I Have Inclusions! Determining the Best Cost Saving Approach
Click here to see this story as it appears in the August issue of Modern Casting
Many foundries have experienced costly outbreaks of high reject rates due to inclusions in castings. An inclusion defect may arise from a single clearly defined cause or might be a result of a combination of factors, so the necessary preventive measures are initially unclear. However, to prevent recurrence it is necessary to correctly identify the inclusion before troubleshooting the process to find the root cause of the defect.
To correctly diagnose inclusion casting defects, metalcasters have to: a) fully examine the general characteristics and occurrence of the inclusion and describe in detail its size, distribution, and appearance; b) document the defect’s location with photographs; and c) understand the casting process. Attempting a corrective action without knowing the type of the inclusion may prove very expensive. Once a corrective action is found, it must be implemented, monitored and re-evaluated.
Typically, foundries use four techniques to evaluate casting inclusion defects:
Simple visual evaluation (“educated guess”).
Optical microscope and/or stereomicroscope.
Optical microscope coupled with computer and image analysis.
Nearly everyone who casts aluminum alloys, regardless of sophistication, has at some time experienced an issue with inclusions from molten aluminum. Even the molten aluminum being used for sheet ingot and beverage cans, which is considered the cleanest molten aluminum, will have inclusions to some extent (smaller sizes and lower concentrations of them). Thus, various types of inclusions such as oxides, nitrides, carbides, fluorides, borides, chlorides and salts may be present in molten aluminum alloys.
During 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.
Better understanding of the cleanliness level of the molten metal being delivered by the melting department and the additional influence that other casting process factors may have on the molten metal quality level would help aluminum foundries implement feasible, practical and robust controls during the molten metal treatment and handling operations to minimize inclusion defects. However, implementation alone would not be sufficient to guarantee success if melting and/or casting operational changes are made, intentional or unintentional.
Typical common process changes and deviations related to the treatment and handling of liquid aluminum that would negatively impact the quality of the molten metal are:
Accepting raw materials of lower quality.
Charging dirtier returns/scrap (i.e., contaminated with oil, and grease due to poor foundry equipment maintenance).
Operating equipment which has inoperative and malfunctioning flow meters, pressure regulators, and RPM counters.
Failing to replace worn off degassing consumables.
Reusing poor quality residual pieces of ceramics and/or graphite materials (that have already exceeded their life expectancy) to make other components to be used in molten aluminum.
Lack of preventive maintenance in both: equipment, and refractory in furnaces and transport ladles.
Failing to follow process procedures during the casting (molding) process. For example, not placing the filter in the gating system and/or not blowing air before closing the mold.
Molten Metal Quality and Molten Cleanliness
The level of quality of a molten aluminum bath is based on the degree to which the chemical properties (chemical element composition) and physical properties (hydrogen content, dissolved chemical impurities, and inclusions) are controlled within a specific foundry’s internal specification, which is established to meet casting requirements. In general, the goal of the melting department is to produce and deliver quality metal, while minimizing dross generation.
Chemical element composition: The chemistry of the alloy affects the surface tension, the viscosity of the molten metal, and the solidification characteristics of the alloy.
Hydrogen content: 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 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.
Dissolved chemical impurities: These impurities fall into two sub-categories: alkali hearth metals and alkali metals that are tramp elements and in excess of the alloy compositional limits.
Although alkali metals are part of the chemical composition, usually they are referred as impurities. Because of the deleterious effects that they could cause, they must be controlled to very low levels.
Based on the potential impurities that might be present in the incoming material, foundries must be aware and should either have robust molten metal practices to remove and control the alkali elements to the desired operating range or 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.
Inclusions: The source of inclusions 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 because of the precipitation and growth phenomena from the liquid state.
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.
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 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, riser 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. Figure 1 depicts the sources of non-metallic inclusions.
Oxides are the most prevalent type of inclusions, 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), calcium silicate (CaSiO), magnesia (MgO), magnetite (Fe3O4), silica (SiO2), and spinel (Al2MgO4).
Sedimentation, flotation, filtration, and fluxing are common techniques for removing and separating inclusions from aluminum alloy melts. 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 walls.
Impurities, Oxides, and Dross
The cleanliness of the molten bath is also greatly affected by the degassing operation, which in turn significantly impacts removal of inclusions and method of flux addition. The steps taken to prevent hydrogen pickup and dross formation will minimize inclusions in molten aluminum alloys.
Chemical impurity means an unwanted chemical element has been dissolved in the molten bath. Inclusion refers to a foreign particle present in the molten metal prior to casting. Dross denotes crumpled aluminum oxide films that encapsulate a significant amount of un-oxidized aluminum, floating on the surface of the molten bath.
With the exception of sludging associated with high levels of Cr, Mn, and Fe that could be considered chemical impurities, other typical chemical impurities do not cause inclusion related scrap defects in castings. Scrap related defects due to chemical impurities are associated with:
The adverse effect that excessive levels of Fe has on tensile properties.
The poisoning effects that several ppm of P or Sb have on Na or Sr, during modification of hypoeutectic al-Si alloys.
The high levels of Ca, and Na that cause edge cracking during hot rolling.
The high levels of Li that produce the “blue” corrosion in aluminum foil.
The high levels of Na that cause embrittlement in 5XX alloys.
Potential methods for removing unwanted elements from molten aluminum alloys include selective oxidation, chlorination, fluorination, and intermetallic compound formation.
Oxide films and particles are introduced and/or generated during the charging and melting practices, as well as during the molten metal treatment and handling operations. Different alloys under similar charging practices can have significantly different oxide skins, and identical alloys from different “heats” will enter the molten metal with different oxide contents. Even primary ingot can introduce oxides. Entrained oxides particles and other inclusions can be floated out with the assistance of inert and/or chlorine gas purging. The non- metallic particles attach to the surface of the rising gas bubbles, collect on the surface and therefore can be skimmed off.
After a meltdown, any molten aluminum alloy will have a large variety of finely divided small quantities of particles suspended in the body of the melt and a layer of wet dross on the surface of the aluminum alloy. The initial thin oxide film that develops on the surface of the melt offers protection from further oxidation. However, the constant movement of the surface of the molten bath due to the different melting practices (charging, skimming, cleaning, degassing, transferring, and ladling) causes the thin alumina films to break, to crumble, to thicken and to encapsulate unoxidized molten aluminum, generating what is known as wet dross. The presence of alkali and alkali earth elements, even in small amounts, could increase the permeability of the film which in turn increases both melt oxidation and dross formation.
The aluminum content of wet drosses is typically reported to be in the order of 60-80% while the remaining 20-40% is aluminum oxide. The amount of trapped liquid metal in the dross varies according to the melting practice. The aluminum oxide is a stable compound that cannot be reduced to aluminum under ordinary melting conditions. However, the amount of suspended liquid metal could be reduced from the 60-80% range to 30% by proper fluxing and drossing techniques. 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 5-10% of the total metal melted. However, the total melt loss throughout the operation can also be influenced by other process steps that are insensitive to the charged material.
Knowing the level of molten cleanliness at a metalcasting facility is just part of a complete solution to eliminating inclusion-related scrap in castings. The other two key factors are establishing a correlation between the defect in the casting and the inclusions in the molten metal, and implementing proper corrective action. These two factors will be discussed in the second part of this series in the September issue of Modern Casting.
This article is based on a paper (17-106) originally presented at the 2017 Metalcasting Congress.