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Theory and practice of grain refining for aluminium alloys

Brian Began, Foseco USA & Pascaline Careil, Foseco Europe

The need for smaller grains is vital to achieving the required properties when pouring most cast aluminium alloys. Whether the desired results are high mechanical properties, leak free castings, a cosmetic appearance or improved structural soundness, smaller grains are impactfully beneficial. Accordingly, there is a desire to improve both grain refining and the ability to quickly and effectively assess grain refinement effectiveness. This article discusses both the need for smaller grains and the principle fundamentals of grain refining. Moreover, it reviews commercially-available grain refiner forms and currently available methods for assessing grain refinement. The article also introduces a new and improved flux form grain refiner (COVERAL* MTS 1582) and documents two recently successful case studies where this was utilised to improve castings in both a low pressure wheel foundry and a high production sand moulding foundry.

 

Introduction

Grain refining is an essential part of the aluminium casting process, with the aim of reducing the size of primary aluminium grains during the solidification phase. This process has many benefits for most hypoeutectic aluminium alloys as it: improves feeding, elongation and mechanical properties, increases resistance to fatigue, improves casting machinability, reduces hot tears, helps disperse micro-shrinkage, decreases the size of porosities and reduces thermal treatment cycles. Historically, grain refinement has been achieved using master alloys, with the most commonly used grain refiner mechanism involving the release of titanium diboride into the melt. Grain refining is especially important in aluminium foundries using investment, sand, gravity die, or low pressure diecasting processes due to the potential for delayed cooling and complex casting designs with varying section thickness. In general, those castings with slower cooling rates and larger variation in casting thickness, require grain refinement more than other casting designs.

There are several casting segments where grain refining is critical including:

·         Wheel foundries where grain refinement and cleaning are crucial for achieving the required feeding and cosmetic surface finishes of the casting.

·         Safety critical automotive castings such as suspension parts, turbochargers, and brake components which require good fatigue properties.

·         General automotive castings like cylinder heads, engine blocks, manifolds in gravity diecastings where an intermediate level of grain refinement might suffice for the mechanical property requirements, but the improved feeding from grain refinement helps prevent leaks.

·         Aerospace and military castings requiring high mechanical properties for difficult applications, where grain refining is highly beneficial.

·         Sand and investment castings where the long solidification times cause large grain growth and difficult feed paths without optimised grain refining.

 

Grain refinement mechanism in aluminium alloys

Target of all melt treatment procedures is an improvement of mechanical properties

Grain refinement affects the α-mixed crystal in the alloy. At decreasing temperature those α-mixed crystals grow. Grain size depends on cooling rate during solidification. The addition or formation of nuclei increases solidification speed and decreases the grain size.

Master alloy and chemical products comparison

Considerations when using master alloy grain refiners are:

·         TiB2 nuclei are pre-formed in an aluminium matrix.

·         Easy to apply.

·         Risk of oxides or impurities in the rod or waffle.

·         Moisture and oxides on rod surface contaminate melt.

Benefits of chemical products are:

·         Contain metallic titanium and boron salt or titanium and boron salt.

·         TiB2 nuclei are in-situ formed in the melt – fresh surface – higher surface energy and lower θ angle.

·         No risk of impurities.

·         Additional cleaning effect.

Reasons for better mechanical strength with chemical products

Several reasons for achieving better mechanical strength with chemical products were identified as follows:

·         Chemical products and master alloys with pre-formed nuclei impact the contact angle θ differently.

·         θ for TiB2 = 60° is a theoretical value for an ideal nucleus.

·         θ for TiB2 from master alloys is significantly higher due to reduced surface energy.

·         θ for TiB2 from chemical products is close to 60° or even below due to fluxing effect from chemicals (fluorides).

 

COVERAL MTS 1582

Foseco has developed a novel granulated flux COVERAL MTS 1582 that is capable of both grain refining and cleaning aluminium alloy melts. COVERAL MTS 1582 is highly concentrated in titanium and boron which form both titanium diboride and aluminium boride in situ leaving fresh nuclei within the aluminium melt. These finely dispersed species are highly efficient nuclei that promote a fine equiaxed grain growth during solidification.

In addition to strong grain refining, COVERAL MTS 1582 flux is also a very good cleaning product that will react to remove oxides and inclusions from the melt. No additional cleaning/drossing flux is required, resulting in lower overall process costs. COVERAL MTS 1582 is a sodium- and calcium-free granulated flux suitable for all types of aluminium alloys except hypereutectic alloys but including those alloys containing large amounts of magnesium.

Application of COVERAL MTS 1582

COVERAL MTS 1582 is specially designed for use with Foseco’s MTS 1500 rotary degassing and melt treatment equipment, whereby controlled flux additions are made directly into a melt vortex and mixed vigorously. PLC controlled additions of the treatment flux are added into the vortex and mixed to complete reaction prior to the vortex breaker baffle board re-engaging the melt, effectively stopping the vortex. After the vortex has been stopped, the MTS completes a standard rotary degassing process and the treated metal in the ladle or crucible is used for transferring and/or casting.

For further information on the MTS 1500 process, review Foundry Practice Issue 247 (2007) or the Foundry Practice Special Edition for CastExpo 2008 (available from Foseco). Both issues feature articles on the MTS 1500 technology(2, 3).

MTS 1582 should be used with the melt at a temperature higher than 720°C. The reaction by-product from this treatment produces an extremely dry ash-like dross that is easily separated from the liquid metal with a coated skimmer or similar tool.

 

Evaluating grain refinement effectiveness

Since grain refinement is critical to achieve the desired properties of aluminium castings, it is important that there are methods for assessing grain refinement effectiveness. The most common methods for evaluating grain refinement effectiveness are as follows:

·         Elemental spectroscopy

·         Thermal analysis

·         Microstructural evaluation

Elemental spectroscopy

Elemental spectroscopy is perhaps the most commonly employed method for assessing grain refinement, but it is also the least effective of the three methods listed. Spectroscopy only determines the total concentration of an element – however titanium is usually present in other forms and phases in addition to TiB2 and these other phases do not impact grain structure. Foundries will measure Ti into the alloy range (typically 0.10-0.25 per cent by weight) and assume that because they are in range, they are achieving sufficient grain refinement. Consequently, given this issue, some foundries will also measure boron (typical range 5-25ppm) as an additional control. Tight controls of Ti and B do typically result in effective grain refining; however, more advanced methods like thermal analysis and microstructural analysis ensure higher probabilities of optimised grain refinement.

Thermal analysis

Thermal analysis is perhaps the fastest growing method for assessing grain refinement as it is quick and more accurate than elemental spectroscopy. The THERMATEST* 5000 NG III (pictured in fig.5) is a widely used thermal analysis unit used to quickly and accurately assess grain refinement effectiveness in aluminium alloys. Thermal analysis involves collecting data of temperature versus time of a solidifying melt sample and comparing the curve to a set of known reference curves algorithmically. The THERMATEST 5000 NG III unit’s algorithm analyses the sample curve liquidus and computes a score on a scale from 1-9 for evaluating grain fineness (GF). A score of 1 references a curve that compares with curves exhibiting no grain refining.

In contrast, a GF score of 9 is achieved when the sample curve compares with those curves known to have produced ‘perfect’ grain refining of melts with the same alloy composition. A pictorial representation of the THERMATEST 5000 NG III grain refinement levels is provided in fig.7. Of note, THERMATEST 5000 NG III unit also provides the additional benefit of helping to assess eutectic modification effectiveness in Al-Si alloys(4, 5).

Evaluation of grain size with thermal analysis

For a given cooling speed, the size of the grain depends on the amplitude and duration of the undercooling, which appears at the formation of primary aluminium crystals.

·         When the undercooling is high and duration medium (fig.6a), grain size is coarse.

·         When there is no undercooling (fig.6b), grain size is very fine.

·         When undercooling is low but duration is high, the grain size is very coarse.

THERMATEST 5000 NG III measures the following liquidus parameters:

·         Temperature θ2 (°C).

·         Undercooling Δθ (°C).

·         The duration of undercooling t1 (in seconds).

Grain refinement is considered fully optimised when the undercooling is nil and grain size index is equal to 9. However, for certain alloys and thin shaped castings in permanent moulds, a lower grain size index (5-9) would be expected and is acceptable due to the higher cooling rate with permanent diecasting.

Setting a minimum grain size index for each casting is recommended, correlated with desired elongation of mechanical properties. For Al-Cu5%MgTi alloys, the absence of undercooling may not be sufficient to avoid hot tears. A stronger grain refinement is recommended to improve the alloy’s performance.

Liquidus curves: comparison of TiB rods with COVERAL MTS 1582

The lower the undercooling at liquidus, the stronger the grain refinement. COVERAL MTS 1582, at much lower addition rate (0.1% vs 0.2% for AlTi5B1 rods), performs better compared to AlTi5B1 rods.

Optical microscopy (Barker test)

Optical microscopy is the final methodology employed by foundries to assess grain refinement. It is considered the most representative method for assessing grain refinement but is time-consuming and resource intensive. Optical microscopy involves grinding and polishing test specimens to microscopic levels to be evaluated for grain size under a microscope. One popular method is the Barker test. The LectroPol-5 from Struers is used for electrolytic etching with Barker reagent consisting of a 5 per cent tetrafluoroboric acid in distilled water. The sample to be tested acts as an anode in a galvanic cell, which removes material from the sample surface and an anodic layer can be formed. With the Barker method, under a polarised light, a coloured representation of the grain structure of aluminium materials is achieved. It is possible to carry out microscopic testing with up to 1000x magnification.

 

Case studies

With COVERAL MTS 1582

1: European foundry

A European wheel foundry was interested in improving its melt treatment practices by utilising COVERAL MTS 1582 with a FDU featuring MTS 1500 technology. This wheel foundry pours a standard AlSi7Mg alloy and historically performed grain refining by making manual additions of TiBor rod into a transfer ladle during degassing. It was the foundry’s target to automate the grain refining process while capturing the typical benefits (drier dross, lower spend, smaller grain) achieved when grain refining with COVERAL MTS 1582. The treatment parameters of the new process featuring COVERAL MTS 1582 can be found in Table 2.

After the new process grain refining was implemented, pictures were taken of the ladle dross (fig.11), thermal analysis curves (fig.13) and microstructures (fig.12).

2: American foundry

Littlestown Foundry is a sand and low-pressure (LP) mould aluminium foundry in Littlestown, Pennsylvania in the USA. The main alloy poured by the foundry is a standard 356 alloy (AlSi7Mg). In the sand foundry, Littlestown makes some difficult castings that are subjected to pressure testing with air to ensure they are leak free for application. After reducing scrap through improved grain refining in the LP foundry from 13.6 to 2.7 per cent by converting from metallic TiBor (10%Ti, 1%B) to COVERAL MTS 1582, a similar project was undertaken in the sand foundry. The objective was that by improving the grain refining using COVERAL MTS 1582 – introduced via an MTS 1500 unit – in place of metallic TiBor rod, the sand foundry would see similar benefits in the form of reduced leakers and lower spend.

The first part of the project involved using a THERMATEST 5000 NG III unit to assess the incumbent procedure and then developing an optimised procedure using the MTS 1500 and COVERAL MTS 1582. The results of the THERMATEST 5000 NG III evaluation are presented in Table 3.

The THERMATEST 5000 NG III evaluation confirmed that the metallic TiBor rod was successful in raising the grain fineness value from insufficient (5.8/9.0) to an improved and more acceptable level of grain refining (6.8/9.0). However, the THERMATEST 5000 NG III unit also confirmed that a huge improvement to a fully optimised level of perfect grain refinement (9.0/9.0) was possible with the COVERAL MTS 1582. Hence, mechanical test bars were poured and evaluated to assess any potential impact of the new process featuring the COVERAL MTS 1582.

The results of the mechanical testing evaluation are shown in Table 4. The results exhibited positive improvement in all three metrics evaluated, i.e, ultimate tensile strength (UTS), yield strength (YS) and % elongation. Accordingly, the decision was made to convert to the new process to make a full assessment of the new process featuring COVERAL MTS 1582 and an FDU featuring MTS 1500 technology.

Finally, after four months in production, the new process change was evaluated economically. The following economic benefits were achieved after implementation:

·         Reduction in annual projected spend on grain refiners and cleaning flux by $276 per day, $1,380 per week, $5,750 per month or more than $69,000 per year.

·         A ten-fold reduction in projected impregnation costs from a starting point that exceeds $1,500 per month to less than $150 per month.

·         The calculated payback for the MTS 1500 unit when factoring in the lower grain refining spend, the lower flux cleaning flux spend and offsetting it with the slightly higher spend on filters is just over six months.

A full peer-reviewed paper (paper #19-015) on the Littlestown case study was published by the AFS in its 123rd Metalcasting Congress Proceedings in April 2019 and is available for a more extensive review.

 

Summary and conclusions

COVERAL MTS 1582 is a universal grain refining and cleaning flux for treating aluminium alloys. It forms in situ aluminium boride and titanium boride which are the most suitable nuclei, within aluminium melts. Creating TiB2nuclei in situ is more effective than releasing pre-made TiB2nuclei into a melt.

Elemental spectroscopy, thermal analysis with a THERMATEST 5000 NG III and optical microscopy are three methods for assessing grain refinement effectiveness within a melt; the latter two methods being the most efficient. Experience in both a low pressure wheel foundry and high production greensand foundry has confirmed the benefits of superior casting mechanical properties and lower overall process costs when grain refining using COVERAL MTS 1582 through an MTS 1500 unit.

 

References

1.     Samsonov G, Panasyuk A, Kozina G, Poroshkovaya Metallurgiya, Nr11, pp42-48, 1971.

2.     Careil P, Simon R. ‘MTS 1500 Automated Metal Treatment Station’. Foundry Practice Issue 247. p15-20, June 2007.

3.     Careil P, Simon R. ‘MTS 1500 Automated Metal Treatment Station’. Foundry Practice Special Edition for Cast Expo 2018. p1-6, May 2008.

4.     Stonesifer J, Began B. ‘Degassing and flux grain refining in a continuous well at Littlestown Foundry’. AFS 123rd Metalcasting Congress Proceedings. Atlanta, GA. American Foundry Society (2019).

5.     Careil P, Kientzler P. ‘Thermatest 5000 NG III: thermal analysis equipment designed to predict and control the structure of aluminium alloys before casting’. Foundry Practice Issue 250. p2-6, September 2008.

Contact: Paul Jeffs, UK technical manager, Vesuvius UK Limited – Foseco Foundry Division, Tamworth, Staffordshire B78 3TL UK. Tel: +44 (0) 1827 289999, email: paul.jeffs@vesuvius.com web: https://www.vesuvius.com/en/our-solutions/international/foundry.html

* COVERAL and THERMATEST are Trade Marks of the Vesuvius Group, registered in certain countries, used under licence.

For copies of the figures referenced in this article, refer to the full printed version of the September 2019 issue.