Anisotropic Dimensional Change Research Articles

Design for sintering – A comprehensive study on anisotropic dimensional change on sintering

This work collects as a whole the main results obtained in the Design for Sintering project, aimed at improving the effectiveness of a design procedure accounting for anisotropic dimensional change on sintering, through a deep investigation of the related phenomena and mechanisms. The project was developed in cooperation with the main European companies producing sintered components, and it is based on a huge experimental campaign. Ring-shaped parts with different characteristics were produced in different process conditions, and dimensional changes were derived from a careful reconstruction of geometry, measured in the green and sintered state. In the first part of the project, parts characterised by different geometrical parameters and different green densities were produced, using many different powder mixes, from highly shrinking to slightly swelling. The first part of the project highlighted the importance of anisotropic dimensional change in the compaction plane, and a new anisotropy parameter was proposed accordingly. In the second part of the project, the study was then focused on the anisotropy of dimensional changes in the compaction plane, as affected by different compaction strategies. The results were implemented in the design procedure, and process conditions were highlighted ensuring good agreement between real and predicted dimensions.

Anisotropic sintering shrinkage behavior of stainless steel fabricated by extrusion-based metal additive manufacturing

Metal additive manufacturing (AM) can be achieved by first extruding polymer-based filaments with densely filled metal particles to create the so-called ‘green’ parts and then experiencing debinding and sintering to produce finished metal components. As a critical step, sintering governs the shape dimension, relative density, and resultant strength of final parts. Thus, there is a growing need for research on understanding and quantifying the sintering-induced anisotropic shrinkage. In this work, the underlying behaviors of anisotropic shrinkage during the entire sintering of extrusion-printed stainless steel 316L parts will be understood and quantified through a combination of theoretical study and experimental investigations including three-directional dilatometry, scanning electron microscopy, and X-ray computed tomography. The three-dimensional size evolution of ‘brown’ parts before sintering will be studied through a macroscopic analysis of shrinkage anisotropy factors and microstructure characterizations. Different filling strategies will be integrated with initial pore distribution characterization for anisotropic dimensional change analysis. The heterogeneous pore distribution will be discussed in conjunction with the anisotropic viscous behavior to eventually unveil the shrinkage performance in the course of sintering. The findings of this work will provide insights into the anisotropic sintering mechanisms and fill the knowledge gap of the relations between extrusion-based metal AM process and sintered structures of metal alloys.

Influence of shape distortion on the precision of holes in parts fabricated by metal binder jetting

Metal binder jetting (MBJ) is an additive manufacturing (AM) technology split into two process steps: printing and sintering. Firstly, product is built up layer-by-layer by the selective deposition of a binder agent on a powder bed. Secondly, a thermal treatment (sintering) consolidates the metal structure. MBJ is currently becoming more and more attractive on the reason of high potential scalability, cost-effective production and wide range of available material feedstocks. However, the transition towards industrial scale production is restrained by the critical control of dimensional and geometrical precision of parts after sintering operation. In fact, product geometry is affected by anisotropic dimensional change or even shape distortion. This study aims at investigating the dimensional and geometrical precision of through holes. Three sample geometries were designed, having a through hole with axis perpendicular to the building direction and located at different levels along sample height. Samples were measured by a coordinate measuring machine before and after sintering, in order to assess the shrinkage and any shape change. Results highlight the inhomogeneous volumetric and linear shrinkage of the three geometries, which is influenced by the printing position in the building plane. A macroscopic deformation of parallelepiped geometry was also evidenced, caused by the superposition of layer shifting originated on printing, and by the frictional forces between sample surface and alumina support during sintering. Such distortion significantly affects the shrinkage and form error of holes.

A study of the anisotropy of dimensional change in the compaction plane during sintering of low alloyed cold compacted steels

ABSTRACT The anisotropy of dimensional change in compaction plane of rings made of three low alloyed steels was investigated as a function of green density and geometry. Increasing green density and (D ext–D int)/H ratio, the anisotropy of both shrinkage and swelling increases. A correlation with springback during ejection of the rings from die cavity after cold compaction was found. The ratio between the dimensional changes of diameters, as a function of the ratio between springback of diameters, describes a linear correlation intersecting point (1,1), representative of isotropic behaviour. This correlation confirms the hypothesis of an effect of micropores, generated in the green parts during ejection from die cavity, on dimensional change anisotropy. An analytical correlation was determined for the anisotropy of dimensional change in the compaction plane as a function of green density and geometrical parameter, which can be implemented in the design methodology accounting for the anisotropic dimensional change previously proposed.

Development of a new model describing the anisotropic dimensional change on sintering

ABSTRACT The anisotropic dimensional changes during sintering were investigated for rings made of eight different materials with different green densities and H/(Dext −Dint ) ratio. Dimensional changes are affected by green density, as shown in previous works, while the geometrical parameter does not display a clear influence. The anisotropy parameter K defined in a previous work does not describe anisotropy of dimensional change unambiguously, due to the anisotropy of shrinkage/swelling in the compaction plane. A new anisotropy parameter (K 3D ) was therefore defined considering the dimensional changes of internal diameter, external diameter and height. This parameter displays an unambiguous dependence on the equivalent isotropic dimensional change and will be used in further work to develop a predictive model for the prediction of the anisotropic dimensional change during sintering of parts with different green densities and geometry.

Dimensional and geometrical precision of parts produced by binder jetting process as affected by the anisotropic shrinkage on sintering

Dimensional and geometrical precision of parts produced by binder jetting is a crucial issue to be considered aiming at promoting the transition to industrial production. The influence of both the printing and the sintering processes has to be evaluated, and the high shrinkage has to be considered. The task is further complicated by the anisotropy of dimensional change on sintering. The aim of this work is to investigate the dimensional and geometrical precision of cylindrical holes, as affected by the anisotropy of dimensional change on sintering after binder jetting. AISI 316 L powder was used to produce five different geometries, characterized by four holes with different orientation with respect to the printing direction. The geometrical features were measured both in the green and in the sintered state with a coordinate measuring machine, and the dimensional changes, as well as the geometrical variations, were calculated. According to the author’s previous experience, a theoretical model has been defined, which aims at predicting the geometry of the holes as derived by the anisotropic dimensional change on sintering. The expected dimensional change of hole diameters, the variation of cylindricity, and the variation of the axis inclination were calculated by the model and compared to those derived from measurement. Good agreement between predicted and measured results has been observed, providing that the influence of printing process parameters is considered.

New interpretation for the origin of the anisotropic sintering shrinkage of AISI 316L rings based on the anisotropic stress field occurred on uniaxial cold compaction

ABSTRACTAISI 316L powder mix was sieved in three particle sizes and successively compacted on rings with two different H/(Dext-Dint) ratios. The height and diameters were measured by a CMM at the green and at the sintered state. The shrinkage was calculated to study the influence of particle size and geometry on the anisotropy of dimensional change. As expected, the volumetric shrinkage increases moving from coarser to finer particle size, and the measurements confirmed the anisotropy of the dimensional variation. The measurements revealed that the lowest rings shrink more than the taller rings. The reference to the radial and axial stress during prior cold compaction shows an interesting correlation, in which shrinkage increases on increasing the stress. The further investigation of the deviatoric stress and the deviatoric shrinkage provided a possible explanation of the anisotropy of dimensional change on sintering as a function of the inhomogeneity of the compaction stress.

Design for Powder Metallurgy: Predicting Anisotropic Dimensional Change on Sintering of Real Parts

Anisotropic dimensional change on sintering may strongly affect the precision of parts produced by press and sinter. In previous work a design procedure accounting for anisotropic dimensional change of axi-symmetric parts (disks and rings) has been developed on the basis of experimental data. In this work the procedure has been applied to predict the anisotropic dimensional change of real parts produced in industrial conditions, providing that coaxial rings were identified in the geometry of the actual parts. Parts were highly different for material, complexity of geometry, green density and process conditions. Parts were measured in the green and sintered state and the measured dimensional changes were compared to the predicted ones, finding a good agreement. The procedure was also adapted to predict dimensional change of an oval feature, and highly satisfactory results were obtained.

Two-step crosslinked liquid-crystalline elastomer with reversible two-way shape memory characteristics

ABSTRACTLiquid-crystalline elastomers (LCEs) possess large and equilibrium reversible anisotropic dimensional change in response to applied stimuli. The deformation behavior demonstrated by current LCE materials under the stimuli are generally determined by their own geometries and the alignment distributions of liquid crystal (LC) units in the LCE matrices. Here we report a LCE whose synthesis was through a two-stage crosslinking coupled with a mechanical reshaping process, where the shape was mechanically reset before the final crosslinking. It demonstrated reversible memory and change between the initial geometries formed during the first crosslinking stage and any reshaped geometries under the stimuli. Its deformation is not influenced by the geometries and the alignment distributions of LC units in the LCE matrix. This characteristic in LCEs holds promise in a wide range of application researches requiring sophisticated functions and smart structures.

Analytical Model of the Anisotropic Dimensional Change on Sintering of Ferrous PM Parts

Abstract This work proposes an analytical model developed from experimental data to describe the anisotropic dimensional change on sintering. Axial-symmetric iron parts differing for geometry and sintering conditions have been investigated, aiming at highlighting the influence of geometry. The specimens were measured in the green and sintered state by a coordinate measuring machine (CMM). The dimensional changes of height, external diameter and internal diameter were derived from measurement results. The anisotropy of the dimensional variations has been studied with reference to the isotropic dimensional change derived from the change in volume of the parts. The influence of geometry and sintering temperature was highlighted. To properly describe the dimensional variations in the compaction plane, the dimensional change of the external diameter versus the dimensional change of the internal one has been analysed. By means of the experimental data, a reliable analytical relationship has been found, dependent on the parts geometry. An anisotropy parameter has been identified, which allows relating the dimensional change in the compaction plane and in the axial direction to the isotropic dimensional change. This parameter depends both on geometry and on sintering conditions. By means of the anisotropy parameter an analytical model for the anisotropic behaviour has been developed.

The Anisotropy of Dimensional Change on Sintering of Iron

The anisotropy of dimensional change on the sintering of iron was investigated by dilatometry. Dimensional changes are different along the longitudinal and transversal directions, and shrinkage is more pronounced parallel to the compaction direction. This phenomenon is particularly pronounced during the early stage of sintering, in the alpha field below the Curie temperature. The results were elaborated according to the kinetics model for shrinkage to calculate an effective diffusion coefficient along the two directions. Such an effective diffusion coefficient is higher parallel to the compaction direction than perpendicular to it, and both are larger than the diffusion coefficient calculated on the basis of the activation energy reported in the literature for pure iron. This discrepancy is attributed to the defectiveness introduced by cold compaction in the particle contact regions, which may enhance diffusivity owing to the dislocation pipe mechanism, which, in turn, is particularly intense below the Curie temperature. This interpretation may also justify anisotropy of shrinkage because the powder particles are inhomogeneously deformed by uniaxial cold compaction. The enhanced diffusion coefficient increases with time and shows a maximum at temperatures below the Curie point. This trend was discussed with reference to the self‐activated sintering mechanism.

Study of the influence of material and geometry on the anisotropy of dimensional change on sintering of Powder Metallurgy parts

In the conventional press and sinter process, dimensional change on sintering determines the precision of the final parts, providing that a good dimensional precision of green parts is ensured. Anisotropic dimensional change on sintering may be detrimental to the precision of Powder Metallurgy (PM) parts, and it should be considered in the design step. The effect of material and geometry on the anisotropic dimensional change is studied in this work. Four different iron alloys and five different geometries were considered. Dimensions were measured both on green and on sintered parts and the anisotropy of dimensional change was evaluated and correlated to the material and geometry. The effect of neglecting anisotropy in the design step was investigated, in terms of dimensional tolerances, which can be obtained with different process capabilities. A model to describe the effect of material and geometry on the anisotropic dimensional change is also being developed.

The effect of anisotropic dimensional change on the precision of steel parts produced by powder metallurgy

The dimensional characteristics of ring-shaped parts produced with an Fe–Cu–P alloy by a conventional powder metallurgy process were measured on both green and sintered parts. The dimensional change was investigated by dilatometry, and the work highlighted that the anisotropy observed is mainly due to the early sintering stage. In this study, where the alloy is characterised by a slight densification, the anisotropic behaviour has a negligible effect on the precision of the height and the internal diameter, while the effect on the precision of the external diameter may be relevant.The results have been generalised, considering larger volume variations and different anisotropy parameters, and some relations between the anisotropy parameters and the attainable ISO tolerance classes have been determined.

Effect of Manufacturing Conditions on the Anisotropic Dimensional Change of STD11 Tool Steel during Heat Treatment

Effect of Manufacturing Conditions on the Anisotropic Dimensional Change of STD11 Tool Steel during Heat Treatment

Stability of 3-D carbon fiber composite to high neutron fluence

The dimensional stability, elastic modulus, and flexural strength of a high quality, three-dimensional balanced weave carbon fiber composite has been evaluated over a range of neutron fluence to ∼32dpa at ∼800°C. Results indicate that while the composite exhibits continuous strengthening over this dose range, this occurs with measurable loss of mass, increased volume, and for the highest dose studied, a large reduction in elastic modulus. While the balanced-weave composite was orthogonally isotropic, a significant anisotropic dimensional change occurred under irradiation. Dimensional change was dominated by fiber dimensional change and the overall shrinkage or swelling in a direction was determined by the extent to which intrinsic fiber shrinkage was capable of restraining swelling of matrix and fiber bundles.

冷間ダイス鋼の熱処理変寸の異方性に及ぼす晶出炭化物の影響

The influence of coarse carbide volume on anisotropy of dimensional change at heat treatment of cold working die steels has been investigated. The influence of carbide shape on anisotropy has been estimated by FEM analysis using very simplified models that contained carbide. Dimensional change increases with increasing carbon content of martensite. Anisotropy isn't affected by carbon content and phase transformation in this experiment. On the other hand, volume of large primary carbides affects anisotropy ; a parallel direction in forging direction is longer than vertical direction. By FEM analysis, Anisotropy is generated including the carbide. The length of test piece increases with increasing aspect ratio of carbide. The result of anisotropy by FEM analysis at a large aspect ratio is same as the experiment result.

Magnetic Properties and Microstructure of High-Density Sintered Iron Formed by Warm Compaction Using Die Wall Lubrication

Sintered iron is a useful soft magnetic material. However, its magnetic flux density is smaller than that of wrought magnetic iron, due to the low density of the sintered material. In order to increase the sintered density, we developed a new warm compaction technique using die wall lubrication (WC-DWL) with lithium stearate. With this method, a very high-density sintered body can be fabricated. Pure iron powder compacted at 1176 MPa and sintered at 1523K showed the following properties: sintered density = 7.76 Mg.m - 3 , maximum permeability (μ m ) = 5300, magnetic flux density; B 1 6 0 = 1.16T, B 2 4 0 = 1.28T, B 4 0 0 = 1.40T, B 2 k = 1.60T, and coercivity ( b H c ) = 110 A.m - 1 . Some of the sintered iron showed an anisotropic dimensional change and anisometric microstructure, which were linked to abnormal grain growth of up to several mm in length. This anomaly was more pronounced the higher the green density of the specimens.

Anisotropic Distortion of Tool Steels D2 and M3 during Gas Quenching and Tempering

Heat treatment of forged high alloyed tool steels produces anisotropic dimensional change. In the present work, the distortion behaviour of tool steels D2 and M3 during gas quenching and tempering was investigated. The influences of material manufacturing processes including casting, forging and powder metallurgy on anisotropic distortion were investigated. Rectangular samples were quenched in a vacuum furnace with high pressure gas cooling. The sample distortion was measured by a coordinate measuring machine. By means of dilatometry and metallography, the possible mechanism of anisotropic distortion was investigated. Experimental results showed that the distortion of cast and powder metallurgical steels was nearly isotropic because of an isotropic carbide arrangement, but the distortion of ingot cast and forged steels was apparently anisotropic during gas quenching and tempering. The carbide bands are the main cause of anisotropic distortion in the investigated steels. Dilatometric experiments showed that phase transformation plays an important role in the mechanism of anisotropic distortion. First simulations of the dimensional change during quenching indicated that transformation induced plasticity is the main mechanism by which anisotropic distortion occurs in this type of steels.

Anisotropic phase transformation strain in forged D2 tool steel

Heat treatment of forged D2 tool steel produces anisotropic dimensional change. It has been observed that tools have a larger dimensional change along the forging direction than perpendicular to it. This paper investigates anisotropic dimensional change by means of dilatometry. The results show that anisotropic phase transformation strain is produced in forged D2 steel during heat treatment. The anisotropic transformation strain is the main reason for anisotropic distortion in heat treatment of forged D2 steel. Anisotropic transformation strain produced during martensite transformation increases with higher austenitising temperature and is little influenced by cooling rate. A suggested mechanism is that transformation induced plasticity is produced under the internal stresses caused by the anisotropic microstructure (carbide bands) in the steel.

Anisotropy of dimensional change and its corresponding improvement by addition of TiH2 during elemental powder sintering of porous NiTi alloy

The paper deals with the influence of TiH2 content and sintering conditions (sintering temperature and sintering time) on the dimensional changes and the radial/axial shrinkage (R/A)-ratio of powder compacts. The purpose was to try to find out how to make precision porous products using a conventional powder metallurgical process. It was found that the dimensions of the compact shrink and anisotropy of dimensional change (ADC) occurs during sintering. Addition of TiH2 markedly minimizes ADC and makes it possible to manufacture the desired final product shape. An R/A-ratio less than 1 is supposed to be attributed to the differences in the degree of mechanical deformation, which occurs on the surface of powder particles parallel to and perpendicular to the direction of pressing during the pressing process. A model of the pore changes during pressing and sintering is proposed. (C) 1998 Elsevier Science S.A. All rights reserved.