Primary Al-Alloy Bulk Production by Retrofitting EMS on Furnace Wall: Part One
by Dr Pradeep K. Maitra (former GM QA & Head R&D, BALCO) LM-IIM, Bhilai Chapter, India
The production of bulk aluminum with increasing batch sizes was driven by an increasing demand and so attempts were made for example with A 356.2, to increase regular batch sizes from 5t to 60t.
Some key issues arose with this increasing batch size including an inconsistency relating to the chemical composition and poor processing capability through issues relating to force stirring.
Abstract
After economic liberalization, the modern Al-smelters and new vehicle production units were started in the country. Consequently the demand of premium grade cast alloy (AlSiMg) was also increased for making lighter stronger & durable auto components for fabrication of alloy wheels, chassis, engine blocks, air frames etc. Looking to the great demand of Al-alloys in large quantities in India & abroad and also to develop indigenous resources, attempts were made, to economically produce A 356.2 alloy in 60t melting cum holding furnace (HF), instead 5t induction furnace. The main hindrances experienced in such attempts were; chemistry inconsistencies, low production & productivity due to the lack of force stirring facility in HF, for rapid dissolution & full recovery of expensive alloying elements. In western countries such problems are resolved by Electromagnetic Stirring (EMS), and there are several primary-Al industries engaged in the bulk production of AlSiMg alloys worldwide, but in developing countries EMS are yet to be recognized as essential cast house equipment. However in recent times, the plausibility of retrofitting EMS system in existing HF of primary Al-smelters is being actively considered. In above context, an attempt has been made to describe the global quality demand of primary AlSiMg alloy used in transport industries and outcome of familiarization studies made for retrofitting EMS in large melting cum HF for bulk production of consistent quality Al-alloys.
Introduction
The primary-Al industries have always tried to develop and optimize both wrought & cast Al-alloys, for the Defense, Space research, Transportation sectors (road, rail, marine, & air) etc. During 1980s BALCO produced primary Cast Al-Alloys as per BS 1490 in induction furnaces (IF) and subsequently the making & trading of cast alloys was handed over to the secondary producers, as a result many minor & small scale Al-foundries appeared all over the country. In the 1990s several commercial & strategic wrought Al-alloys including Afnor-7020 for defense, aeronautics & space research, were developed in melting & holding furnaces by mechanical stirring but with low productivity.
During the past decade the capacity of Al-smelters grew phenomenally & to cope with the metal flow, the capacity of HF was also increased to three to four folds and even more, and the availability of cast unalloyed Al-ingots also increased, as a result the surplus Al has to be exported, provided it is in conformity with the global standards.
In the past Al-Si-Mg cast-alloy components produced from secondary-Al, suffered from low ductility due to the presence of tramp impurities. Since the mechanical properties of the heat treatable machine components are significantly influenced by the quality of the base metal, the demand of primary-alloy ingots also increased and were progressively employed in production of aerospace, automotive and biomedical components and a new market appeared before primary-Al producers for selling cast Al-alloy instead unalloyed ingots, for durable & safe machine part manufacturing.
In order to meet the rising demand, premium grade alloys, the alloying elements are required to be dissolved quickly in large HF by force stirring of the melt. To comply with the requirement, the retrofitting of EMS in existing HF is becoming inevitable for primary Al-producers. The metal purity, casting facilities and logic behind the EMS retrofitting is described below.
1) The Primary Metal Quality - During recent times the quality of primary metal has improved significantly in the country. The common impurities in the smelter hot-metal are %Fe between 0.03 – 0.20 and % Si between 0.02 to 0.15 %. The aggregate metal purities are classified as follows:
1. High purity Al ≥ 99.86% ... [~10%]
2. Commercial purity Al between-99.0 to 99.85% ... [~85%]
3. Low purity Al < 99% ... [~5%]
2) The Casting Facilities - In cast house the liquid metal is poured into HF installed in series (in tandem with their respective casting machines), for production of cast bodies in the following shape and weight proportionalities; Flats for Rolling-(~50%), Rounds for Extrusion-(~10%), Rods for Wire drawing- (~20%) and Ingots for Re-melting-(~20%) for making downstream semi-finished products.
"From above metal planning data, it may be logically said that >20% ingots are least valuable in comparison to other products and any attempt to convert unalloyed ingots into value added alloy ingots is always be welcomed"
The Background of Bulk Production
In general the actual end users of cast-Al alloys use small furnaces for the remelting of alloy ingots and consume small tonnages where as in primary Al industries, the small tonnage of alloy production is uneconomical because of lower capacity utilization. The metal distributors also prefer to receive bulk supplies from few reputed sources instead multiple supplies from small producers because of the following:
i) According to the metal planning the schedule for hot metal arrival before HF should match with the alloy casting cycle. The production of alloy in bulk is advantageous when dissolution of alloying elements are fast, since charge preparation & process control activities are common irrespective to the quantity of alloy produced. Further the alloy is produced in one stroke, with one inspection/testing & traceability documents.
ii) The bulk alloy buyers distribute the metal among component producers. The residual alloy ingots in the stock yard, if it happened to be from different cast numbers then it creates a problem for certification & distribution. Further after sale services, record keeping & custom clearance is convenient with single cast numbers with large tonnage.
iii) It facilitates the end users to hold technical transactions with alloy producers & melt correction becomes simple since chemistry does not fluctuate.
Genesis of A 356.2 alloy
Cast Al alloys are cost-effective, due to their low melting point & excellent cast-ability; in spite of little lower tensile strengths than wrought alloys, hence it is possible to cast components of complex shapes, in "poorly fed sections" with acceptable mechanical properties. Among Al-cast alloys Al-Si alloys are most versatile, comprising 85% to 90% of the total aluminum parts cast for the transport-industries. Few pertinent aspects of Al-Si alloys relevant to the present discussions are given below.
1) The Al-Si alloys based on Si content, fall into three major categories: hypereutectic (Si>13 up to 25%), eutectic (Si>12 up to 13%) and hypoeutectic (Si<12 %),
2) Commercially three types of hypoeutectic-alloys are produced viz (Al-Si-Cu) & (Al-Si-Mg/Cu), & (Al- Si-Mg,); among these the alloy Al- Si-Mg, with low- to medium-strength & good corrosion resistance, has the bulk share in production of automotive & aerospace vehicle components.
Commercially the popular hypo eutectic Al-Si-Mg cast-alloys can be produced either from: Primary metal (code A356) or from Secondary metal (code 356). The buyer’s choice between the Primary & Secondary alloy ingots depends on the end uses like [Sand castings; Die castings (Low pressure, High pressure, Gravity, Vacuum. Squeeze, Semi solid casting); Lost foam castings; Lost wax castings etc.], design parameters and production technology employed by them for component making.
In India the cast Al-alloys are generally produced by secondary Al-producers in rotary oil fired furnaces by melting Al-scraps which invariably contains some attachments. Consequently the quality often found to be not enough for high-tech applications. The production of alloys from above two sources involves different processing techniques and product specifications. Alloy A356 with low impurity level & higher degree of metal cleanliness, has higher strength and ductility than 356. A few applications of A 356 alloy are discussed below;
a) Because of above quality realizations, the high-tech industries are demanding premium quality Al-alloys for producing alloy wheels, engine blocks, chassis etc. An engine block is the heaviest part of the vehicle, A356 alloy benefits not only from the lower density compared to cast iron, but also from the high specific modulus of elasticity and better thermal conductivity. The engines like; Rover, BMW, Nissan, Jaguar, Mercedes, North star, Peugeot etc. established that engine blocks made of primary Al-alloys is quite suitable.
b) The alloy A356-T6 is frequently used in aerospace, inclusive of airframe castings, aircraft and missile components. Both A356 and A357 alloys are used in aircraft crankcases, gearboxes, housings supports, impellers for superchargers etc. Besides local buyers, the bulk demand of A 356.2 alloy aroused from North America during 2012 due to the declining primary Al-production in western countries. The buyers were intended to establish an alternative bulk supplier of primary Al-alloy initially with 5000 t/month. They wanted to get familiarized with the primary metal-quality & casting facilities available in primary Al-industries, the salient points of discussions with the buyer is given below.
Quality of Alloy A356.2
1] The molten Al-alloy is prone to lose Mg and pick up Iron, which can affect the mechanical properties, hence needs attention.
2] Buyers however concluded that, the primary metal quality & casting facilities available were found to be in conformity with the global standards, except force stirring facility in the holding furnace which is required to be addressed.
3] The buyers usually prefer alloy ingots with stricter control limits than stipulated in AA standard, as given under;
i) Fe-0.10%, Si- 7% & Mg- 0.4% with minimum standard deviations (very close to the mean),
ii) Sr-modified between (0.01 - 0.025%),
iii) Grains refined with TiB2,
iv) Melt filtration with ceramic foam,
v) Degassing & testing for Hydrogen content, the melt must be sufficiently free of alkali metals, and hydrogen gas.
The above control is necessary because; Fe reduces the ductility of alloy, in presence of Si forms compounds Al8Fe2Si (α-phase), Al5FeSi (β-phase) & Al8FeMg3Si6 (π-phase) which often show morphologies as either long needles or large plates, which drastically deteriorate the ductility of the alloys. The alloys are normally modified with Sr, it improves ductility which remain effective for many hours and through numerous re-melts. The TiB2 addition in melt aid grain growth, with smaller grain sizes. Ceramic foam filtration is used to remove non-metallic inclusions effectively. The chemical compositions of these alloys are given in Table-1.
Table 1: Showing chemical compositions of alloys A 356.0, A 356.1 and A 356.2 alloys