Supersolidus Liquid Phase Sintering Research Articles

Effect of microwave and conventional heating on sintering behavior and properties of Al–Mg–Si–Cu alloy

Effect of heating mode and sintering temperature on sinterability and properties of the 6711 (Al–1Mg–0.8Si–0.25Cu) alloy was investigated. Alloy compacts were consolidated in conventional and microwave furnace at 570, 590, 610 and 630 °C under vacuum (10 −6 torr). Microwaves coupled with the compacts and resulted in ∼58% reduction in processing time along with higher heating rates when compared to conventional sintering. However rapid heating rate resulted in inhomogeneous microstructure with larger melt fraction at grain boundaries. XRD analysis showed absence of intermetallics due to insufficient time for diffusion. With increasing sintering temperature, all compacts did undergo supersolidus liquid phase sintering (SLPS), accordingly higher densification and shrinkage was observed. The electrical conductivity and hardness followed similar trend as sintered density in both modes. Alloy compact sintered at 630 °C resulted in significant improvement of mechanical properties (TRS: 57%↑; UTS: 27%↑and ductility: 41%↑) in conventional mode. Age hardening treatment under T6 temper enhanced the tensile strength by 136% with drastic reduction in ductility as compared with sintered compacts.

Mechanical Properties of Powder Injection Molded Ni-Based Superalloys

This paper describes the microstructural and mechanical properties of powder injection moulded (PIM) Ni based superalloys. Ni-based superalloys were mixed with a polymeric binder (paraffin wax, carnauba wax, stearic acid and PP) and injection moulded as standard tensile bars. The critical powder loading for injection moulding were 62.5 vol. % for standard samples. Binder removal, debinding, was performed using a two-step solvent/thermal method. After debinding the samples were sintered at 1100°C and at 1285°C under high level vacuum. Metallographic studies were conducted to reveal the extent of densification and the corresponding microstructural changes. Ni 718, Ni 625 and Nimonic90 samples achieved high sintered densities via supersolidus liquid phase sintering. The mechanical properties of Ni 718, Ni 625 and Nimonic90 are determined and compared with the properties of previously reported studies on similar PIM materials.

The impact of processing on microstructure, single-phase properties and wear resistance of MMCs

Abstract High mechanical loads and abrasive wear are boundary conditions for many tooling materials used in modern economy. One of the standard materials used in high abrasive environments is tool steel, or more specifically cold work tool steel. Wear resistance can be increased by adding hard phases like titanium carbides (TiC) to obtain a particle-reinforced metal–matrix composite (MMC) that can be produced either by hot isostatic pressing (HIP) or super solidus liquid-phase sintering (SLPS). Starting from the same raw materials, these two processes lead to different microstructures. The amount and dispersion of the hard phases as well as their hardness and fracture toughness control the wear resistance. In this study the hardness, Young's modulus and the fracture toughness of titanium carbides in Fe-base MMCs have been investigated by nanoindentation measurements. Owing to the small size of the TiC particles, the matrix properties have a pronounced effect on the results. This effect was studied in dependence on the indentation depth to enable the discussion of both the apparent and the real changes in the properties resulting from the production process and the heat treatment. This contribution also discusses the link between the microstructural properties and the macroscopic wear behavior.

Corrosion properties of a plastic mould steel with special focus on the processing route

AbstractApplications in plastics processing bear increased requirements for the used materials, especially with respect to their corrosion and wear resistance. For this reason, special powder metallurgical tools steels were developed that fulfil these demands. The common processing route for their production is hot isostatic pressing (HIP) of pre‐alloyed powders which is followed by hot working if semi‐finished parts are to be produced. As an alternative to HIP, super solidus liquid phase sintering (SLPS) permits the consolidation of pre‐alloyed tool steel powders to near net‐shape parts. It can be performed in different sintering atmospheres. In this work, the plastic mould steel X190CrVMo20‐4 was processed by SLPS in vacuum as well as under nitrogen atmosphere. The resulting materials were analysed with respect to their microstructure, tempering behaviour and corrosion resistance in 0.5 molar sulphuric acid in dependence of the heat treatment. As a reference, the HIPed and the HIPed and worked state were also investigated. The results show that different heat treatments alter the ranking of the sintered and the HIPed state with respect to corrosion resistance. As expected, a high tempering for maximum secondary hardness causes a significant loss of corrosion resistance. The experimental findings were supported by thermodynamic calculations based on slight alterations in chemical composition that result from the different manufacturing processes.

Preservation of Geometrical Integrity of Supersolidus-Liquid-Phase-Sintered SKD11 Tool Steels Prepared with Powder Injection Molding

The powder injection molded SKD11 tool steels often manifest shape retention problems during supersolidus liquid phase sintering due to the difficulties in controlling the amount of liquid phase. The typical temperature range for the sintering of SKD11 is only 10 K, between 1503 and 1513 K (1230 and 1240 °C), and this narrow sintering range demands a special furnace with very uniform temperature distribution. Through the addition of carbides, in particular TiC, this problem is resolved by enlarging the liquid + γ + carbide region in the phase diagram and by impeding the grain growth with the carbides. The resulting sintering window is broadened to 40 K, between 1513 and 1553 K (1240 and 1280 °C). The relevant mechanisms on the improvement of shape retention are discussed with a focus on the effect of carbide addition on the changes in the phase diagram and the microstructure. A guideline for the selection of effective carbides is also proposed based on the experimental results and the phase diagram analyses.

Master sintering curve concepts applied to full-density supersolidus liquid phase sintering of 316L stainless steel powder

Supersolidus liquid phase sintering (SLPS) is an important liquid phase sintering variant that is used to densify prealloyed powders. The densification behaviour of a boron doped 316L stainless steel during SLPS is treated using a master sintering curve (MSC) concept based on the integral time–temperature work of sintering. With this approach, the MSC analysis is able to identify a fundamental change in the densification rate that is indicative of a mechanistic shift during SLPS.

Comprehensive Investigations of the Supersolidus Liquid-Phase Sintering of Two Plastic Mold Steels

The processing of plastics, particularly reinforced composites, necessitates the use of corrosion- and wear-resistant materials for tools that come into contact with the polymer. For such applications, plastic mold steels were developed that offer not only a good wear resistance due to the presence of carbides in a martensitic matrix, but also good corrosion resistance provided primarily by a sufficient amount of dissolved chromium. The common processing route for these high-alloyed materials is the hot isostatic pressing (HIP) of gas-atomized powders (PM-HIP). In this context, sintering plays an insignificant role, except for the processing of metal-matrix composites (MMCs). The development of novel wear- and corrosion-resistant MMCs based on plastic mold steels requires knowledge of the sintering behavior of prealloyed powders of such tool steels. It is well known that alloyed powders can be processed by supersolidus liquid-phase sintering (SLPS), a method leading to almost full densification and to microstructures without significant coarsening effects. In this work, two different gas-atomized powders of plastic mold steels were investigated by computational thermodynamics, thermal analysis, sintering experiments, and microstructural characterization. The results show that both powders can be sintered to almost full density (1 to 3 pct porosity) by SLPS in a vacuum or a nitrogen atmosphere. Experimental findings on the densification behavior, nitrogen uptake, and carbide volume fractions are in good agreement with calculations performed by computational thermodynamics.

Abrasive wear resistance of starch consolidated and sintered high speed steel

The abrasive wear resistance of starch consolidated (SC) and super solidus liquid phase sintered (SLPS) M3/2 high speed steel (HSS) samples have been evaluated by a two-body micro-abrasion test (low stress abrasion), using 6μm diamond abrasive particles, and a three-body abrasion test (high stress abrasion), using significantly larger abrasive particles of blast furnace slag (600 HV) and silicon carbide (2400 HV), respectively. In the tests a commercial powder metallurgical (PM) HSS was used as a reference material.The results show that the microstructure of the SC and SLPS HSS samples is strongly dependent on the sintering temperature used. With increasing temperature the microstructure ranges from a porous (∼5% porosity) relatively fine grained low temperature sintered microstructure to a fully dense relatively coarse grained high temperature sintered microstructure with eutectic carbides/carbide networks. However, despite the pronounced microstructural differences displayed by the as-sintered HSS microstructures these show a relatively high abrasive wear resistance, comparable with that of a HIPed HSS reference, both under low and high stress abrasion contact conditions. The characteristic features of the low and high temperature sintered microstructures, i.e. the pores and coarse eutectic carbides/carbide networks, only show a limited impact on the wear rate and the wear mode (dominant wear mechanism). The results obtained imply that near net shaped components manufactured by starch consolidation and super solidus liquid phase sintering might be of interest in tribological applications.

Sintered Intermetallic Reinforced 434L Ferritic Stainless Steel Composites

The present study examines the effect of aluminide (Ni3Al, Fe3Al) additions on the sintering behavior of ferritic 434L stainless steels during solid-state sintering (SSS) and supersolidus liquid-phase sintering (SLPS). 434L stainless steel matrix composites containing 5 and 10 wt pct of each aluminide were consolidated at 1200 °C (SSS) and 1400 °C (SLPS). The effects of sintering and aluminide additions on the densification, microstructural evolution, mechanical, tribological, and corrosion behavior of sintered ferritic (434L) stainless steels were investigated. The performances of the 434L-aluminide composites were compared with the straight 434L stainless steels processed at similar conditions. Supersolidus sintering resulted in significant improvement in densification, mechanical, wear, and corrosion resistance in both straight 434L and 434L-aluminide composites. Fe3Al additions to 434L stainless steels result in improved wear resistance without significant degradation of corrosion resistance in 3.56 wt pct NaCl solution.

Wear resistance of in situ MMC produced by supersolidus liquid phase sintering (SLPS)

The resistance to abrasive wear can be effectively raised by hard particles embedded in a metal matrix. Powder metallurgy allows to admix larger particles than those precipitated from the matrix. To save costs ferrotitanium (FeTi) particles are admixed to a tool steel powder and are transformed in situ into titanium carbonitrides during compaction by supersolidus liquid phase sintering (SLPS) instead of hot isostatic pressing. Pin-on-abrasive paper-tests as well as scratch tests reveal an increase in wear resistance of in situ SLPS–metal-matrix-composites (MMC) over hot isostatic pressing (HIP)–MMC, which is linked to the large size of precipitated hard particles in the former. The application of these laboratory results is seen in reinforcing inserts of wear parts.

Nickel base superalloy GH4049 prepared by powder metallurgy

The process of preparing nickel base superalloy GH4049 from atomized prealloyed powder is studied. The results show that the compacts with relative density over 99% can be made by sintering at 1350 °C for 2 h in vacuum. Sintering in vacuum is better than that in argon atmosphere. The sintering way is supersolidus liquid phase sintering. The tensile strength and yield strength and ductility are about 1113 MPa and 760 MPa and 13% respectively. They are near the wrought superalloy GH4049. There are two kinds of γ′ phase precipitated in the grains. The rectangular γ′ phase is about 200 nm and the global γ′ phase is about 40 nm. The average grain size is below 80 μm. Its property of the compression stress relaxation at 850 °C is better than wrought cobalt base superalloy GH605 (L-605). The results lay the foundation of near-net-shape forming for the powder nickel superalloy GH4049.

Starch consolidation of M3/2 high speed steel powder – influence of process parameters on resulting microstructure

The possibility to produce near net shape high speed steel components with an adequate microstructure by the combination of starch consolidation (SC) and super solidus liquid phase sintering (SLPS) has been evaluated using a gas atomised M3/2 high speed steel (HSS) powder. Characterisation of the green body and as sintered microstructures using light optical microscopy (LOM), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX) reveals that both the SC and SLPS processes strongly influence the resulting microstructure and thus the properties of the high speed steel material. The results obtained show that the morphology and distribution of starch to a high extent affect the green body surface strength and that the large pores remaining after sintering originates from entrapped air introduced in the fabrication process of the green body. The results illuminate the possibility to combine SC and SLPS to produce HSS components with fully dense microstructures and retained green body geometry from a powder with particle size distribution 50–150 μm.

Liquid Phase Sintering of Ferrous Powder Metallurgical Materials

Abstract Liquid phase sintering is an attractive means of reaching near full densification of sintered powder metallurgical materials. In order to successfully realise the desired densification and intended properties of the final sintered material, the tailoring of the powder composition, powder mixture and sintering cycle is decisive. One attractive way is to use an appropriate mixture of powder where at least one powder constituent - the master alloy powder - is given such composition that it fully or partially melts while another powder constituent - the base powder -remains solid. This classical way of persistent liquid phase sintering has been developed for various combinations of alloys of the Fe-C-P-Cu-Si system and results demonstrate the capabilities of reaching near full density for mixtures of powder of sizes commonly used for traditional uniaxial die pressing. Another way explored has been the design of sintering approaches for supersolidus liquid phase sintering of high speed steel powder. This approach has been extensively studied by several research groups and the benefit of in-situ nitrogen alloying during sintering to yield high density and the precipitation of vanadium nitride has been realised. In this study, this approach is explored for a high-vanadium alloy and the results show how almost full density with appropriate microstructure can be reached from pressureless-shaped specimens using so-called starch consolidation.

Sintering behaviour and microstructure development of T42 powder metallurgy high speed steel under different processing conditions

High speed steel powders (T42 grade) have been uniaxially cold-pressed and subsequently densified through different sintering routes including: supersolidus liquid phase sintering (SLPS) under vacuum and different nitrogen pressures (0.2, 0.9, and 8 bar) and through solid state sintering (SSS) by hot isostatic pressing (HIP). HIP temperatures as low as 850 °C led to near full densification of the material (>98% theoretical density) with average size of M 6C and MC carbides lower than 1 μm and grain size ≈3 μm. Pressureless sintering under different nitrogen pressures (up to 0.39 wt.%N absorption) led to a significant reduction of the optimum sintering temperature (OST) and a pronounced increase in the sintering window (SW) as compared to vacuum sintering. Pressureless sintering under 8 bar N 2 led to a further reduction in OST together with the precipitation of massive eutectic structures. Therefore, the SW was judged to be negligible. The response of the as-sintered materials to the heat treatment is basically determined by the amount of C available in the matrix prior to quenching and the grain size. The highest hardness achievable for the sintering conditions evaluated ranges 700–1100 HV2 after austenitizing at 1100 °C, oil quenching and multitempering at 500–550 °C.

Possibilities and constraints of implementing starch consolidated high speed steel in prototyping

In the starch consolidation (SC) process, a water-based slurry containing powder, starch, dispersant and thickener is used to fabricate near net-shape green bodies that are de-binded and further consolidated by sintering. In this study, gas atomized M3/2 as well as high and low carbon V-rich M4 type high speed steel powder (<150 μm) are considered. Both material types undergo high volumetric shrinkage during super-solidus liquid phase sintering enabling them to reach near full density. The analyses and the review cover different process aspects like: recipe optimisation, post-gelatinization drying, de-binding and sintering. A SC recipe consisting of 58 vol.% powder, 3 vol.% starch, 1 vol.% dispersant and a thickener solution resulted in a density of >98% than what is theoretically stated after sintering. It is found that the success of the post-gelatinization drying procedure depends on the smoothness of mould material and controlling powder oxidation. The best combination was freeze drying the slurry in a silicon rubber mould. For V-rich alloys a total or partial control of eutectic carbides in the final microstructure could be realized for vacuum and nitrogen sintering atmospheres, respectively.

Sintering of High Wear Resistant Metal Matrix Composites

Particle reinforced metal matrix composites (MMC) based on cold work tool steels are increasingly used as wear resistant materials. Superior wear properties compared to commonly used wear resistant materials justify their application. Usually such materials are produced by means of powder metallurgy (PM) and hot isostatic pressing (HIP), a comparatively cost-intensive process. Against this background a sintering route based on super solidus liquid phase sintering (SLPS) was developed to obtain MMC with properties comparable to those produced by means of HIP. This publication points out the processing route in detail and focuses on the resulting microstructures. Finally, some results of wear tests are presented in comparison to MMC produced by HIP (HIP-MMC).

Fabrication of Gas-Permeable Die Materials Having Orthogonally Arrayed Pore Channels

Defects of components as a result of entrapped gases during an injection process could be minimized with the utilization of a gas-permeable metal die material in the mold, due to its excellent permeability of air. Conventional gas-permeable die materials employ low temperature sintering of loosely packed steel powders with or without the addition of pore-forming polymers, whose microstructures are usually weak and their gas permeability values are also low. In this study, gas-permeable metal die materials are developed using tool steel powder, packed in a mold having the insertion of orthogonally arrayed polymer wires. Linear gas-permeable channels in orthogonal array are thus developed by the burning out of the polymer wires, which yield a large value of air permeability. The value of air permeability can be adjusted by changing the diameter and number density of the polymer wires. The tool steel powder can be made fully dense by supersolidus liquid phase sintering, yielding a microstructure with a wear resistance value much larger than that of the conventional gas-permeable die material.

Optimised sintering route for cold work tool steels

Two different powder metallurgy cold work tool steels, D7 and a nickel alloyed D7 modification, were compacted to full density by utilisation of super solidus liquid phase sintering in vacuum and nitrogen atmospheres. The experiments were carried out with respect to the fabrication of wear resistant metal matrix composites. These materials exhibit a higher durability than cast materials and are increasingly used in the mineral and mining industry. In the present paper, sintering exhibits a more cost effective processing of these materials compared with hot isostatic pressing. The experiments were supported by thermodynamic calculations with Thermo-Calc. Significant differences of the sintering behaviour in vacuum and nitrogen atmospheres were detected. Full density could be obtained at a sintering temperature as low as 1210°C.

PM processing of elemental and prealloyed 6061 aluminium alloy with and without common lubricants and sintering aids

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 influence of MoS 2 additions on the densification of water-atomized HSS powders

Molybdenum disulphide (MoS 2) might be a useful addition in the production of composite self-lubricating high-speed steel parts, which are used in different anti-wear applications. MoS 2 might also has a favourable influence on the synthesis of sintered high-speed steels (HSSs) improving the compressibility of the metal powder mixture during its cold compaction, as well as acting as a sinterability enhancer of powder compacts during their sintering. In view of this, the compressibility and the sinterability of M3/2 and M35 types of HSS powder mixtures with MoS 2 additions were investigated. In this article, the influence of MoS 2 additions on the densification during cold compaction and vacuum sintering of selected HSSs is presented. It has been established that the MoS 2 promotes a consolidation of HSS powders during automatic die compaction, as well as final densification during sintering. Sintering of HSS-MoS 2 powder mixtures proceeds by the combination of a reactive and supersolidus liquid-phase sintering process. The added MoS 2 reacts with the steel matrix, enhancing densification. Fine dispersion of complex sulfides (Cr, V, Fe)S is formed, which improves the machinability and tribological properties of sintered steel.