of heterogeneous microstructure are slow diffusion of Ni in Fe matrix and interaction of other alloying elements with Ni.  The results of trial 2 showed that addition of Mo and Cu to Ni-containing PM steels improves the distribution of Ni in Fe matrix. Mo results in improved uniformity of microstructure by lowering the chemical potential of carbon. In Ni and Cu containing alloys, the interaction between Ni and Cu is responsible for enhanced distribution of Ni. However, the improved Ni distribution is achieved at the expense of non-uniform distribution of Cu. In Ni-containing PM steels, improved microstructure homogenization can be attained by increasing Ni-Cu interaction, lowering the surface energy of Ni-Cu liquid and decreasing the chemical potential of carbon.

No description available.

In the foundation of this work is the issue for production of non-magnetic materials with predetermined weak magnetic properties. Preferable solution is to dope these materials with ferromagnetic impurities in low concentrations. One of the efficient ways to obtain such materials is the use of powder metallurgy which gives an opportunity to produce complex powder compositions with variety of components in precisely specified concentrations.

The powder metallurgy (PM) process route is very competitive for mass production of structural steel components with complex shape, due to efficient material utilisation, low energy consumption, and short overall production time. The most commonly used alloying elements are the processing friendly metals Cu, Ni and Mo. However, the prices for these metals are today high and volatile, which threatens to make the PM process less competitive compared to conventional metal forming processes. Consequently, there is a strong desire in the PM industry to increase the use of less costly alloying elements. Cr is an attractive alternative since it, besides low cost, provides high hardenability and also recyclable components. The drawback is that Cr has high affinity for oxygen, which makes oxidation and oxide reduction in PM processing of Cr-alloyed materials a challenging issue. Furthermore, the interaction between nitrogen and Cr-alloyed powder during processing is important to consider, since Cr also has high nitrogen affinity and is prone to form nitrides. The aim of the research work presented in this thesis was to study oxide reduction and nitrogen uptake in sintering of Cr-alloyed steel powder. Water-atomized powder grades pre-alloyed with 1.5-3% Cr were used as test materials. Sintering experiments were performed in N2/H2 (90/10) atmospheres with test bars pressed to density 7.0-7.2 g/cm3. The oxygen content of the sintering atmosphere was varied and different sintering temperatures and cooling rates were applied. The experimental study has been complemented with thermodynamic calculations using the software Thermo-Calc. The oxygen partial pressure should be below 4 x 10-18 atm in order to have reducing conditions during sintering at 1120°C of steel powder pre-alloyed with 3% Cr. With graphite added to the powder, conditions are reducing at higher oxygen partial pressures (up to 10-16 atm) due to favourable conditions locally in the material. Sintering at 1120°C for 30 minutes leads to incomplete reduction of Cr-oxides in the Cr-alloyed PM grades, but remaining oxides are not detrimental for mechanical properties of the PM components. Increased sintering temperature is beneficial for the oxide reduction kinetics and practically all oxides are reduced after sintering for 30 minutes above 1200°C. Nitrogen uptake by Cr-alloyed steel powder from N2-based sintering atmospheres is strongly dependent on the cooling rate applied after sintering. No nitrides appear in the sintered material and mechanical properties are not affected when normal cooling rates (0.5-1°C/s) are applied. Very low cooling rates (such as 0.05°C/s) may lead to grain boundary precipitation of Cr-nitrides in the sintered material. / QC 20101105

M. Tech. (Department of Metallurgical Engineering, Faculty of Engineering and Technology), Vaal University of Technology. / Elemental titanium (Ti), niobium (Nb), zirconium (Zr), and iron (Fe) powders were used to fabricate four near-β alloys with non-toxic of composition Ti-34Nb-25Zr, Ti-34Nb-25Zr-0.4Fe, Ti-34Nb-25Zr-1.2Fe, and Ti-34Nb-25Zr-2Fe (wt. %) (TNZ and TNZF) using spark plasma sintering (SPS) of nano-crystalline powders attained by high energy ball milling. The fabricated alloys were compared to Ti-34Nb-25Zr (used as a benchmark alloy in this study) and comparison was made with the commercially used Ti base alloys produced either by conventional methods or powder metallurgy. The powder mixtures were milled for 5 hours using a Simoloyer high energy ball mill with a ball to powder ratio of 10:1 and a rotational speed of 1000 rpm. This was followed by sintering the mechanically alloyed powders at 1100 ºC for 10 minutes with a pressure of 50 MPa and a heating rate of 100 ºC/min using an H-HP D25 spark plasma sintering furnace (FCT System, Germany). The powders were characterised for particle size and crystal structure using SEM and XRD. The consolidated components were characterised with regards to density, microstructure, mechanical properties. The electrochemical behaviour of the alloys was investigated using a Digi Ivy DY2300 series potentiostat. Three corrosion medium, Sodium chloride (NaCl), phosphate buffered saline solution (PBS) and Dulbecco’s modified eagle’s medium that mimic the conditions in the human body were used. Mouse myoblast cell line (C2C12) was used to investigate the biocompatibility of the sintered alloys in 1010x5 mm specimens using standard colorimetric assay MTT. Both electrochemical and biocompatibility test were conducted in triplicates and the results compared with that of the benchmark. Results of mechanical alloying of powder mixtures demonstrated an inhomogeneous structure. Milling for 5 hours resulted in agglomeration of small Fe and Zr particles. Milling for 3 hours resulted in a better distribution of elements compared to longer milling times. Therefore, sintering powders milled for 3 hours would have yielded better results. The densification results were acceptable and ranged between 97-99% of theoretical densities. Although some porosity was observed, especially on the un-etched microstructure. An insignificant decrease in density was observed when 1.2 (wt. %) Fe was added. The sintered samples had microstructures which were not homogenous. However, the addition of Fe yielded a more homogeneous microstructure compared to the one with less Fe. Therefore, TNZF with 2 (wt. %) Fe had a more homogenous microstructure. Sintering at 1100 ºC resulted in undissolved niobium and titanium which were observed in the microstructure as dark and white areas. The hardness of the TNZF alloys were comparable and lied between 373 and 432 Hv. These hardness values are higher than other similar titanium-based alloys fabricated using conventional methods. The addition of Fe to TNZ showed an insignificant decrease in hardness. The addition of Fe was found to decrease the Young’s Modulus of TNZ from 119.1 to 80 GPa with an addition of 2 wt.% Fe. However, an unacceptable reduction (230.91 to 158.2 MPa) in strength was also noticed. Pseudo passivation was observed when the alloys were immersed in 0.9 % Sodium Chloride (NaCl) which could be attributed to the inhomogeneity in the microstructure. The possibility of pitting corrosion was also observed. The alloy containing 2 Fe (wt.%) was found to be more corrosion resistant than the other alloys. The TNZF alloys exhibited better corrosion resistance in 0. 9% NaCl compared to phosphate buffered solution (PBS) and DMEM. The corrosion behaviour in PBS and DMEM cannot clearly be explained from the graphs. The morphology of the corroded samples was almost the same for all the alloys in different corrosion media. The microstructures showed pits which could have been from the pores that acted as initiation sites for pitting. In cell culture for 1 and 7 days, the cell viability for TNZF alloys was greater than that of the control group (TNZ). A significant decrease in cell viability for TNZF was observed in cell culture for 4 days. The addition of Fe on TNZ do not cause toxic effects and show good cell adhesion, indicating in-vitro cytocompatibility. The greatest cell viability of 102±3.0 % for Ti-34Nb-25Zr-2Fe. The analysis of cell morphology indicated good cell-substrate interaction. The TNZF alloys developed in this study can be suitable candidates for orthopaedic implant application due to their low Young’s modulus, corrosion resistance and superior biocompatibility. However, the strength needs significant improvement. The advantage of this biomaterial, when compared to commercial alloys, is the absence of cytotoxicity elements such as Al and V.

The purpose of this bachelor thesis is to design and optimize a powder metal gear through FEM-analyzes. The moment of inertia and weight of the gear shall be reduced at the same time as the demands on tension and bending stiffness are met. The gear that is used as reference and will be optimized is the standard gear in the FZG-rig at the department of Machine Design at KTH. The work is initiated with a literary study. After that a CAD-model of the gear and its pinion is created in Solid Edge. Then the reference gear is analyzed in the FEM-program Ansys and control calculations are made according to standards and handbooks. Thereafter different geometry and density variations are made and tested. The tests are then compared to the reference gear. By varying the density of the gear and varying the geometry of the waist of the gear several different optimization proposals could be made. By cutting material off the waist of the gear the weight can be reduced by 6 % with an increase in bending stress of 1 %. If a bigger increase in bending stress is allowed more material can be removed. If the bending stress increase is allowed to be 5 % a decrease in weight and moment of inertia of 14 % is obtained. With an increase in deformation of 5 % a decrease of 11 % in weight and moment of inertia was obtained. The different tested geometries behave relatively equal up to a 3 % decrease in weight with respect to deformation but begins to vary considerably if the weight is further decreased. Removal of material should be symmetric around every gear tooth to avoid transmission failure which also leads to increased noise. The results show that it is better to remove material under each gear tooth rather than under the root. It is also advantageous to remove material close to the flange. Further work is required to analyze and optimize the gears even more. Fatigue tests as well as different load cases should be analyzed. / Syftet med kanditatexamensarbetet är att ta fram och egenskapsoptimera ett pulvermetallurgiskt kugghjul med hjälp av FEM-analyser. Tröghetsmomentet och vikten på kugghjulet ska minimeras samtidigt som kraven på spänningar och böjstyvhet uppfylls. Det kugghjul som används som referens och ska optimeras är standardkugghjulet i FZG-riggen på institutionen för maskinkonstruktion på KTH. Arbetet inleds med att en litteraturstudie görs. Sedan tas en CAD-modell för kugghjulet och dess tillhörande drev fram i Solid Edge. Därefter analyseras referenskugghjulet i FEM-programmet Ansys och kontrollberäkningar görs enligt standarder och handböcker. Därefter konstrueras och analyseras olika geometrier och densitetsvariationer och jämförs med referenskugghjulet. Genom att variera densiteten på kugghjulet och variera geometrin på kugghjulets liv kunde flertalet optimeringsförslag tas fram. Genom att göra en utskärning i livet kan vikten minskas med 6 % utan att böjspänningen påverkas mer än 1 %. Om en större ökning i böjspänning tillåts kan ytterligare material avlägsnas. Om böjspänningsökningen tillåts vara ca 5 % kan en viktminskning och tröghetsmomentsminskning på ca 14 % åstadkommas. Vid en deformationsökning på 5 % erhölls en viktminskning och tröghetsmomentsminskning på ca 11 %. De olika geometrier som testas beter sig relativt lika upp till 3 % viktminskning med avseende på deformation och börjar därefter variera kraftigt. Borttagning av material måste ske symmetriskt kring varje kuggtand för att transmissionsfel och därmed ökat buller ska undvikas. Det visar sig att ta bort material under kuggen är bättre än att ta bort material under kuggroten. Det gynnsamt att göra geometriska förändringar närmast flänsen på kugghjulet. Vidare arbete krävs för att analysera och optimera kugghjulen ytterligare. Utmattningstester och fler lastfall bör analyseras.

The KTH Department of Materials Science and Engineering has lacked powder metallurgy research for many years, and as this field is constantly gaining in importance, such research needs to be reestablished. This requires the department to be able to accurately and efficiently characterize the properties of a powder, such as size distribution and composition, and in the short term, this needs to be done using non-specialized equipment. This project aimed to assess the availability and usefulness of both traditional and novel characterization methods by way of trial characterization experiments as well as a literature review.   The experiments resulted in some data about three sample powders, as well as the conclusions that size distribution could be effectively characterized by automatized image analysis, composition could be characterized using Energy Dispersive X-ray Spectroscopy and that sample preparation was key to good results. It was concluded that the department could conceivably evaluate the most important properties, but that sampling and sample preparation routines need to be established to ensure efficient characterization and representative data. / Materialinstitutionen på Kungliga Tekniska högskolan har i många år helt saknat pulvermetallurgiforskning och eftersom detta fält ständigt blir mer relevant behöver denna forskning återetableras. Detta kräver att institutionen med tillräcklig noggrannhet och effektivitet kan bestämma egenskaper hos ett pulver, såsom storleksfördelning och sammansättning, och på kort sikt behöver detta ske med ickespecialiserad utrustning. I detta projekt har tillgänglighet och användbarhet hos både traditionella och innovativa analysmetoder utvärderats med hjälp av experimentell karaktärisering av pulver samt en litteraturstudie. Experimenten gav data om de tre undersökta pulvren och resulterade även i slutsatsen att storleksfördelning kunde bestämmas med automatiserad bildanalys, att samansättning kunde bestämmas med Energi Dispersiv Röntgen Spektroskopi och att provpreparering äravgörande för bra resultat. Av detta följer att institutionen rimligtvis kan bestämma de viktigaste egenskaperna hos ett pulver, men att rutiner för provtagning och provpreparering behöver etableras för att säkerställa effektiv analys och representativ data.

No / A comparison has been made between compaction, sintering, microstructural and mechanical properties of the 6061 aluminium alloy prepared via premixed elemental (EL) and prealloyed (PA) powders (as received and degassed) with and without additions of sintering aids and various solid and/or liquid lubricants. Both EL and PA powders were cold pressed at different pressures, ranging from 250 to 770 MPa, and sintered under vacuum in the range 580-640°C for 30-120 min. and then under pure nitrogen atmosphere for comparison. Vacuum degassing of the PA powder provided better compressibility and thus higher green densities than those for the as received PA or the premixed EL powder compacts pressed at compaction pressures ¿340 MPa. Near full sintered densities of ~98%TD were obtained for both EL and PA 6061 Al alloys. Degassed PA Al with 0·6 wt-% paraffin wax (PW) or with only 0·12 wt-%Pb addition as sintering aid and no lubricant, and premixed EL with only 0·12 wt-%Pb addition and no lubricant gave the best optimum properties. It became apparent that additions of some solid lubricants such as lithium stearate (LS) and acrawax to both the premixed EL and PA powders provided reasonable green densities, but had deleterious effect on sintered densities and microstructures, particularly under vacuum sintering. Heating data curves during the sintering cycle, revealed formation of both transient and persistent liquid phases for the EL and mainly supersolidus liquid phase sintering (SLPS) mechanism for the PA. Tensile properties of the degassed, vacuum or nitrogen sintered PA Al alloy in T6 condition were higher than those of the equivalent alloy prepared by EL mixing with the former giving a tensile strength of 330 MPa and 6-8% elongation to failure, which are similar to those of the commercial (wrought) 6061 Al alloys.

The historical success of friction stir welding (FSW) on materials such as aluminum and magnesium alloys is associated with the absence of melting and solidification during the solid-state process. However, commercial adoption of FSW on steels and other non-ferrous high-strength, high-temperature materials such as nickel-base and titanium-base alloys is limited due to the high costs associated with the process. In this dissertation, the feasibility of using an FSW approach to fabricate certain structural components made of nitrogen containing austenitic stainless steels that go into the vacuum vessel and magnetic systems of tokamak devices was demonstrated. The FSW weldments possessed superior application-specific mechanical and functional properties when compared to fusion weldments reported in the technical literature. However, as stated earlier, the industrial adoption of FSW on high temperature materials such as the ferrous alloys used in the present study is greatly limited due to the high costs associated with the process. The cost is mainly dictated by the high temperature FSW tools used to accomplish the weldments. Commercially available high temperature FSW tools are exorbitantly priced and often have short lifetimes. To overcome the high-cost barrier, we have explored the use of integrated computational materials engineering (ICME) combined with experimental prototyping validation to design next-generation tool materials with high performance and relatively low cost. Cermet compositions with either tungsten carbide or niobium carbide as the hard phase bonded by high entropy alloy binders were processed via mechanical alloying and spark plasma sintering. The feasibility and effectiveness of the newly developed cermet tool materials as potential next generation high temperature FSW tool materials was evaluated.

A systematic and scientific approach has been applied to the study of codeformation processing. A series of composites having mechanically-dissimilar phases were developed in which the high temperature flow behavior of the reinforcement material could be varied independent of the matrix. This was accomplished through the use of a series of intermetallic matrix composites (IMCs) as discontinuous reinforcements in an otherwise conventional metal matrix composite. The IMCs are produced using an in-situ reaction synthesis technique, called the XD™ process. The temperature of the exothermic synthesis reaction, called the adiabatic temperature, has been calculated and shown to increase with increasing volume percentage of TiB2 reinforcement. Further, this temperature has been shown to effect the size and spacing of the TiB2, microstructural features which are often used in discontinuous composite strength models. Study of the high temperature flow behavior of the components of the metal/IMC composite is critical to the development of an understanding of codeformation. A series of compression tests performed at 1000° to 1200°C and strain-rates of 10-3 and 10-4 sec-1. Peak flow stresses were used to evaluate the influence of material properties and process conditions. These data were incorporated into phenomenologically-based constitutive equations that have been used to predict the flow behavior. It has been determined that plastic deformation of the IMCs occurs readily, and is largely TiB2 independent, at temperatures approaching the melting point of the intermetallic matrices. / Ph. D.