Colour Metallography Research Articles

Advantages of colour etching in quality control of graphitic cast irons

Abstract Graphitic cast irons are the most important and most widely used materials for the production of castings. Increasing requirements for the quality of these castings lead to increased demands on the control of foundry production from raw materials through technological operations to the finished product. An integral part of this control process is structural analysis, because the properties of graphitic cast irons depend mainly on their structure (on the shape, size and number of graphitic particles and on the character of a metal matrix in which graphite occurs). The paper deals with the structural analysis of graphitic cast irons using the classical black and white contrast, as well as the use of colour metallography, which enables to obtain additional information about the structure of cast iron. The article contains photographs of microstructures of graphitic cast irons, obtained using black and white etching, as well as colour etching. It deals with the advantages of colour etching compared to the use of classical black and white methods.

Use of colour etching in the structural analysis of graphitic cast irons

Colour metallography, especially colour etching, is a widely used technique for visualizing different phases in the structure of metals. Its advantage to the traditional etching techniques is that it gives additional information about the structure of material. The paper deals with the structural analysis of graphitic cast irons with a focus on colour etching. The possibilities of extending information about the structure of cast irons using colour contrast compared to classic black-and-white contrast is discussed. The paper is supplemented by photographs of microstructures of graphitic cast irons, obtained using classic black-and-white methods, as well as colour etching. Colour photographs of the structure of cast irons were obtained using the light metallographic microscope Neophot 32 with a digital camera Nikon DS-Fi3. Selected etchants for traditional black-and-white etching and for colour etching were used.

Microstructural characterization of the A356 alloy in different processing routes used in the thixoforming process

This work aims to evaluate different routes of raw material production to be used in thixoforming process. The semisolid materials exhibit particular structural characteristics, such the characteristic non dendritic structure. The studies for the production of near-net-shape parts have progressed together with the studies of material weight reduction on the automobile industries production lines. In general the advantages are energy saving, manufacturing steps reduction, productivity and quality increasing. Five processing routes have been investigated (conventional casting, electromagnetic stirring, grain refinement technique and different cooling rates) through microstructural characterizations, conventional metallographic and colour metallography. The material used in this study was the A356 alloy (commercial) having a hypoeutectic structure and grain size of about 750 mm and primary dendrite arm spacing size of around 80 mm, the combination of electromagnetic stirring + grain refining + high cooling rate provided the best route processing to obtain a refined structure, with grain sizes of around 240 mm, being feasible at the thixoforming process. Note also that using conventional metallography (black and white) all routes tested showed almost the same primary dendrite arm spacing size values ie had identical characteristics. Importantly, only with the characterization via polarized colour metallography is that was achieved in fact the best processing route definition. Keywords: A356 alloy, casting, electromagnetic stirring, grain refinement, thixoforming.

Use of Microscopy in the Study of Self-Hardening Al-Alloy for Automotive Application

Self-hardening secondary Al-Zn-Si-Mg alloys represent an innovative class of light Al-Si alloys. The most important and relevant feature of the self-hardening alloys is related to their good performance, without the need of any heat treatment. Mechanical properties depend upon the morphologies, type and distribution of the phases. It is therefore important to study the intermetallic phases occurring in the secondary Al-alloys, where they found more than in the primary-Al alloys. Study of intermetallic phases in the alloy was performed on an optical microscope using classical etching technique (by 0.5 % HF) to invocation black-white contrast as well as unconventional methods for invocation colour contrast (etching by Weck-Al) to the observed surface of the sample. The colour metallography results were correlated with the information obtained by standard etching. The microstructures of the samples were analysed by scanning electron microscopy after standard etching and deep etching (with HCl) too. After deep etching we are able to see the 3D morphology of eutectic Si particles and intermetallic phases.

Comparative analysis of structure and hardness of cast and direct metal laser sintering produced Co-Cr alloys used for dental devices

Purpose – The purpose of this paper is to analyse structure and measure hardness of Co-Cr dental alloy samples made with two different technologies, conventional casting method (CCM samples) and additive direct metal laser sintering technology (DMLS samples), and to compare the results. Design/methodology/approach – CCM samples were made in a conventional casting machine, using remanium 800+ Co-Cr dental alloy (Dentaurum, Ispringen, Germany). DMLS samples were fabricated out of EOS CC SP2 Co-Cr alloy (EOS, GmbH, Munich, Germany) using DMLS technology. Samples for structural analysis were plate-shaped (10 × 10 × 1.5 mm3) and for the hardness test were prismatic-shaped (55 × 10.2 × 11.2 mm3). Structure was analysed via an inverting microscope and colour metallography method. Findings – CCM samples have a dense, irregular dendritic mesh, which is typical for the metallic phase of the Co-Cr dental alloy. DMLS alloy has a more homogenous and more compact structure, compared to CCM. Metals, the alloy basis consists of, form semilunar stratified layers, which are characteristic for the additive manufacturing (AM) technique. Hardness values of DMLS (mean value was 439.84 HV10) were found to be higher than those of CCM (mean value was 373.76 HV10). Originality/value – There are several reports about possible use of AM technologies for manufacturing dental devices, and investigation of mechanical properties and biocompatibility behaviour of AM-produced dental alloys. Microstructure of Co-Cr alloy made with DMLS technology has been introduced for the first time in the present paper.

The Use of Colour Metallography and EDS for Identification of Chemical Heterogeneity of Selected Aluminium Alloys Copper and Zinc Alloyed

Aluminium alloys with higher content of alloying elements are very susceptible to the emergence of crystal segregation that significantly affects the mechanical, physical and chemical properties of these alloys. Crystal segregation is called chemical heterogeneity in microscale and is formed during crystallization. Crystallization of alloys does not occur at a particular temperature, as is the case of pure metals, but in a certain temperature interval. During cooling of the melt occurs to formation of different regions within the dendritic cell that differs in chemical composition. Generally is crystal segregation defined as chemical heterogeneity formed during alloy crystallization that is enriched or deprived of alloying elements and impurities that segregate unevenly across the surface of the dendrites. In the central area of the dendritic cells is alloy deprived of alloying elements, while the outer parts of dendritic cells and in space between dendrites is the concentration of alloying elements richer. This concentration has a hyperbolic course, when the central region of dendritic cells has the lowest concentration of the alloying elements and the outer part of dendrite boughs and interdendritic space have the maximum. Distribution of individual elements has a recurring character and can be described by sine function. The distance between two main axes of dendritic cells is affected by the temperature interval between the liquidus and solidus for the given alloy, the cooling rate of the melt and temperature gradient during solidification. Formation of the crystal segregation in aluminium alloys rich in alloying element and additives cannot be prevented, it is possible only influence its scope and with the right choice of heat treatment parameters can be suppressed. To suppress the crystal segregation the castings are subjected to heat treatment which is called homogenization annealing. It is the diffusion process in which there occurs to a balancing of chemical composition of alloy and the uniformity of its structure.

Vanadium and Chromium Impact to Microstructure of AlSi10MgMn Alloy with Elevated Iron Content

In Al-Si alloys iron as an impurity causes decreasing of mechanical and foundry properties of castings. Nowadays is paid attention to adding different elements into aluminium alloys to increase the properties of final castings. Some elements eliminates iron by changing iron intermetallic phase morphology, decreasing its extent and by improving alloy properties. Also there is a possibility of using lower amounts of more elements, what can lead to change of morphology and to improve casting properties at the same time. The contribution is devoted to vanadium and combined vanadium and chromium impact to AlSi10MgMn alloy with high iron level. This effect is evaluated through microstructural analysis using different etchants. Colour metallography is also used to find the better and faster identification of the intermetallic phases.

Colour Metallography of Cast Aluminium Alloys

Abstract For better observation of the microstructure of cast aluminium alloys under an optical microscope, the contrast obtained for the different micro constituents was enhanced by making use of a copper phthalocyanine based colorant. The proposed method allows distinguishing structural details which would remain hidden or poorly resolved when studying the conventionally etched sample. The method produces high relief for silicon precipitates with the tint further accentuating the outlines, allowing detection of morphology change of the aluminium-silicon eutectic due to chemical modification with strontium. Magnesium micro segregations in the α-aluminium phase, caused by incorrect thermal treatment during ageing, can also be clearly distinguished. The coloured images obtained by optical microscopy are very sharp and provide similar or better information than the corresponding Scanning Electron Microscopy images. The method proved to be suitable for Quality Control in the fabrication of cast aluminium alloy compounds, sourcing down possible errors to different stages of the process.

Application of colour metallography in the examination of ancient metals

The use of colour in the metallographic examination of ancient metals is useful both for research and teaching purposes. Application of different optical methods of examination include: polarized light, with or without a first-order red compensator, and differential interference contrast microscopy (DIC). The use of DIC can be combined with the other techniques mentioned in the text for the examination of etched or unetched metal surfaces, which are best observed microscopically using a metallograph. Colour techniques are now used routinely in industrial metallography, and are available for a variety of metals and alloys. Colour tint etching can be used to enhance grain structure or different phases present in the metallic artefact which may not be visible in conventional etched samples, or which lack optical contrast. Many more applications of these techniques in the examination of ancient metals will be published in the literature as they become better known and second-hand equipment with DIC lenses enter the market at a price suitable for conservation laboratories to purchase them.

Effect of Cooling Start Temperature on Microstructure and Mechanical Properties of X80 High Deformability Pipeline Steel

The effect of cooling (laminar cooling) start temperature on the phase constitution was analyzed by quantitative metallography. The martensite/austenite (M/A) island distribution was fixed by colour metallography. The strength and uniform elongation of the steels were tested with quasi-static tensile testing machine. The in-coordinate deformation of the soft and hard phases was analyzed using FEM. The results indicate that when the cooling start temperature is 690 °C, the mechanical properties are the best, meeting the requirements of X80 high deformability pipeline steel.

Metalografía a color en aleaciones Al-Si comerciales. Optimización de las técnicas de caracterización microestructural mediante microscopía óptica de reflexión

The present demand on alloy production with improved quality requires the optimization of the metallographic procedures used on its characterization. Traditional etching techniques commonly employed for phase identification by optical metallography in aluminium alloys are not always suitable for a detailed analysis of existing phases, nor to accurately predict the mechanisms that govern the solidification process in certain detail. This work explores the potential of colour metallography to reveal at its best as-cast microstructures in Al-Si 12 alloys. For this purpose a colour etching technique, specifically developed for aluminium alloys and based on the Weck reagent. The application of such etchant has allowed the qualitative characterization of the microstructure. And it has also shown the advantages of colour metallography over black and white (B&W) etching techniques.

Effect of homogenisation treatment on segregation of silicon in ferritic ductile irons: a colour metallographic study

Microsegregation strongly affects the properties of ferritic ductile irons. A homogenisation treatment was attempted to decrease such segregation. The severity of segregation can be determined with microprobe analysis, but this technique cannot reveal the morphology of segregation on a larger scale. The present investigation was undertaken to explore the use of colour metallography for revealing the severity of segregation both qualitatively and quantitatively during the homogenisation treatment. The modification of the morphology of segregation of silicon during homogenisation treatment for different times was revealed clearly by colour metallographic techniques. The proportion of interdendritic areas to the total matrix area was quantified. A semi-quantitative relationship between the difference in the sequence of colour and the degree of segregation of silicon in ferritic ductile iron was developed with the use of a combination of a colour metallographic technique and microprobe analysis.

The effect of vanadium in as-cast ductile iron

The effects of adding different amounts of vanadium (0.02–0.54%) to ductile irons have been investigated. Optical microscopy revealed that that the addition of vanadium led to the formation of a phase with a particle size of up to 6 μm in the as-cast microstructure and SEM—EDX microanalysis showed that this phase is vanadium-rich. In addition to these coarse particles, TEM studies using carbon extraction replicas revealed a wide range of sub-micron vanadium-containing particles. Selected area diffraction patterns of vanadium-rich particles with a size of < 100 nm showed that they were cubic and corresponded to V4C3. Image analysis and colour metallography have been used to measure the volume fraction of the coarse particles and have shown that about 75–80% of the vanadium carbide was present in the coarse form.Quantitative metallography showed that, as the vanadium content was increased to 0.54%, the average graphite nodule count decreased, the pearlit content increased by about 20–30%, the pearlite interlammellar spacing decreased and the ferrite grain size decreased. In spite of its strong tendency to promote the formation of eutectic carbide, it has been found possible to add up to 0.54% vanadium without forming eutectic carbide in sections equivalent to a 6 mm thick plate, so long as late inoculation is used to add sufficient inoculant to give a silicon increment of 0.8%.The addition of 0.54% vanadium increased the modulus of elasticity, proof strength, tensile strength and hardness but reduced the elongation.

Revealing the solidification structure of nodular iron

The effectiveness of several metallographic techniques used to reveal the solidification structure of nodular iron has been evaluated by quantitative metallography. A colour metallography reagent, ...

Texture Determination in Aluminium Alloys Using Colour Metallography

Texture components in aluminium alloys can be distinguished by colour metallography using anodic oxidation and polarization microscopy. This method is particularly useful for texture inspection in correlation with other structural parameters such as grain size, orientation correlation and texture inhomogeneity. The method reveals, however, only two of three orientation parameters. It is therefor not suitable to obtain complete orientation distribution functions.

Microstructural investigations of a nickel-based repair coating processed by liquid phase diffusion sintering

Modern stationary gas turbines are designed for the best possible efficiencies and outputs. However, as temperatures and stresses are increased, the design and manufacture of cooled blading in gas turbines becomes complex. This results in higher costs and longer delivery times for cast components and it becomes worthwhile to repair damaged components. The microstructure of samples processed by liquid phase diffusion sintering of pre-alloyed atomized and mixed powders is investigated by electron microprobe analysis, X-ray diffraction and optical microscopy involving colour metallography. The melting and remelting behaviour is analysed by differential thermal analysis. Materials obtained from powders based on the cast superalloys NIM-75, IN-939 and IN-738LC and the braze materials based on the systems Ni-Cr-B, Ni-Cr-Fe-Si-B are shown to consist of a dispersion of borides and/or silicides in the base metal matrix and a more or less eutectic grain boundary precipitation depending on the thermal cycle and the composition. The porosity exhibited does not exceed that of the cast base materials IN-738LC and IN-939. The thermal fatigue proved to be excellent. The corrosion resistance was only sufficient in blends with a high chromium content as boride precipitation caused chromium depletion. Liquid phase diffusion sintering meets in many cases the requirements of repairing cracks and erosion damage by filling and sintering with a braze mix consisting essentially of a superalloy base material and a braze component containing a melting point depressant.