Aluminum gravity casting production continues to grow, in many cases replacing ferrous materials, with increasing demand in industrial sectors such as energy, defence, medicine and transport. The reduction in the weight of the components is a very attractive point, and the latest technological advances, which have helped aluminium alloys to provide more mechanical strength, has opened up even more opportunities for cast aluminium components, say Enrique Pardo and Luis Merchan of Foseco Iberia.
This article focuses on the partnership between Foseco and Spanish aluminium foundry Grupo Aluminio de Precision (GAP) to optimise casting methodology to better serve customers.
The majority of aluminium gravity cast components for industrial applications require a very high level of quality control and quality assurance procedures. To ensure sound castings it is essential that strict procedures are followed in the casting methods (feeding, filtration and simulation), manufacture and preparation of the moulds and cores and the pouring of molten metal, with its special treatments, to obtain final castings without inclusions, defects or shrinkage. Aluminium components produced by gravity casting are manufactured in medium or large series, where if a defect appears, it affects many parts and directly incurs an increase in manufacturing cost. Therefore, it is essential that within all parts of the process the correct raw material choices are made, and these decisions are critical to the successful and repeatable production of defect free castings and must be combined with the correctly applied application, technical and manufacturing process knowledge.
Introduction
Grupo Aluminio de Precision (GAP) is an aluminium gravity, sand foundry based in Burgos, Spain, producing castings ranging from 0.2kg up to 300kg using either the greensand or no-bake moulding process. Production capacity is in the region of 12,500 tonnes per annum and castings are supplied to a diverse range of market sectors on a global basis. To serve these demanding industries, GAP is providing its customers with a full supplier service, from casting design and development to production and delivery of fully machined, painted and tested cast components. Throughout the design and implementation stage, GAP works together with Foseco to optimise the casting methodology and subsequently utilises the superior properties of Foseco consumable products to optimise casting quality, improve yield and reduce overall production costs. Here we focus on the partnership between Foseco and GAP throughout the development process and across several internal departments, and in doing so demonstrate the value created by undertaking such close co-operation.
Design and development
When a new casting is to be produced in the foundry Foseco’s involvement starts in the design and development stage. The main criteria being how to produce a sound casting without defects and with optimum yield to ensure cost-effectiveness. Initial discussions focus on the method of producing the casting; how the metal will flow into the casting with minimal turbulence, where filters will be placed to reduce turbulence and eliminate inclusions and where to place feeders / feeding aids to ensure there is no shrinkage in the casting itself. There are many variables to consider, including the orientation of the casting and the subsequent implications on moulding and core making.
Typically, there is more than one potential solution and by using metal flow and solidification modelling these solutions can be evaluated and optimised to focus in on the most cost-effective method of production, whilst delivering the customer’s quality and integrity requirements. It is at this stage that many of the consumable products that enhance the casting process will be selected to optimise the casting process, such as:
- KALMIN* insulating sleeves – to prevent shrinkage defects.
- SIVEX* foam filters – to remove inclusions and reduce turbulent metal flow.
Moulding and coremaking
After the design stage is complete the required designs are passed to the pattern workshop where skilled patternmakers will produce the pattern equipment to make the moulds and the coreboxes for core production. The high-quality production of pattern tooling is essential to the casting process, in that any discrepancies at this point will be reflected in the ease of production of the moulds and cores and subsequently in the dimensional accuracy of the final casting.
For the production of cores GAP has a highly automated manufacturing line, which allows the mass production of cores without compromise of integrity or dimensional accuracy. The sand cores are bonded using a special polyurethane coldbox binder system supplied by Foseco (POLITEC* AL320/420). The benefit of this system is that it requires very low addition rates to give high strength cores. The low binder addition rate minimises the production of gases during thermal decomposition on pouring, thereby avoiding the potential for gas pinhole defects at the surface of the finished casting. To improve productivity, the coreboxes are coated with a highly effective release agent (ACMOS* 118-63) that has superior release properties without the build-up of residues that need to be removed on a regular basis, taking the corebox out of service. Where cores are joined together a refractory putty (TAPA PLAST 41) is used to seal any gaps or small breakages. To further enhance the surface finish of the casting and to avoid sand adherences, the cores are coated with a refractory coating from Foseco (TENOTEC* 7804A). After casting the binder breaks down readily, allowing for the easy removal of the core material during the shake-out process, ensuring no damage to the casting. The casting surface finish is free from sand adherence, gas pin-hole defects or any other metal/mould reactions. GAP uses two modern moulding lines; the first utilises a no-bake binder system based on polyurethane chemistry (POLISET* AL 6A & 6B), for high dimensionally accurate castings up to 300kg in weight.
The POLISET AL binder system has been specifically developed for the needs of aluminium sand foundries to reduce gas emissions, provide high production rates through reduced stripping times and easy core breakdown and shake-out after casting. The second line is for smaller castings up to 80kg in weight and is based around a greensand system. To avoid sticking of the greensand to the pattern plates, small amounts of a highly effective release agent (PARTISAL* 421) are sprayed on to the pattern at regular intervals. PARTISAL is an environmentally friendly product without aromatic solvents.
Meltshop
The quality of the molten aluminium alloy is paramount to producing consistent castings to the correct metallurgical specification and free from defects such as inclusions and gas porosity. GAP uses a Foseco degassing unit (FDU) melt treatment system for the degassing and cleaning of the aluminium alloys. The FDU rotary degassing units operate patented rotor designs to ensure rapid and efficient distribution of nitrogen or argon as finely dispersed bubbles to ensure effective degassing in short treatment cycles. Advantages are:
- Reproducible results.
- Short treatment time.
- Reduced gas porosity and hard inclusions in castings.
- Reduced machining costs.
- Consistent mechanical and physical properties.
- Environmentally friendly.
The benefits of best practice in terms of melting, melt treatment and pouring of the molten aluminium are observed in the non-destructive testing laboratory with the cast materials having optimum metallurgical structures and soundness. Foseco supplies testing equipment and consumables that support the quality assurance procedure such as GASTEC PRO and DENSITEC devices for measuring density. GASTEC PRO allows for the creation of a partial vacuum (up to 10mBar) under one glass bell, to highlight the presence or absence of dissolved hydrogen, which has a negative influence on the tightness and mechanical characteristics of aluminium castings. DENSITEC provides a direct measurement of density, a ticket can be printed, or values are recorded to a computer for traceability.
GAP is also using Thermatest 5000 NG III thermal analysis equipment. These units are designed to predict and control the structure of aluminium alloys before casting, including the grain refinement and the type of eutectic structure that will be formed. Within only a few minutes, it assesses the melt quality, allowing for specific additions to be made to the melt before casting, therefore avoiding costly scrap due to shrinkage, leakage, porosity, and hot tears.
Conclusion
Foseco offers the aluminium foundry a full range of equipment and consumables that support the production of quality castings. In the sand foundry, binders, coatings and moulding materials supplied by Foseco ensure the integrity of the moulding process and enable complex castings to be manufactured with superior surface finish. The preparation and transfer to the mould of the liquid aluminium is equally important and again Foseco products are used to improve metal cleanliness and metallurgical structure, with product performance ensuring energy costs are minimised and casting quality is maximised. The benefits of this focus on process and consumable technology is observed in the high quality, defect free castings produced, with Foseco developed test methods providing the end user with quality assurance data. Above all the benefits of the products themselves, it is the relationship with foundries such as GAP that elevates performance to a higher level. By working in partnership to deliver ‘best in class’ solutions and creating value for the end user, a sustainable relationship is developed that ensures future success for all parties.
Contact: Paul Jeffs, UK technical manager, Vesuvius UK Limited – Foseco Foundry Division, Tamworth, Staffordshire B78 3TL UK. Tel: +44 (0) 1827 289999, email: [email protected] web: www.foseco.com
* KALMIN, SIVEX, POLITEC, ACMOS, TENOTEC, POLISET, PARTISAL are Trade Marks of the Vesuvius Group, registered in certain countries, used under licence.
For copies of the figures from this article, refer to the full printed version in the July/August 2020 issue of Foundry Trade Journal.