Benchmarking Aluminum Melt Treatment

By AFS Aluminum Division; Benjamin Groth, Brian Began

Click here to see this story as it appears in Modern Casting

Aluminum casting personnel often ask about best practices for grain refinement and eutectic modification in aluminum alloys. A wide variety of practices are utilized in the aluminum casting industry and many technical papers have been published, as well as multiple discussions conducted on the issue. Yet, debate continues regarding best practices for refinement and modification, particularly when it comes to strontium additions. And there have been few documented statistics on what practices are being followed in metalcasting facilities.

Two separate surveys, one 10-question survey on grain refinement and one 15-question survey on eutectic modification, were distributed electronically to AFS membership, specifically those categorized as casting aluminum. Forty-six members responded to the grain refinement survey, while 50 responded to the eutectic modification survey, with some overlap between responding members. Respondent demographics were similar between each survey.

It would be helpful to at least define the intended goal of each melt treatment.

Grain-refining: Generally speaking, a grain refiner will act on the size and structure of the grains within an aluminum casting, resulting in a finer grain size. This leads typically to reduced porosity and a reduction in the size of pores, as well as improved feeding and a reduced tendency for shrinkage formation. This is typically accomplished with the addition of titanium and boron in an aluminum master alloy.

Eutectic modification: Modification of the silicon is typically achieved through a strontium or sodium addition. The additive acts on the silicon in the alloy system, transforming the morphology of the silicon from long strands to small particles. The result of this transformation is an improvement in mechanical properties, specifically tensile strength and elongation.

Given below are the compiled results from each survey, with brief discussion for most questions. Some of the responses have been combined for brevity. This study can help aluminum melting personnel benchmark their operation to see how it compares to other in the aluminum casting facilities.

AFS Grain Refinement Survey Results
What kind of alloy does your metalcasting facility use?
Perhaps unsurprisingly, nearly 95% of the responding metalcasters are consistently casting A356 aluminum alloy. The 300 series alloys, in general, were the most cast by those surveyed (66%), with 500 series and 700 series alloys being cast as well. Some higher copper and zinc content alloys were mentioned as having been previously run within certain plants. Of those surveyed, almost half (47%) have steady jobs running four or more different alloys. This trend was across all plant sizes.

Which grain refinement techniques do you use? Which do you use most often?
The dominant technique for refining used today is controlled additions of cut “TiBor” rod (titanium plus boron master alloy). Seventy percent of the survey respondents said they use chopped rod on a daily basis as their most prevalent refinement method, with 10% saying they utilize purchased ingots with grain refining elements added to them. The latter was exclusive to metalcasting facilities smaller than 100 people, though 22% of all surveyed have made use of these ingots in the past. Other addition techniques mentioned but not used as often included: continuous additions with scrolling TiBor wire, TiBor “waffle” additions, grain refining fluxes or pills, and use of a combined strontium plus titanium master alloy. Of those surveyed, 7% said they did not grain refine at all.

How do you test for grain refinement effectiveness?
Multiple methods exist that help the metalcaster determine whether the refiner additions have had an effect on the alloy. These range from direct testing of mechanical properties, to indirect spectrographic analysis that look for titanium or boron content within the alloy. Of the methods available, spectrographic analysis of titanium is used by 54% of those surveyed, followed closely by testing of mechanical properties at 43.5%. The larger facilities surveyed (greater than 250 employees) tend to test for effectiveness via multiple methods, with 60% using three or more methods compared to only 20% in plants with less than 250 employees. Additionally, larger metalcasting facilities focus on the use of metallography, thermal analysis, and the “hockey puck” technique while smaller plants will more readily use spectroscopy, go by mechanical properties, or do no testing at all (22%).

In most cases, when do you add grain refiner relative to other additives? 
The addition of a grain refiner relative to other additives was seen to be quite process dependent, with many responses showing a variety of timelines. Half (50%) of those surveyed said they added grain refiner at the same time or after additions of both magnesium and strontium, while the rest offered some variation of before “X” but after “Y”. “Other” responses to this question included making additions 15 minutes into the degassing cycle, continuous additions by hand rather than coil, using only refining pills, and making additions to the ladle at the time of pour.

When do you add grain refiner compared to other treatments (degassing/fluxing)?
A near equal split was seen in responses with regard to grain refiner addition before, during, or after degassing and fluxing. 31% of respondents said they add before, 27% during, and 24% after, with the remainder either adding continuously throughout production or they do not degas or flux (14%). While this survey does not necessarily define best practices, it would be interesting to know what sort of yield or effect is seen among these metalcasting facilities based on these differences in process.

Why do you add grain refiner?
The primary reason stated for adding grain refiner is to reduce grain size, leading to an increase in mechanical properties (70%). Improved flowability and better heat-treatability each were cited as primary reasons for 10% of respondents, with only 2% citing a customer requirement for refiner additions. Interestingly, 5% of respondents said they did not know why they added the refiner, but it resulted in better castings.

Approximately how long before pouring is the grain refiner added?
Most metalcasters are in agreement when it comes to the timing of additions relative to pouring, saying refiners should be added between 0 and 15 minutes prior to casting (70%) and no earlier than 30 minutes before casting (92%).

When asked to provide one tip about grain refining, some respondents answered as follows:

“The most important thing about grain refining is the amount of lapsed time between the addition made and time of pour; about 45 minutes beginning to end.”

“Do not over grain refine. Keep Ti levels below 0.12% in A356 alloy but above 0.10%.”

“All manufacturers are not equal; you get what you pay for.”

Additionally, respondents indicated it is best to make sure the part being cast needs the addition; simple, non-structural components may not require refiner additions. Small additions at the proper time and in the proper location (ladle vs melting furnace) would help deter any particle buildup and avoid “over refining” the microstructure with larger additions at one time. Finally, know your source of refiner regardless of the method in which it’s introduced.

AFS Eutectic Modification Survey
A similar distribution of alloys being cast was seen from respondents to this survey as in the grain refiner survey.

Which eutectic modifier do you use most often?
Of the response options to this question (Sr, Na, Sb, CuP, Other), 100% of the survey takers that use a modifier said that they use strontium. Sodium is an effective modifier at low addition rates, but fades exceptionally fast. Antimony is more widespread in European metalcasting facilities and shows little sign of fade even through re-melting, but has the effect of poisoning any additions of Sr or Na that are subsequently made. This makes it a tricky additive unless highly controlled. Several respondents who cast hypereutectic aluminum-silicon alloys utilize copper phosphate, but this was a small selection (1%) of respondents. Fourteen percent of respondents said that they do not perform any eutectic modification.

What eutectic modification technique do you use most often?
Eutectic modifiers are available in many forms similar to the commonly used grain refiners, and like the grain refiners, adding cut strontium rod is the most prevalent addition technique used by the survey takers (51%). Strontium rod is typically available as a master alloy, AlSr10, with Sr15 also available for higher addition rates per kg of rod used. Again, similar to the grain refiner the next most commonly employed method of eutectic modification is through the purchasing of ingot with eutectic modification elements added to them (21%). Unlike with grain refiners, these practices appear to be independent of foundry size.

How do you test for eutectic modification effectiveness?
Much of the same tests used to test the effectiveness of grain refiners can also be used for testing eutectic modification effectiveness. Spectrographic analysis for strontium is the most widely used method in this case (56%) followed by mechanical properties testing (40%). This was true for all metalcasting facilities, regardless of size. Half (50%) of the larger plants surveyed are using three or more test methods to check for effective modification, compared to only 5% within smaller companies. Of the respondents using eutectic modifiers, 26% of them do not test at all for effective modification.

When do you most often perform modification relative to other additions? 
Responses to this question were similarly varied to the grain refiner relative addition question.  With eutectic modifiers, 60% of respondents said they add them at the same time or before Mg and Ti additions. Figure 3 shows the complete breakdown of responses. One respondent made the comment that “our additions are relative to the end product, the casting’s process, the furnace type, and a host of other variables.”

When do you add eutectic modifiers compared to other treatments (degassing/fluxing)? 
In the case of modifying the silicon eutectic with respect to degassing and fluxing, the most common practice is seen to be “after degassing and fluxing”, with 50% of surveyees responding as such. Making additions before degassing or during degassing were close, with 21% and 18%, respectively. Of the respondents, 11% said that there was no definitive chronology to the addition, with respect to degassing. A follow up question indicated that 70% of the foundries will stick to the same processing chronology, with 25% saying they will add after degassing and fluxing as a variation to their standard procedure.

Approximately how long before pouring is the eutectic modifier added?
As with grain refiner additions, the majority of metalcasters (90%) responded that modifier additions should be made within 30 minutes of pouring.

When asked to provide tips on adding eutectic modifiers, respondents had this to say:

“Monitor additions, it’s easy to add too much strontium (and you CAN add too much). It needs time to take effect, but will also fade over long periods of rest.”

“Add after fluxing, but before degassing so the flux does not remove the strontium, and the degassing can remove added gas from the strontium.”

“Experience with modification is limited, foundry personnel don’t know why they add Sr.”

The last few questions on this survey were open ended questions with regard to using strontium as a modifier. Specifically, those that do not, why not? And those that do, why? For those that do not, the majority said that it was just not required for their castings, while others saw no benefit from the additions or negative effects in the form of too much porosity. Some even answered that they didn’t know why not, it just hasn’t been done in their plant.

For those that add strontium as a modifier, 7% add based on customer requirements, 30% find it helps counteract shrink by adding in gas porosity, 13% are adding for the improved flow-ability, heat-treatability, mechanical properties benefit, or a mix of similar reasons that they see within the end casting. The rest, 48%, cited “modifying the eutectic” as the main reason. While this is clearly the goal of a eutectic modifier, what seems to be unclear to the respondents is what that actually means in terms of the resulting casting. Perhaps it stems from a lack of information on the “best practices” for the metalcaster and relying on experience and “as it’s always been done” mentality. In either case, seeing what procedures and practices others in the industry are following may help those who are curious about improving their practices find a place to start.