I Have Inclusions: Identifying the Source
Click here to see this story as it appears in the September 2017 issue of Modern Casting
Finding the root cause of inclusion defects in castings represents a challenge because of the wide range of interdependent molten aluminum and casting process contributing factors.
Molten metal factors are linked to the level of molten aluminum cleanliness, which depends on the degree to which the chemical properties (chemical element composition), and physical properties (hydrogen content, dissolved chemical impurities, and inclusions), are controlled within the foundry operation.
Sedimentation, flotation, filtration, and fluxing are common techniques being used to remove and separate inclusions from aluminum alloy melts. 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.
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.
The level of inclusions in molten aluminum alloys can be substantial. The inclusion concentration might be in the range of parts per million (ppm) to fractional percentage (by volume). For example, a “low” inclusion concentration of 1 ppm per pound of molten metal would contain around 5,155 inclusions if all the inclusions were considered to be spheres of 40 µm (0.0016 in. or 0.04 mm) diameter. The assessment of the level of inclusion present in the melt has been a very important parameter that needs to be controlled by proper inclusion removal and detection techniques. In addition to inclusion particle size, a significant attribute of molten metal cleanliness is inclusion size distribution. Furthermore, while some inclusions particles may have the same dimensions, they could have significant differences in their chemical properties. Thus, it is of primary importance that inclusions evaluations distinguish differences in chemical and physical properties among them. In general, the larger the inclusions are, the greater their deleterious effects to casting quality.
With present technology commercially available for removing inclusions, a wide range of levels are achievable (0.1-10 ppm). Common techniques for the removal of inclusions involve settling during holding of the melt, flotation during the injection of gases, filtration, and fluxing. The question to ask is to what level of treatment should a foundry commit for a given casting and/or process. It appears most crises due to inclusion casting defects are caused by unforeseen generation of inclusions.
Sedimentation processes are effective for particles whose density is significantly greater than aluminum. Particles greater than about 90 µm (0.09 mm or 0.0036 in.) settle at the bottom of the bath within 30 minutes. Due to the typical purging bubble sizes: ≤ 5 mm (5,000 µm) encountered during rotary degassing, inclusions greater than 30 to 40 µm may be reliably separated from the melt by flotation.
Furthermore, inclusion particles larger than 80 µm (0.08 mm or 0.0031 in.) can be removed with rising de-gas bubbles of 10 mm (10,000 µm) diameter. Filtration methods can further remove inclusions smaller than 30 µm (0.03 mm or 0.0012 in.). However, filtration efficiency depends on the type of the filter used, filter size, pore structure, initial molten cleanliness and metal velocity.
Inclusion Defects in Castings
Although foundries still have difficulty accurately assessing aluminum melt cleanliness prior to pouring, most understand that molten metal handling and treatment techniques would have an impact on the melt cleanliness prior to pouring. Having a notion of the level of molten cleanliness is just a third of the solution to eliminate inclusion related scrap in castings. The second third of the solution is to establish a correlation between the inclusion defect (s) in the casting (s) and the inclusions present in the molten metal (molten cleanliness level). The last third of the solution is the implementation of the proper corrective action to eliminate the root cause and the continuous monitoring of the solution.
There appear to be two different schools of philosophy with respect to defining inclusion limits in molten metal for foundry applications:
1. The inclusion content in molten aluminum alloys has to be several volume parts per billion and the average particle size in the population can be no more than 50 µm (0.002 in. or 0.05 mm) to produce quality castings.
2. The extent to which inclusions render a molten alloy “unfit for use” is considered a function of the casting application and therefore of suitable molten metal practice. This notion is a very practical outlook from a foundry perspective.
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) in the casting are smaller than 60 µm (0.0024 in. or 0.06 mm). However, particles larger than 60 µm would not necessarily damage the quality of the castings. 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 µm (0.030 in. or 0.762 mm) and 1,524 µm (0.06 in. or 1,524 mm).
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.
It is not uncommon to find that more than 50% of the inclusion scrap defects that a foundry experiences occur after the machining operation. Such castings would be rejected because of poor machinability due to hard spots and/or because of failure to meet stringent cosmetic requirements on machined surfaces. Cosmetic requirements may cause a casting to be scrapped if inclusions are larger than 400 µm (0.0157 in. or 0.4 mm). This size is considered to be about the smallest defect that could be seen by the naked eye on a machined surface. Thus, many inclusions are only discovered when the inside and/or the outside customer complains.
The negative effect of inclusions in mechanical property evaluations is first noticed most commonly 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 100 µm (0.1 mm or 0.004 in.) start affecting mechanical strength and fatigue life.
Standard foundry melting and handling procedures, which are considered to be “good and sound practices,” include proper melting, degassing, fluxing, and refinement practices. Molten cleanliness evaluations show that molten A356, C355, A357, A206, and 319 aluminum alloys subjected to such practices will have inclusions (particles) from 20 µm (0.0008 in. or 0.02 mm) to 60 µm (0.0024 in. or 0.06 mm) suspended in the molten bath prior to casting. These particle sizes can be regarded not as making the melt unfit for use but rather as inherent and characteristic of the alloy and melting practice. Such particles would be normally dispersed throughout the casting.
Foundry melting and handling processes without the best degassing, fluxing, and refinement practices, will cause have inclusions (particles) over 60 µm in A356, C355, A357, A206 and 319 aluminum alloys.
Yet the question remains as to what is the effect of the different inclusion sizes present in the molten bath on the final composition, morphology and size of the inclusion(s) defect found in the casting(s)?
Molten Metal Inclusion Assessments and Inclusion Casting Defects
Molten metal cleanliness assessments with metal analyzers could provide the foundry with practical information about their melting process. If properly done, in conjunction with optical microscopy and SEM analysis, a metalcasting facility would be able to correlate inclusion defects in castings with molten metal quality and/or better understand where the inclusions in the castings are coming from. Any foundry that at least from time to time audits the melting process and/or uses such technology during inclusion casting issues (after properly identifying the inclusions in the castings) would have a strong knowledge that would facilitate finding the root cause of the inclusion defect.
An ultrasonic inclusion analyzer (Analyzer 2) can provide continuous cleanliness level measurements while particle size measurements are continuously measured. The instrument displays a graphical representation of the molten cleanliness as a function of time and a histogram of the various quantities, and the relative number of particles in each of 10 size ranges (from less than 20 µm to over 160 µm). By looking at the screen, one could monitor any change in the melting process and/or disturbance in the molten bath.
Another type of inclusion analyzer (Analyzer 1) provides batch evaluation, obtained at one particular point in time.
A detailed metallographic examination of the corresponding Analyzer 1 filter cross-section provides information on the overall metal cleanliness by inclusion content given in sq.mm/kg, oxide films (number/kg) and inclusion type given by sq.mm/kg and in percentage of the respective total content. Typical total inclusion counts have been found to vary from 0.037-8.1 sq.mm/kg and over 1,500 films/kg depending on an individual foundry’s melting practices. However, the higher inclusion count does not necessarily make the molten metal unfit for the casting process. In this case, the microstructure is used to reveal aluminum oxides.
As a closing argument to emphasize the importance of properly identifying inclusion defects in castings before blaming the molten metal right away after visually assessing scrap rejects, three different castings sections are presented. Each section is from a different metalcasting facility. Each of these sections is a representative sample of specific castings defects that foundry personnel visually identified as inclusions due to bad metal. Two of the defects were detected in the as-cast condition, and one after machining.
After the casting defects are detected, corresponding microphotographs from the SEM results for each of the defects will be given. Such information is enough to properly identify what type of defect the castings had and if one knows the process, the solution is obvious.
It is up to the metalcaster to decide what approach to use to solve inclusion defects in the casting. Sporadic outbreaks of high casting scrap rates due to inclusions indicate lack of understanding and/or constantly monitoring of not just the melting process but also the casting process.
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). The boundary in which inclusions (sizes, types, and concentration) render a molten alloy “unfit for use” is settled based on the casting process.
Having a notion of the level of molten cleanliness is just a third of the solution to eliminate inclusion related scrap in castings.
The second third of the solution is to establish a correlation between the inclusion defect (s) in the casting (s) and the type of inclusions present in the molten metal (molten cleanliness level).
The last third of the solution is the implementation of the proper corrective action to eliminate the root cause, and the continuous monitoring of the solution.
When dealing with inclusion defects or casting scrap issues, rules of thumb solutions must be avoided and replaced by careful and detailed analysis.
Molten metal cleanliness assessments with an inclusion analyzer could provide a foundry with practical information about its melting process. If properly done, in conjunction with optical microscopy and SEM analysis (from both samples: the molten bath and the casting defect) a foundry would be able to correlate inclusions defects in castings with molten metal quality.
A foundry request of “give me the cheapest and best flux for cleaning my melt” will not be the best driven cost saving solution to eliminate inclusion defects in castings if neither the inclusions in the castings nor in the molten metal are not properly identified, nor if the selection and application of the flux are not properly understood.
This article is based on a paper (17-106) originally presented at the 2017 Metalcasting Congress.