Sinter Forging Research Articles

Fabrication of high-performance pure alumina ceramics with defect-rich structures by dynamic sinter forging

In this study, α-Al2O3 was sintered by dynamic sinter forging (DSF). The grain growth, mechanical properties and microstructures of the resultant ceramics were investigated in detail. The results showed that the grain growth during DSF was dominated by a dynamic grain growth process, in which grain size increased with an increase in pressure. The flexural strength of the ceramics increased with increasing pressure and reached 770 MPa in the sample prepared at 1300oC and 100 ± 10 MPa, which was significantly higher than that of the samples prepared by hot pressing and hot oscillatory pressing. The high strength was contributed to the severe plastic deformation induced by the oscillatory pressures, which strengthened both grain boundaries and grains. The current study suggests that DSF is a promising technique for preparation of high-performance Al2O3 ceramics, better than hot pressing and hot oscillatory pressing.

Effect of applied stress on microstructural evolution during the sintering of nanocrystalline tetragonal zirconia below 1000 °C

Microstructural evolution during the sintering of tetragonal ZrO2 nanoparticles was investigated using stress-free sintering and sinter forging. The green body prepared via gel casting had a densely packed structure with a narrow pore size distribution. Special attention was paid to the evolution of pore structures in the initial and intermediate stages of sintering. Pore coarsening, which is considered an important issue in the nanoceramics densification, was not remarkable in sinter forging. The accelerated densification rate, coupled with the retention of finer pore size than that in stress-free sintering throughout densification, lowered the sintering temperature requirement. This temperature reduction effectively inhibited grain growth, allowing samples prepared via pressure-assisted sintering to maintain a nanometric structure.

A novel sintering method of Al2O3/SiCw ceramic composites with improved wear resistance: Oscillatory pressure‐assisted sinter forging

As a typical ceramic tool, the wear resistance of Al2O3/SiCw composites is a prominent parameter in engineering applications. Therefore, how to improve its wear resistance has becomes a research emphasis of ceramic tools. In this work, the Al2O3/SiCw composites with improved wear resistance were prepared by a novel oscillatory pressure-assisted sinter forging (OPSF) process. The composites prepared by OPSF process not only exhibit higher density and significantly improved mechanical properties, but also own lower coefficient of friction than conventional sinter forging and hot oscillatory pressing processes. EBSD results indicated that the OPSF process could improve the texture of (0001) crystal face, low-angle grain boundaries, high-angle grain boundaries with special angles in the Al2O3 matrix and enhance shear deformation at micro-scales, which are multiple contributed to enhanced wear resistance. Therefore, OPSF process is a promising technique to prepare ceramic-based cutting tools with excellent mechanical properties and wear resistance.

Super strong B4C ceramics prepared by dynamic sinter forging

In this study, super strong B4C ceramic was prepared by dynamic sinter forging (DSF). The flexural strength and hardness of the sample are 935 MPa and 39.2 GPa, respectively. The strength is much higher than any value previously reported, while the hardness is one of the highest ever reported. The superior mechanical properties are attributed to the particle rearrangement and plastic deformation generated by the oscillatory pressure during dynamic hot forging. The current results suggest that DSF is a promising technique for preparing high-performance B4C ceramics.

Ultrafine WC-Co cemented carbide with enhanced mechanical properties prepared by oscillating sinter forging

A novel oscillating sinter forging (OSF) was used to prepare ultrafine WC-Co cemented carbides for the first time. The mechanical properties of the resultant samples were characterized and compared with those of cemented carbides prepared by conventional sinter-forging (CSF). The results show that the samples prepared by OSF have higher hardness and fracture toughness. The sample prepared by OSF at 1125 °C has a hardness of 1911 kg/mm2 and fracture toughness of 17.52 MPa·m1/2. These values are much higher than those obtained from the sample prepared by CSF and outperform the general trade-off between hardness and toughness observed in cemented carbides processed by other processes. Microstructural analysis suggests that OSF samples exhibited much better microstructure, such as higher relative density, finer grains, more uniform microstructure, more fcc-structured Co phase, and more ∑2 low-energy grain boundaries. The effects of microstructure on the mechanical properties were discussed. The current study suggests that OSF has excellent synergistic hardening and toughening effects and is a promising technique for preparing high-performance cemented carbides.

A new route to fabricate high‐performance binderless tungsten carbide: Dynamic sinter forging

AbstractIn this paper, we report the synthesis of fully dense and ultrafine‐grained binderless tungsten carbide (WC) by novel dynamic sinter forging. A nearly fully dense sample was obtained at 1500°C and 220 ± 30 MPa@90 min. The hardness and fracture toughness of the sample with forging time of 180 min are 29.6 GPa and 7.3 MPa·m1/2, respectively, much higher than the counterparts prepared by conventional sinter forging and other techniques. The enhanced densification and mechanical properties are attributed to the intragranular plastic deformation accommodated grain‐boundary sliding induced by oscillatory pressure during dynamic sinter forging. The current results suggest a promising approach for the manufacturing of high‐performance binderless WCs. Also, the method is likely applicable to other material systems.

Oscillatory pressure-assisted sinter forging for preparation of high-performance SiC whisker reinforced Al2O3composites

In this work, a novel process, oscillatory pressure-assisted sinter forging (OPSF), for the preparation of high-performance ceramic composites was reported. Compared with the samples made by conventional sinter forging (SF) and hot oscillatory pressing (HOP), the SiC whisker reinforced Al₂O₃ composites (SiC_w–Al₂O₃) prepared by OPSF at the same temperature exhibited a higher density and significantly improved the mechanical properties. The improvements in densification and performance are attributed to simultaneous enhanced shear deformation at both macro- and micro-scales, resulting from the combination of die-free configuration and oscillatory pressure of OPSF. And the strength of grain boundary is greatly increased when the temperature reaches 1600 ℃ of OPSF, due to that the grain-boundary sliding became pronounced at higher temperatures. The current results shed light on a powerful technique for preparing ceramic composites, which is likely applicable to other systems.

Transparent tetragonal zirconia prepared by sinter forging at 950 °C

Transparent tetragonal ZrO2 (3-mol% yttria-stabilized zirconia [3YTZ]) was fabricated by sinter forging of nanoparticles at 950 °C with applied stress of 200 MPa. In this study, the initial compact prepared by gel-casting had a densely packed structure of nanoparticles, and the calcined compact exhibited slight translucency. The grain size of the resulting dense sample was approximately 47 nm. In-line transmittance for a 0.9-mm-thick sample was 28% at a wavelength of 640 nm, representing 37% of the theoretic value. Meanwhile, the sample with the best transparency in this study was slightly whitish. This study demonstrated that combining nanoparticle gel-casting with external pressure to lower the sintering temperature is an effective method for obtaining transparent tetragonal zirconia.

Modelling of elimination of strength-limiting defects by pressure-assisted sintering at low stress levels

The structural reliability of sintered products depends on large defects introduced during powder processing, which cannot be removed by pressureless sintering. Here, we present a model how a large single ellipsoidal void is deformed, and finally disappears by pressure-assisted sintering. Taya-Seidel’s model is applied to predict the shrinkage of a large void in a compressible linear viscous material by using bulk viscosity, shear viscosity, and sintering stress that are determined experimentally for sintering of alumina powder at low stress levels. The application of mechanical stress promotes the densification rate. Its effect is maximum for hot isostatic pressing (HIP) and minimum for sinter forging. The effect is intermediate for hot pressing (HP) and spark plasma sintering (SPS), because the hydrostatic component of stress varies with densification. While a crack-like defect can be removed during densification, a spherical void must be eliminated by shear deformation in the final stage during dwell time.

A Lamination Model for Pressure-Assisted Sintering of Multilayered Porous Structures

This work describes a lamination model for pressure-assisted sintering of thin, multilayered, and porous structures based on the linear viscous constitutive theory of sintering and the classical laminated plate theory of continuum mechanics. A constant out-of-plane normal stress is assumed in the constitutive relation. The lamination relations between the force/moment resultants and the strain/curvature rates are presented. Numerical simulations were performed for a symmetric tri-layer laminate consisting of a 10% gadolinia doped ceria (Ce0.9Gd0.1O1.95-δ) composite structure, where porous layers were adhered to the top and bottom of a denser layer under uniaxially-applied pressures and the sinter forging conditions. The numerical results show that, compared with free sintering, the applied pressure can significantly reduce the sintering time required to achieve given layer thicknesses and porosities. Unlike free sintering, which results in a monotonic decrease of the laminate in-plane dimension, pressure-assisted sintering may produce an in-plane dimension increase or decrease, depending on the applied pressure and sintering time. Finally, the individual layers in the laminate exhibit different stress characteristics under pressure-assisted sintering.

Application of Electric Current‐Assisted Sintering Techniques for the Processing of Advanced Materials

Highly efficient energy conversion and storage technologies such as high‐temperature solid oxide fuel and electrolysis cells, all‐solid‐state batteries, gas separation membranes, and thermal barrier coatings for advanced turbine systems depend on advanced materials. In all cases, processing of ceramics and metals starting from powders plays a key role and is often a challenging task. Depending on their composition, such powder materials often require high sintering temperatures and show an inherent risk of abnormal grain growth, evaporation, chemical reaction, or decomposition, especially in the case of long dwelling times. Electric current‐assisted sintering (ECAS) techniques are promising to overcome these restrictions, but a lot of fundamental and practical challenges must be solved properly to take full advantage of these techniques. A broad and long‐term expertise in the field of ECAS techniques and comprehensive facilities including conventional field‐assisted sintering technology/spark plasma sintering (FAST/SPS), hybrid FAST/SPS (with additional heater), sinter forging, and flash sintering (FS) devices are available at the Institute of Energy and Climate Research: Materials Synthesis and Processing (IEK‐1). Herein, main advantages and challenges of these techniques are discussed and the concept to overcome current limitations is introduced on selected examples.

Process investigation and mechanical properties of electro sinter forged (ESF) titanium discs

Classified as an electric current–assisted sintering (ECAS) process, electro sinter forging (ESF) represents a sintering process following the resistance heating approach. The powder is simultaneously compacted and heated in a closed-die setup. The heating is generated by the Joule effect from the electrical current. Near net shape components of conductive materials are made in the closed-die setup within a short process time (100–400 ms). The final relative density is an important quality measure for the sintered parts. In the present work, samples of commercially pure titanium are produced with up to 98% relative density by optimisation of the main process parameters, namely electrical current density, compaction pressure and sintering time. Metallographic observations revealed that porosities were mostly found at the perimeter of the sintered samples. Mechanical testing by μ-Vickers hardness test, uniaxial compression and indirect tensile tests showed improved properties of the material with increasing density. The achieved mechanical properties were compatible with the theoretical values for bulk titanium.

Effect of the sintering technique on the ferroelectric and d33 piezoelectric coefficients of Bi0.5(Na0.84K0.16)0.5TiO3 ceramic

In the search of lead-free piezoelectric materials, ceramic processing techniques offer potential tools to increase the piezoelectric and ferroelectric properties in addition to new chemical compositions. Powders of pure BNKT16 (Bi0.5(Na0.84K0.16)0.5TiO3) phase were synthesized by sol-gel method with a low crystallization temperature (750 °C). Ceramic samples were sintered by pressureless sintering (PLS), sinter-forging (SF), and spark plasma sintering (SPS) techniques. Structural, morphological, and chemical characterizations were performed by XRD, Raman, EDS, and SEM. Sintered samples by PLS and SF exhibit rod-like grains associated to bismuth volatility. The highest remanent polarization (11.05 μC/cm2), coercive field (26.2 kV/mm), and piezoelectric coefficient (165 pC/N) were obtained for SF sample. The piezoresponse force microscopy (PFM) analysis shows that the crystallites at the nanoscale exhibit piezoelectric phenomenon and the highest piezoelectric response is reported for PLS sample. The presence of the rhombohedral phase, the increase in grain and crystallite size, and the oriented rod-like inclusions favoring the crystallographic texture are facts that enhance the piezoelectric coefficient for BNKT16 piezoceramics.

On the Process and Product Fingerprints for Electro Sinter Forging (ESF).

Electro sinter forging (ESF) represents an innovative manufacturing process dealing with high electrical currents. Classified in the category of electrical current assisted sintering (ECAS) processes, the main principle is that Joule heating is generated inside the compacted powder, while the electrical current is flowing. The process is optimized through the analysis of the main process parameters, namely the electrical current density, sintering time, and compaction pressure, which are also evaluated as process fingerprints. The analysis was conducted on commercially pure titanium powder. Small discs and rings were manufactured for testing. The influence of the process parameters was analysed in terms of the final material properties. The relative density, microstructures, hardness, and tensile and compressive strengths were analysed concerning their validity as product fingerprints. Microstructural analyses revealed whether the samples were sintered or if melting had occurred. Mechanical properties were correlated to the process parameters depending on the material. The different sample shapes showed similar trends in terms of the density and microstructures as a function of the process parameters.

Lubricant Influence on the Ejection and Roughness of In-Die Electro Sinter Forged Ti-Discs

Electro Sinter Forging (ESF) is a new sintering process based on Joule heating by high electrical current flowing through compacted metal powder under mechanical pressure. The whole process takes about three seconds and is based on a closed-die setup, where the sample is sintered inside a die. A near-net shape component is therefore manufactured. One of the challenges associated with this process is the ejection of the sample after sintering. Due to powder compaction and axial loading during sintering, a radial pressure is generated at the die/sample interface. Consequently, the ejection can be difficult, and the final quality of the sintered component in terms of roughness and surface defects may be affected. In the present work, four different lubricants and non-lubricated conditions were tested to investigate the effects on the final part quality. The sintered sample is a disc made of commercially pure titanium powder. The force was measured while ejecting the samples by using a speed-controlled press. The surface roughness parameter Sa was measured by using a laser confocal microscope.

Effect of nickel addition on mechanical properties of powder forged Fe-Cu-C

Fe-Cu-C system is very popular in P/M industry for its good compressibility and dimensional stability with high strength. Fe-Cu-C is a structural material and is used where high strength with high hardness is required. The composition of powder metallurgy steel plays a vital role in the microstructure and physical properties of the sintered component. Fe-2Cu-0.7C-Ni alloy with varying nickel composition (0%, 0.5%, 1.0%, 1.5%, 2.0%, and 3.0%) wt. % was prepared by powder metallurgy (P/M) sinter forging process. The present work discuss the effect of varying nickel content on microstructure and mechanical properties.

Comparison between sinter forging and X-ray microtomography methods for determining sintering stress and bulk viscosity

The macroscopic sintering parameters, sintering stress, bulk viscosity, and shear viscosity, were measured by the discontinuous sinter forging experiment for viscous sintering of calcium aluminosilicate (CAS) glass. The calculated results were compared with experimental values from the microstructural evolution during viscous sintering of spherical soda lime glass particles by X-ray microtomography. The sintering stress of CAS glass normalized by surface energy and the initial particle size was in good agreement with values estimated from the microtomography data of soda lime glass, despite the differences in particle shape and chemical composition. The bulk viscosity obtained by discontinuous sinter forging agreed fairly well with that estimated by X-ray microtomography observation, when they were normalized by glass viscosity.

サーボプレス付き放電プラズマ焼結機によるメカニカルミリングTi-Al粉末の反応焼結鍛造とその鍛造体の熱的安定性

サーボプレス付き放電プラズマ焼結機によるメカニカルミリングTi-Al粉末の反応焼結鍛造とその鍛造体の熱的安定性

Evolution of anisotropy in hierarchical porous ceramics during sinter-forging

When a body is sintered under non-hydrostatic stress (or strain) fields, anisotropic microstructures may develop which would have an impact on the performance of sintered products. In this study, we focus on the development of pore shape anisotropy during sintering forging. Several parameters were used to characterize the anisotropy of both intrinsic (small inter-particle pores) and extrinsic (large pores from the burnout of pore-formers). The effect of applied stress on grain growth was also investigated. As expected during sinter forging, the pore shape becomes anisotropic and the pores orient preferentially. The intrinsic pores preferentially align parallel while the extrinsic pores align perpendicular to the applied stress. For both intrinsic and extrinsic pores, the degree of anisotropy increases with applied stress, reaches a maximum and then decreases with further increase in stress. Applied stress leads to finer grain microstructures at a particular density. Possible explanations are proposed to explain these observations.

Erratum to: Closed Die Deformation Behavior of Cylindrical Iron–Alumina Metal Matrix Composites During Cold Sinter Forging

Erratum to: Closed Die Deformation Behavior of Cylindrical Iron–Alumina Metal Matrix Composites During Cold Sinter Forging