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Differentiating Inclusions in Molten Aluminum Baths and in

Rafael Gallo Pyrotek Inc. OH, USA ABSTRACT It is well accepted that a variety of inclusions are present in liquid aluminum baths. Their source derives from the type of charge, alloying additions, melting practices, liquid metal treatments, and molten handling practices. The presence of unwanted second phase particles in solidified castings such as non-metallic inclusions or undesirable foreign material particles usually causes castings to be rejected. Successful elimination of inclusion related defects in castings requires proper collection and analysis of data. Finding the root cause of inclusions defects poses great complexity, because of the wide range of interdependent molten metal and process contributing factors. Having a notion of the level of molten cleanliness is just a part of a solution to eliminate inclusions related scrap in castings. Equally important parts of the solution are to establish a correlation between the inclusion defects seen in the castings with the inclusions present in the molten metal (molten cleanliness level), and to correlate in which part of the casting process the defect occurred. Without a proper correlation, it is very typical that the molten bath be incorrectly blamed for casting defects. Thus, the relevant questions that have to be answered at that time are: A) Are the inclusions from the molten bath the same as the inclusions seen in the casting? B) How does the casting defects correlate to the molten metal cleanliness? C) When was the date, and how it correlated to casting process information? With present technologies commercially available for removing inclusions from the molten bath, a wide range of levels are achievable. Current analytical techniques for assessing the cleanliness of molten aluminum baths provide a with practical information about the melting process. Metallurgical analysis by chemical analysis, microscopic examination, destructive testing, and non-destructive testing allow correct diagnosis on inclusion defects in a casting. The four objectives of this article are: 1) to improve the understanding of the difference between metal quality and molten cleanliness level, and their relationship with casting process and molten metal treatment variables, 2) to discuss recent technological advancements made in assessing inclusion levels in molten baths, 3) to present several examples of inclusions in molten metal, and in actual castings, and 4) to provide food for thought to value the benefits of using quantitative and qualitative inclusion data on molten metal and actual castings, to find the root cause of the scrap. Keywords: Inclusions, , Molten Cleanliness Assessment, Molten Quality, Casting Defects, Metallographic Analysis.

1 INTRODUCTION Molten Quality absorption, formation of films, , metallic and non-metallic inclusions, and oxide build up are inherent characteristics when melting and handling molten aluminum, regardless of the melting and/or holding furnace design, or the energy used (gas or electricity). Therefore, it is critically important to understand and control such intrinsic characteristics, because they will greatly affect the overall quality of the molten bath, which in turn impacts the final casting quality with respect to porosity, shrinkage, oxides and inclusions. The level of quality of a molten aluminum bath is based on the degree to which the chemical and the physical properties are controlled to meet specific customer’s requirements. While the term chemical properties refers to the chemical element compositional levels, the physical properties imply hydrogen content, dissolved chemical impurities, and inclusions. For the majority of , the concept of molten metal cleanliness is a segment of the overall melt quality of the bath. It takes into account the amount of “impurities” present in the molten bath before it is cast. The term “impurities” also implicate hydrogen gas, dissolved chemical impurities, and inclusions. To avoid misinterpretations and misunderstandings, the word “impurities” must not be confused with inclusions only. The molten metal cleanliness level is determined by the molten metal processing treatments performed to minimize, remove, and control hydrogen gas levels, the percentage of dissolved chemical impurities, and the amount of oxides and inclusions present in the molten bath before it is cast. Aluminum foundries could implement easier practical, feasible, and robust molten metal treatment and handling processes to minimize inclusion defects by advancing their comprehension on the molten cleanliness level being delivered by their melting department, and by better understanding how their casting technology influences the molten cleanliness level as the melt is poured. However, implementation of any given molten process would not be sufficient to guarantee success if intentional or unintentional changes in the melting and/or casting operations occur without revaluating the established process.

Degassing Basics Typical foundry techniques for lowering the concentration of dissolved hydrogen to satisfactory low hydrogen content before casting include three different methods. The most popular method includes the use of and/or components to inject a purging inert or reactive gas or a combination of them via lances, porous plugs, rotary impellers and molten metal pumps. The most effective and efficient techniques to degas are via rotary impellers and/or molten metal pumps. While rotary impeller is the most popular technique being used by high-pressure casting, low pressure, permanent mold, green sand, and dry sand foundries, molten metal pump degassing is the preferred method being used by aluminum smelters. Depending of the particular rotating impeller used by each technique, the introduced purging gas could be sheared into a variety of fine bubbles and distributed at different rates and uniformity patterns throughout the molten bath. Foundries need to take into account that a degassing operation not only reduce the hydrogen content in the melt, but also that a robust degassing process will greatly impact the flotation of inclusions, and in addition, if a rotary degassing process is properly combined with injection, the molten cleanliness will improve further as a result of more inclusions removal.

Alkali Elements Although alkali elements are part of the chemical composition, usually they are referred as chemical impurities. Because of the deleterious effects that they could cause, they have to be controlled to very low levels. Due to the difficulty in separating these impurities, they are best handled by a policy of avoidance (i.e., foundry having and adhering to a well –defined incoming material specification sheet with respect to elements and levels accepted). Nevertheless, there are generic treatments that can be employed to deal effectively with potential contamination by each element.

Inclusions in Molten Aluminum It has already been established in the technical literature that a variety of inclusions exists in molten aluminum alloys. Their source derives from the type of metal charge, alloying additions, melting practices, and liquid metal treatments and handling practices. Inclusions can be broadly classified as intermetallic and non-metallic. Intermetallic inclusions are primary compounds that result as a consequence of the precipitation, and growth phenomena from the liquid state. However, not all of them are necessarily detrimental to the quality of the molten bath. While presence of TiB2, and AlB2 for grain refining in hypoeutectic alloys, and AlP for modification hypereutectic alloys are not detrimental, TiAl3 particles in excess of 10µm can cause inclusion related defects. In addition, sludge formation (Cr-Fe-Mn) is also detrimental. Non-metallic inclusions can be present in the form of films, fragments, particles, and clusters. The inclusions can have different composition, texture, morphology, and appearance. Common types of non-metallic inclusions are: borides, carbides, nitrides, oxides, and salts. While the existing types of inclusions that would be present at the different molten stages will vary from foundry to foundry, their removal is essential for proper molten metal cleanliness. A number of commercially accepted melt treatment techniques are being used to remove and separate inclusions from the molten aluminum prior to casting. These include various different methods of fluxing, degassing, and filtration in the furnaces and in the gating system. Any of these techniques will have an impact on the melt cleanliness of the molten aluminum alloy. However, the effectiveness to evaluate their removal would rely on the melt cleanliness measurement technique being used. Ideally, the optimum technique would provide assessment of three inclusions parameters: size, distribution, and composition.

Inclusions in Castings Early in the casting process, foundries may scrap castings due to inclusions (foreign particles) when they are relatively easily seen by the naked eye during visual inspection on the as-cast condition. As the castings flow upstream in the process, inclusions could be detected during radiographic and/or fluorescent penetrant inspection. The next operation at which the majority of inclusions are detected is after the operation. It is not uncommon to find out that more than 50% of the inclusion scrap defects are reported after this operation. The castings are typically rejected either because of poor machinability (due to hard spots) or because they failed to meet stringent cosmetic requirements on machined surfaces. Cosmetic requirements may cause a casting to be scrapped if inclusions are larger than 400µm in purely machined surfaces, or if they are larger than 200µm in mirror polished surfaces. An inclusion defect may arise from a single clearly defined cause, or may be a result of a combination of factors, so that the necessary preventive measures are initially unclear. However, to prevent recurrence, it is necessary to correctly identify the inclusion before troubleshooting the process. As the castings fail to meet customer requirements, it is important to avoid the trap of applying “rule-of-thumb” solutions. Instead, it is highly recommended that foundries: A) fully examine the general characteristics and occurrence of the castings’ inclusions, B) understand and assess how the casting process influences the cleanliness of the liquid metal as the metal is delivered to the mold, C) correlate inclusions’ defects with molten cleanliness data, and D) understand the metallurgical terms and concepts with respect to chemical impurity, oxides, bifilms, inclusions and dross.

2 MOLTEN CLEANLINESS Intrinsic Properties It is already known that molten aluminum alloys have two inherent characteristics: the tendency to absorb hydrogen gas, and the ability to readily oxidize. When aluminum alloy melts react with the atmosphere or moisture, they form amorphous continuous alumina

(Al2O3) films on the surface of the bath, according to the following reactions:

2Al + 3H2O → Al2O3 + 3H2 (1)

4Al +302 → 2Al2O3 (2)

The alumina films are an intrinsic part of the melting process: they protect the metal underneath the film from further oxidation. It has been reported1) that the rate of thickening of the film on holding furnaces grow at a relatively fast rate of approximately 7 X 10-7 (kg/m2)/s. Nevertheless, in the process of melting and handling liquid aluminum alloys, the surface of the molten bath is not only exposed to the hostile gaseous environments of the products of the combustion, but it is constantly disturbed due to one or more of the following melting practices: 1) charging, 2) skimming, 3) cleaning, 4) degassing, 5) transferring, and 6) ladling. Any of these melting practices causes the thin alumina films to crumble, to break, to fold, to re-oxidize, and to encapsulate un-oxidized molten aluminum, generating wet dross, and causing rapid alumina film thickening (oxide build-up). These oxides films and dross are continuous over the surface of the melt. Dross formation is an unavoidable consequence of melt treatment and melt handling operations. The dross is considered to be the main contributor in influencing the total metal loss during melting. Depending upon the efficiency of the melting furnace, and melting practices, the amount of dross generated may be from 5 to 10% of the total metal melted. In addition to the generation and growth of oxide films and dross that are induced by melting practices (good or bad) associated with handling molten aluminum, there are two other thermodynamic principles involved within the operation of the furnace that can increase the oxidation rate and as a consequence create additional operating issues. The first one is the logarithmic increase in aluminum oxidation as a function of holding temperature as shown in Figure 1. The second one, is the decrease on the amount of heat that the bath is able to absorb as the thickness of the oxide layer (dross) increases on top of the molten surface. If the surface is clean, it will act as a mirror reflecting heat away. If the bath is covered with dross, that dross will act as an insulating blanket, slowing the entry of heat into the bath2). A 12.7mm dross thickness layer will reduce radiation heat transfer by 19%. The temperature of the dross layer has been reported to be 38 to 66 deg. C higher than the melt temperature3). Another problem that aluminum foundries face in molten baths is the non-metallic and the metallic impurities that are suspended and floating in the bath. Impurities and aluminum oxides will remain suspended and floating in the liquid bath because they are porous and contain some gas adhering to and trapped in the pores. In this case, the oxide’s density is similar to that of the aluminum alloy.

Fig.1 Oxidation of Molten Aluminum2).

Hydrogen Content Hydrogen absorption is common to all casting aluminum alloys. The hydrogen, which is absorbed, is made available at the surface of molten aluminum alloys through the reaction of the molten bath with water vapor (moisture) present in the melting environment. The reaction between water vapor and molten aluminum yields not only hydrogen gas but also the formation (within milliseconds) of amorphous aluminum oxide films (Al2O3) on the surface bath. The thickness of these films is less than 1µm. By the time the melting has been completed, the skin layer is a mass of randomly oriented oxide skins with metal and gas trapped in between, floating on top of the melt. During that time, the amorphous Al2O3 films crystallize first into gamma alumina (γ-Al2O3) films (10µm thick), and then they will transform to a much denser alpha alumina (α-Al2O3) films, given sufficient time and temperature. The alpha alumina films favor more oxidation at a faster rate. Since virtually all molten aluminum contains some level of hydrogen in solution, hydrogen removal is often necessary. The amount of hydrogen gas allowable in a molten bath at the time of pouring is established on “engineered critical hydrogen concentration ranges” that take into consideration specific casting quality requirements. The most effective method to degas molten aluminum is by injecting an inert gas, such as or , or by a combination of an inert gas with a small amount of a halogen gas (i.e., Cl). The most effective method to inject the purging gas is by using rotary impeller technology. Furthermore, rotary degassing also assists in floating particles, which are swept to the melt surface by flotation, where they accumulate in the dross.

Dissolved Chemical Impurities These impurities fall into two sub-categories: alkali hearth, and alkali that are tramp elements and in excess of the alloy compositional limits. Alkali earth elements relevant to molten aluminum are: Be, Mg, Ca, and Sr. Alkali metals are: Li, Na, and K. Foundries must be aware that while alkali elements usually derive from two distinctly different aluminum making sources: reduction of alumina in the Hall-Heroult process (virgin or primary metal) and recycled aluminum, their respective concentration levels vary among them. Based on the potential impurities that may be present in the incoming material, foundries must be aware of them and should either have robust molten metal practices to remove and control the alkali elements to the desired operating range, or to pay upfront for reducing and tightening control limits in the specification. Thus, foundries must pay attention to lower scrap grades because of the greater probability of poorly defined composition and content of deleterious contaminants. Alkali elements can be removed during the degassing process if the treatment contains a reactive gas such as chlorine. The presence of chlorine also assists the removal of non- metallic solid particles, which are swept to the melt surface by flotation, where they accumulate in the dross.

Inclusions in Molten Baths Inclusions can be isolated particles or agglomerates of different phases in the melt. Usually they are grouped by source, type and morphology. The size of the inclusions may vary from less than one micron to a hundred microns, and larger. Non-metallic inclusions are typically grouped as exogenous, or as in-situ. Inclusions that are imported to the molten metal from external sources are referred to as exogenous, while inclusions that arise from either chemical reactions within the melt itself or as a result of a melt treatment are considered indigenous. Sources for exogenous inclusions include refractory particles, usually from degradation of furnace walls, transfer ladles, launders, tubes, filling funnels, and in some instances, from pieces of the sand mold. In addition, inclusions derived from charging materials are also considered exogenous. Sources for in-situ inclusions are oxides, fluxing products, alloying elements, and intermetallic compounds. Inclusions present in aluminum melts are mostly oxides of aluminum, , and spinel, from either direct melt oxidation, or the oxidation of certain elements during alloying. Because of the nature of molten aluminum to readily oxidize, different oxides can form during different stages of the melting and liquid metal handling processes. Typical examples are:

Alumina (Al2O3), silicate (CaSiO), magnesia (MgO), magnetite (Fe3O4), silica (SiO2), and spinel (Al2MgO4). Typical inclusions that may be present in aluminum melts are portrayed in Table 1, which is a compilation of previously published data4,5,6,7). It is well accepted that the most commonly oxide inclusion formed in aluminum baths is Al2O3 which can be present on three different phases: amorphous, gamma, and alpha.

Table 1 Classification of inclusions in molten aluminum.

Size range Density Type Phase Shape thickness, cross section lb/in3 °F µm

γ-Al2O3 Films ? 1, 10-500

α-Al2O3 Particles, films 1-5, 10-1000

Al2O3 (Corundum) Particles, skins 3.97 0.2-30, 10-5000 3717 MgO Particles, skins 3.58 0.1-5, 10-5000 3839 Al MgO (Spinel) Particles, films, lumps 3.6 0.1-100, 10-5000 5117 Oxides 2 4 AlMgO4

SiO2 Particles 2.66 0.5-30 3002 CaO Particles 3.37 ?5, 4766 CaSiO Lumps, particles , 10-100

Fe3O4 Clusters, films 50-1000, 0.1-1 TiB Particles, clusters 4.5 , 1-50 5054 Borides 2 AlB2 Particles 3.19 0.1-3, 20-50 3920 Al C Particles, clusters 2.36 0.5-25 3812 Carbides 4 3 SiC Particles 3.22 0.5-5 4604

Nitrides AlN Particles, films 3.26 0.1-3, 10-50

NaCl, KCl, CaCl , MgCl Liquid droplets 1.9-2.2 0.5-1 1300-1472 Salts 2 2 Na2SiF6 Spheres 3 , 2-60 1832

Sludge (Cr-Fe-Mn)Si Particles ? 4.0

Inter TiAl, TiAl , NiAl Particles, clusters , 10-100 metallics 3

For quality castings, it is essential that inclusions be removed and/or that the size and number of inclusions be minimized. Common techniques for the removal and separation of inclusions involve settling during holding of the melt, flotation (based on rotary degassing), filtration, and fluxing. Any of these techniques will have an impact on metal cleanliness. However, fluxing is the first step for ensuring molten cleanliness, by preventing excessive oxide formation, removing non-metallic inclusion from the melt, and preventing and/or removing oxide build up from furnace walls8,9).

3 MOLTEN FLOW All exposed molten streams coming into contact with air are prone to re-oxidation and inclusion formation. Melting, molten treatment and handling operations usually rupture the protective oxide skin of the molten surface, causing freshly exposed surfaces to oxidize instantaneously. As the oxide films are overrun with liquid metal, and incorporated into the melt, they become more difficult to separate due to the smaller differences in densities between the oxide and the melt. Disruption of the initial protective oxide skins occurs typically during the molten stream flows encountered in different processes and/or stages of the foundry operation. Molten aluminum alloys may be subjected to bulk and surface turbulence.

Bulk turbulence is commonly characterized by the Reynolds number: 푉휌푑 푅푒 = (3) µ Surface turbulence is characterized by the Weber number: 푉2휌푟 푊푒 = (4) 훾

Where, V is the velocity of the bulk melt, ρ is the density of the melt, d some characteristic dimension of the passage in the path, µ is the absolute viscosity of the melt, r is radius of curvature of surface, and γ is the surface tension of melt/oxide-atmosphere interface.

It is important to be aware that molten bulk turbulence is a common practice in foundries using pumps to circulate molten aluminum in reverberatory and stack melter furnaces. The type of flow exiting the pump is always turbulent, i.e., the Reynolds number is always well above the critical value of 2,300. The circulation of the molten bath takes place by underneath currents that do not rupture the molten surface of the bath. Among several of the benefits reported10), circulation pumps provide: forced convectional heat transfer in the molten bath to facilitate faster melting, mass circulation to improve molten quality metallurgical homogenization, and reduction of dross formation. Unless bulk turbulence be excessive such that the surface of the melt be disrupted, bulk turbulence should not be of any concern. Foundries could benefit more from evaluating and assessing surface turbulence. A Weber number between 0.2 and 0.8 has been proposed1) as a range in which a molten surface would be free of surface turbulence. A good example of damaging molten surface turbulence is typically seen during improper rotary degassing, when excessively large bubbles of purging gas disrupt the oxide skin of the molten surface due to excessive gas flow rate and/or higher RPMs. While inclusions derived from exposed molten streams can be controlled and minimized by good metal handling operations that include carefully controlling molten aluminum flows, inclusions derived from refinement or alloying operations can be controlled by proper degassing. Inclusions left in the molten bath after the final molten treatment operation can be eliminated by a proper and final filtration operation.

4 QUANTIFICATION OF INCLUSIONS IN MOLTEN BATHS Commercially available techniques Over the last 50 years, several techniques have been developed and used for assessing the cleanliness of molten aluminum casting alloys. These include qualitative, quantitative, and analytical laboratory procedures, as well as on-line and off-line techniques such as: Reynolds 4MTM (The Mansfield Molten Metal Monitor), PoDFA (Porous Disc Filtration Analysis), LAIS (Liquid Aluminum Inclusions Sampler), LIMCA (Liquid Metal Cleanliness Analyzer), Qualiflash (Clogging of Extruded Ceramic Filter), PREFIL-Footprinter (Pressure Filtration Technique), RPT (Reduced Pressure Test), the K-Mold (K-Fracture Test), Fluidity Mold, Microscopic Examination, Computer-Aided Image Analysis, MetalVision MV20/20 (Ultrasonic Technology), and LIBS (Laser Induced Breakdown Spectroscopy). The strengths and weakness of these methods with regards to equipment requirements, sampling, sensitivity, timing, and practical means of assessing molten metal cleanliness and/or inclusion levels in the foundry floor have been fully discussed and published in the literature. Nevertheless, since some of the data that will be discussed in the following final sections of this article was obtained using the MetalVision technology, a brief description of the technology is pertinent.

MetalVision MV 20/20 MetalVision MV 20/20 is a very sophisticated, state of the art instrument, which is based on ultrasonic technology to provide continuous, in-line monitoring of inclusions in molten aluminum. It operates at a frequency of 2.25 MHz. Although the MetalVision system is the result of over 50 years of research and development, the current technology is based on improvements that have been made in the last 20 years11,12,13,14,15). Cleanliness level measurements are calculated every twelve seconds, while particle size measurements are continuously performed over a ten second period, followed by a two second pause. The cleanliness level is based on an arbitrary scale between 0 and 10. The cleanliness level is a measure of the clarity of the molten metal. It is inversely proportional to the number of suspended particles in the melt, and proportional to the energy of the reflected pulse. The cleanest metal will be at level 10 while the dirtiest metal will be at level 0. The smallest particle that can be detected and counted is 20µm, while the largest detectable particle is 160µm. The instrument displays a graphical representation of the molten cleanliness level as a function of time, and a histogram of the particle size distributions grouped in 10 different size ranges. In addition, it also provides the largest particle measured during a given monitoring period. While the graphical representation of the measurements can be printed in a continuous strip chart, the document files can also be imported to a personal computer for a more detailed analysis of the results.

Number of inclusions in the molten metal The level of inclusions in molten aluminum alloys can be substantial. The inclusion concentration may be in the range of parts per million (ppm) to fractional percentage (by volume). To illustrate this concept, let’s analyze how many inclusions could exist in a typical 1,200 pound furnace, if we were to implement a high molten cleanliness quality standard, to allow a residual inclusion content of 1 ppm per pound of molten metal. It is for certain that any quality control department in a foundry would agree that a 1 PPP inclusion content would be indicative of an outstanding molten quality. Now let’s also speculate that there is a rotary flux injection process in place, and that is being operated efficiently. Therefore, it would be expected that the sizes of the remaining inclusions floating in the molten bath should be ≤ 40µm. However, to facilitate the calculation, let’s grant that all of them are spherical in shape with a 40µm diameter. Doing the math, the results will show that there would be around 5,155 inclusions16). Thus, this number would represent 6,186,000 inclusions for the 1,200 pound melt. With the present technology commercially available for removing inclusions, a wide range of levels are achievable (0.10 to 10ppm). The question to ask is to what level of treatment a foundry should commit for a given casting/melting process. In general, the larger the inclusions are, the greater are their deleterious effects to casting quality.

Prefil and MetalVision Evaluations Molten metal cleanliness assessments with Prefil and/or MetalVision could provide the foundry with practical information about their melting, melt treatment and/or molten metal handling process. If properly done, in conjunction with optical microscopy and SEM analysis, a foundry would be able to: A) recognize the sizes, concentrations and types of inclusions present in their molten baths, and B) correlate inclusions (molten cleanliness) with inclusion defects (or related defects) in actual castings, as long as the inclusion defects in the castings were properly identified. To illustrate how the information provided by Prefil and MetalVision could be used not only to analyze molten cleanliness, but equally important, how to correlate it with casting defects, some examples from recent cleanliness evaluations will be presented. The intention of this section is not to describe how Prefil nor Metalvision work, but to draw attention to how the information provided by them can bring light to the foundry floor. While it is expected that the reader has a basic knowledge of the Prefil technology, it is not anticipated that the reader be familiar with the MetalVision technology. The intent of sharing the information is to expose the reader to a new technology and/or perhaps to a different methodology for inclusion defect analysis.

Prefil Evaluations Figure 2 shows some of the characteristic Prefil curves, based on the accumulated weight versus elapsed time, that were obtained during evaluations of different degassing and fluxing trials. The tangible benefit of such curves is that enables the foundry personnel to obtain immediate information on how each different molten treatment affects the cleanliness level of the molten bath. Steeper curves represent cleaner molten metal, since more metal is able to flow through the filter per unit time. Therefore, the black curve indicates cleaner metal than the blue curve. As the number of oxide films increases and/or the number of fine particulate inclusions due to grain refinement become substantial, the slope decreases.

Fig.2 Illustration of Prefil curves.

Table 2 depicts a typical summary of metallographic results that could be obtained after evaluating the corresponding solidified Prefil samples. In this particular case, only 3 results, from a previous trial of 21 samples, are shown. Such samples represented the results from three different degassing/fluxing trials. Such results correspond to the molten metal before any treatment (12B) as well as to the best (33A) and worst (27A) new molten treatment processes evaluated. Total inclusion content is first of all expressed in mm2/kg, and then broken down into individual species, each expressed in mm2/kg, and as a percentage of the total. In this particular trails, no oxide films where quantified because of the grain refiner particles. Important general facts to consider before interpreting Prefil results are: A) molten metal is cleaner as the total inclusion count (mm2/kg) gets lower, B) grain refiners generally tend to mask the presence of real inclusions, and C) molten cleanliness depends not only on the total inclusion content, but also, on the type of the inclusions and their sizes.

Table 2 Typical summary of metallographic evaluation results of Prefil samples.

Inclusion Types

Total Sample Magnesium Spinel Boride Aluminium Ti-Aluminide Sample Location Alloy Inclusion No. Oxide (Al O ) Oxide (MgO) (MgO.Al O ) (TiB ) Carbide (Al C ) (TiAl ) Count 2 3 2 3 2 4 3 3

2 2 2 2 2 2 2 (mm /kg) (%) (mm /kg) (%) (mm /kg) (%) (mm /kg) (%) (mm /kg) (%) (mm /kg) (%) (mm /kg) 12B Before degassing and fluxing A356 0.5709 2% 0.0114 85% 0.4853 1% 0.0057 ------12% 0.0685 ------27A After treatment system 1 A356 0.8896 2% 0.0178 ------5% 0.0445 ------93% 0.8274 33A After treatment system 2 A356 0.0579 ------99% 0.0573 1% 0.0006 ------

Table 2 shows that while sample 27A was the worst with a total inclusion count of 0.8896mm2/kg, sample 33A was the best with a inclusion count of 0.0579mm2/kg. Inclusion content of sample 12B was 0.5709 mm2/kg. In sample 27A, the main type of inclusions were titanium aluminide (93% of the total inclusion count), and then followed by titanium boride (5%). Sample 12B, which is a typical representation of the molten cleanliness before any degassing or fluxing treatment, shows that the main type of inclusions were MgO. Figure 3 are micrographs at different magnification from Prefil sample 27A. The metallographic analysis was conducted at the interface between the filter and the metal just above it (inclusion band) as depicted in Figure 3A. The metallographic analysis revealed that for this particular sample the inclusions were titanium aluminide, which were different from the typical and expected “good inclusions” TiB2, as the ones from sample 33A. Figures 3B and 3C are microphotographs at different magnifications highlighting the TiAl3 particles, which are well known to be deleterious to the quality of the molten bath, and the quality of the final casting. Such inclusions are dense, have high melting point, are not soluble, and therefore cause excessive crystal sedimentation at the furnace floor, inclusions in the castings, and lower mechanical properties in the casting end product.

Titanium Aluminide

Inclusion Phase Area A IB

3A. Filter grains, inclusion band 3B. Enlarged view of area A 3C. Microscopic aspect of (IB), and base metal. from figure 3A. titanium aluminide. Fig.3 Microstructures from Prefil sample 27A (treatment system 1).

MetalVision Evaluations Figures 4A, and 4B depict the MetalVision system evaluating cleanliness of molten aluminum at two different foundries. While Figure 4A depicts the system in a transfer ladle, Figure 4B depicts it in the dip out well of a reverb furnace. Figure 4C shows the probes being preheated above the melt surface before being submerged into molten aluminum. Figure 4D portrays the front view of the MetalVisison cabinet, and the screen displaying the histogram of inclusion size distributions (red color bars at the bottom of the screen).

Fig.4A MetalVision system evaluating Fig. 4B MetalVision system evaluating molten cleanliness in a transfer ladle. molten cleanliness in a reverb furnace.

Fig. 4C MetalVision probes being Fig. 4D MetalVision cabinet and screen preheated on a crucible melt surface. displaying distribution of inclusion sizes.

Figure 5 is a screen shot, from the MetalVision system, representing the quality of the molten metal in the transfer ladle before and after the treatment with system 2 (rotary flux injection). The empty space between the colored areas is the result of taking the probe out of the molten bath while putting the MetalVision system on pause during the fluxing and degassing operation. Figure 6 is a photograph of the color bar coding designated to identify the particle sizes during the analysis. Based on the results shown in Figure 5, and the corresponding color coding from Figure 6, the following interpretations can be drawn: A) the majority of the inclusions’ sizes before the fluxing and degassing treatment were above 160µm, and B) the inclusions’ sizes four minutes after the treatment were: 80% (≤ 20µm), 10% (20 to 30µm), and 10% (30 to 40µm). The corresponding total inclusion content from the Prefil samples that were taken during the MetalVision evaluations, are also depicted in Figure 5. The inclusion content after the treatment is the one shown in Table 2 for sample 33A.

Prefil: 0.1552 mm2/kg Prefil: 0.0579 mm2/kg

Fig.5 Screen shot of the MetalVision display for sample 33.

Fig.6 Photograph of color bar coding to identify inclusion sizes.

Figure 7 is a screen shot, from the MetalVision system, representing the quality of the molten metal in the transfer ladle before and after the treatment with system 1 (different rotary flux injection than system 2). As before, the empty space between the colored areas is the result of taking the probe out of the molten bath while putting the MetalVision system on pause during the treatment cycle. Based on the Metalvision results shown in Figure 7, and the corresponding color coding from Figure 6, the following interpretations can be drawn: A) the starting molten quality for sample 27 was better than the one for sample 33, since the majority of the inclusions’ sizes before the fluxing and degassing treatment were around 90-120µm, and B) the inclusions’ sizes four minutes after the treatment were: 12% (≤ 20µm), 11% (20 to 30µm), 44% (30 to 40µm), 22% (40 to 50µm), and 11% (50 to 60µm).

Prefil: 0.2642mm2/kg Prefil: 0.8896mm2/kg

Fig.7 Screen shot of the MetalVision display for sample 27.

It is pertinent to highlight that the higher total inclusion content of 0.2642mm2/kg with inclusions’ sizes of 90-120µm, as compared to the lower total inclusion content of 0.1552mm2/kg with inclusions’ sizes above 160µm, is an indication of the relationship between the total number of inclusions and the sizes of them. Are bigger and less number of inclusions better or worse than smaller sizes but lots of them?

5 IDENTIFICATION OF INCLUSIONS IN CASTINGS The elimination of inclusion defects in castings require proper collection and analysis of data. There are many statistical techniques to help us control process variables, correlate the effects of variables, and establish priorities for problem solving. Yet, all of these techniques are inconsequential if the defect is improperly diagnosed. Undertaking melting process changes without identifying accurately the type and source of an inclusion in a casting may prove very expensive. A solution for one type of inclusion may not be necessarily the same solution for a different inclusion. Despite the fact that quantitative metallography is essential to effectively evaluate casting inclusions defects, not all the foundries are willing to invest time, effort, and money for proper inclusion assessment either in the molten bath or the castings. Typically, 4 techniques are being used in foundries to evaluate casting inclusion defects: 1) Simple visual evaluation (“educated guess”). In the majority of the cases, when inclusion defects are considered a minor nuisance, this type of evaluation is conducted. 2) Optical and/or stereomicroscope. 3) Optical microscope coupled with computer and image analysis. 4) SEM analysis. Unfortunately, many foundries would limit the usage of this technique to a few samples that in many instances are not enough to properly identify the different inclusions defects that they are having in their castings.

Foundries may scrap castings due to inclusions after radiographic and/or fluorescent penetrant inspection. Quality casting requirements on inclusions could usually be met if foreign particles (inclusions) sizes in the casting are smaller than 60µm. However, particles larger than 60µm would not necessarily damage the quality of the castings16). An important evidence to take into consideration is the fact that the acceptable foreign material discontinuity sizes (width and length) established by the ASTM E155 standard for radiographic inspection for plate 1 varies from 762 to 1,524µm. Castings having internal inclusions not exceeding such limits are considered to be acceptable castings while meeting such quality standard. Larger inclusions sizes established for plates 2 and 3 are also used to define lower acceptable quality castings. The negative effect of inclusions in mechanical property evaluations is first of all most commonly noticed during the tensile testing of separate cast test bars, and then from test bars designated from specific casting locations. However, the negative effects of inclusion occurrence in the test bars are almost never related to castings being scrapped due to inclusion defects. Flaws in test bars due to inclusions in the fracture surface do not necessarily cause rejection of the castings because the test bar can be replaced with another one and retested per ASTM B 557. Regarding potential harmful discontinuities sizes, past studies have revealed that porosity defects of 100µm start affecting mechanical strength and fatigue life. Yet, the challenge for every foundry rely on defining their own threshold limits on the different inclusion sizes that could be tolerated in their molten baths, before they start affecting the final composition, morphology, and size of the inclusions defects found in the castings.

Example of castings with inclusions In the next and final section of this article, four different castings, representing the casting processes of green sand, permanent mold (static, and tilting), and low pressure are used to place emphasis on the value of properly identifying the inclusion defects on castings, before blaming the melting operation, the molten treatment process, and/or before attempting to change the melting and casting processes. Three photographs are included for each casting. Figures 8 to 11 show the castings being considered. In each of these four castings, the inclusions that were found in their machined surfaces, after visual evaluation of the machining operation, were initially diagnosed solely as inclusions due to “bad metal” or “dirty metal”. The analysis and results that follow were all based on SEM and EDX analysis. Figure 8A shows a cylinder head in which two black circles indicate the two defects found during visual inspection after machining. Both defects were from a section of the casting that is in contact with the green sand mold. A closer view of one of those defects is given in Figure 8B. The size of such defect varied from 1,500 to 2,000µm. Figure 8C clearly shows that the defect has nothing to do with the quality of the metal, since the defect was found to be caused by sand grains from the operation.

Fig.8A Cylinder head. Fig.8B Defect size: 1500-2000µm. Fig.8C Defect: sand grains.

Fig.8 Inclusion defect in a cylinder head.

Figure 9A shows a section of a brake caliper which has two similar defects (shown in black color), which appear to be dispersed in an area of approximately 3.2 by 4.0mm. A higher SEM magnification (500X) at the bottom of the defect, indicated by the arrow in Figure 9A, is given in Figure 9B. As shown in Figure 9C, the SEM evaluation indicated that the bottom surface of the defect was made out of overlapping plate-like features. Such plate-like features were typical representation of the overall topography seen in the bottom surfaces of both defects. EDX spectrums taken at the location marked in Figure 9B as “B” (bottom of defect) revealed that, in general, the chemical composition of the lowest bottom surface not only contains high levels but also high levels of Mg, Al, and Sr. Based on such chemical composition, while the plate-like features could be associated with Al2O3 and/or MgAl2O4 based-films, the Sr particles could be attributable to SrO. Consequently, it could be considered that the casting defect was caused by a combination of irregular entrained air layers due to oxide films (the defect did not show a continuous, unbroken surface), and by presence of Sr induced inclusions.

Defect 1 shown in Figure 9B

Fig.9A Brake caliper section Fig.9B SEM (500X) at Fig.9C SEM (750X) bottom with similar defects. bottom of the defect. surfaces rich in Sr (white particles).

Fig.9 Inclusion defect in brake caliper. Figure 10A shows a machined piston with a defect on the edge of the skirt. The defect measured approximately 640 by 1,000µm. The defect was detected after the machining operation. Figure 10B, which shows a SEM overview of the defect, also revealed that the apparent “inclusion defect” consisted mainly of an irregular contoured void. The SEM overview image (1,000X) shown in Figure 10C provides a better interpretation of the defect (cavity) topography that shows that: A) the bottom of the cavity was formed by continuous folded films, and B) the lateral walls were made out of overlapping plate-like structures. The several EDX spectrums obtained in different locations revealed that the bottom surface, and the lateral wall of the defect had similar chemical compositions. Thus, by considering the form and the shape of the defect (well defined smooth and continuous bottom surface),and the plate-like lateral walls with high silicon levels, the defect was considered to be due to air entrapment during mold filling, and/or due to solidification issues.

Fig.10A Piston with Fig.10B SEM overview of Fig.10C SEM (1,000X) continuous defect on the skirt. surface topography of defect. folded films at bottom surfaces.

Fig.10 Inclusion defect in a piston.

Figure 11A shows the center of an aluminum wheel in which two minute black particles identified as #1 and #2 (red circle) were found during the visual inspection of the machining and operation. The size and shape of embedded particle #2 is depicted at different magnifications in Figures 11B and 11C. As seen in Figure 11C, the particle had two different surface topographies (“A” and “B”). After EDX analysis of such topographies, the conclusion was that the particle was a combination of SiO2, salt particles (flux residues), and intermetallic titanium.

Fig.11A Wheel section. Fig.11B Defect at 1,000µm. FiG.11C Different topographies of defect.

Fig.11 Inclusion defect in an aluminum wheel.

After going throughout the analysis and the discernment of the form, shape and type of the inclusions found in each of the four castings described, it is evident that the initial diagnosis and identification of the defects was inaccurate. To eliminate inclusions in castings, it is required not only to avoid blaming the molten metal (bad metal”, dirty metal”) as the “culprit”, particularly if there is no evidence to sustain it, but most importantly, to correlate molten metal variables with the casting process to quantify the effects of the whole process on the molten cleanliness level. Once the melting, and the casting processes be well understood, and documented, it would be easier to find the root cause of the inclusions in the castings. After grasping the benefits of SEM and EDX analysis, and after being exposed to benefits of measuring molten cleanliness via Prefil and MetalVision, the reader should be able to envision the potential value of having tangible quantitative and qualitative inclusions documentation on molten metal cleanliness, as well as on the actual inclusions in the castings. The reader also would remember that to prevent non-metallic inclusions there are a minimum basic guidelines that should be followed. Most of them are related to the need of minimizing as much as possible the breakage of the molten surface film. Particular attention must be placed on establishing a careful transferring of molten metal from furnace to ladle, and from ladles to the subsequent destination to minimize molten surface breaks. Gentle molten handling practices are essential. Especially the one related to minimizing the breakage of molten surfaces films from the time the drossing operation (subsequent to the fluxing and degassing treatment) was done, to the time the molten metal gets into the mold. Eliminating turbulence in molten transfer, and during casting pouring will reduce the formation and generation of the gamma alumina (γ-Al2O3) films, as well as of the much denser alpha alumina (α-Al2O3) films responsible for casting defects. Once a foundry has proper quantitative documentation on the molten cleanliness levels in the melting and in the molten metal treatment processes, foundries may question if inclusion defects in castings are being caused by oxide films generated by improper molten handling and/or pouring practices. Still, even at such further molten upstream processes, foundries could benefit from Prefil, MetalVision and/or SEM analysis. Different metallurgical solutions could be implemented based on what type of inclusions are present, and from where they are coming from.

6 CONCLUSIONS 1) The level of inclusions in molten aluminum alloys can be substantial. The inclusion concentration may be in the range of parts per million (ppm). 2) The boundary in which inclusions (sizes, types, and concentration) render a molten alloy “unfit for use” is settled based on the casting process. Melts with similar levels of cleanliness may exhibit different effects on casting quality. 3) To remove non-metallic inclusions, it is necessary to use all molten metal treatments techniques that are commercially available (fluxing, degassing, and filtration). 4) Molten metal treatments and handling operations need to be more robust to minimize inclusion defects. 5) Information on melt cleanliness obtainable with commercially available equipment could be extremely useful to foundries willing to: learn about the process, and to dedicate time, effort, and money to analytically solve casting defects. 6) The availability of information on melt cleanliness determined by Prefil and MetalVision can help foundries to stablish and to optimize molten metal treatments and handling practices, and as a consequence to reduce casting defects, including inclusions. 7) When dealing with inclusion defects or casting scrap issues rule of thumb’s solutions must be avoid and replaced by careful and detailed analysis. 8) Finding the root cause of inclusion defects in castings represent a challenge because of the wide range of interdependent molten aluminum and casting processes. 9) Condemning the molten metal as the only “culprit” for inclusions in castings, would not be conducive to find the root cause of inclusions in castings. 10) Molten metal cleanliness assessments with Prefil and/or MetalVision provide a foundry with practical information about its melting process. If properly done, in conjunction with optical microscopy and SEM and EDX analysis, a foundry will be able to correlate casting inclusions defects with molten metal cleanliness to facilitate finding solutions to eliminate inclusions from castings. 11) Eliminating turbulence, and cascading in molten transfer, and during casting pouring

will reduce the formation and generation of the gamma alumina (γ-Al2O3) films, as well

as of the much denser alpha alumina (α-Al2O3) films responsible for casting defects. 12) Foundries need to investigate deeper to understand if the inclusions found in their castings are being caused by oxide films generated by improper molten metal treatment and handling practices, and/or by improper pouring practices, and/or by week casting technology . The use of Prefil, MetalVision and/or SEM analysis will facilitate finding the root cause of defects.

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