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Magnesium Alloy R&D Challenges – Aerospace Spinoff

S. Sundarrajan Defence Research and Development Laboratory (DRDL) Hyderabad, India

Abstract Magnesium alloy castings are emerging as major players in global business by replacing aluminium alloys in aerospace and automobile applications. International trend indicates upward demand for Magnesium alloy die castings. Indian aerospace programme has gone in for designing Magnesium components directly and establishment of production technology through networking of industries and research institutions. The paper deals with an outline of applied experimental research carried out on casting characteristics like fluidity, mould filling ability and shrinkage behaviour and implementation of recommendations by adopting Taguchi techniques of design of experiments. Experimental findings have very good practical significance for reduction of rejection levels. The Indian scenario on Magnesium Research provides an outline of R&D capabilities established in the country. Spin off recommendations as strategies for future have been provided for value added and commercial products.

Key Words Magnesium alloy, casting characteristics, spinoff

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Introduction Magnesium alloy castings are emerging as major players in global business by replacing Aluminium alloys in aerospace and automobile applications. Many components in transportation equipments, consumer products, machine tools, electronic products, construction and material handling sectors are currently produced out of Magnesium alloys and there are many more identified as potential users of Magnesium castings [1]. The growing market is due to the lower weight, good damping capacity and excellent thermal diffusivity characteristics. Indian missile programme provided the nucleus for the growth of R&D in this field with demanding requirements from the component designers. While the global attention is on substituting Aluminium alloys with Magnesium alloys, Indian approach has been towards the design of components with Magnesium alloys, thus exploiting its full technical potential for aerospace programme. Apart from carrying out applied R&D in the areas of product development and productionisation through technology transfer, the programme has also established a network of academic institutions, R&D laboratories and industries, thus laying down a strong technical foundation for techno commercial exploitation of spin off potential. This paper provides a review of R&D work carried out in this area towards product development for the missile programme and also outlines the Indian R&D capabilities. With global networking for commercial application, India has strategic advantages of serving the international community in this area.

Application Oriented Experimental Research Products Outline & Design Approach The products for aerospace applications fall under the following categories. • Cylindrical outer shells of aerospace vehicles mainly from weight

reduction point of view • Contoured plate structures for wings from vibration / flutter damping

point of view • Electronic package components for heat dissipation / thermal

diffusivity • Vibration fixtures and tables for elimination of unwanted modal

vibration during environmental testing of aerospace components Photographs of some of these components are given in Fig. 1. The design for all these components follows AVP 32 aerospace standards, satisfying the requirements of operations at hot [+ 60°C] and cold [-40° C] temperatures, vibration, rain and electromagnetic protection requirements. The ‘fail-safe’ design calls for specification of defect levels as per radiographic standard ASTM E-155. While designing for operation life of 10 years, corrosion protection factors are taken into consideration. The electronic package housings contain printed circuit boards and other items which emit heat during operation. The heat, if not diffused out

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affects the overall performance of the system. In addition, high precision components like gyroscopes and accelerometers perform well only in particular thermal environment. Transient iteration optimisation analysis of thermal elements is carried out during thermal analysis. Conduction, convection and radiation parameters are defined by boundary conditions and the temperature acquired by the chips and heaters are calculated through experimental data. Experimental Research Based on the component requirements, a major research programme has been executed with studies on casting characteristics [fluidity, mould filling ability and shrinkage] as the focus. Process parameters like pouring temperature, moulding method, sand fineness, mould coats and alloy additions have been optimised using Taguchi method of design of experiments. Issues arising out of corrosion have been tackled by launching a time bound R&D on corrosion schemes and sequence of corrosion protection methods. Mg-Al systems form the basis for all investigations. Salient research findings are outlined below: Flow and Filling Characteristics : Large size thin walled castings (where length to thickness ratio is greater than 100) require adequate flow of liquid metal. Reproduction of contours is important for the electronic package castings having heat dissipation fins. Spiral fluidity test method (Fig. 2) and pin test piece (Fig. 3) have been used to evaluate the fluidity and mould filling characteristics. Aluminium addition up to 5% decreases fluidity and thereafter it increases whereas the mould filling ability values increase with increase in Aluminium addition [2]. The decrease in fluidity values is attributed to change in solidification morphology from exogenous to endogenous shell up to 5% and to endogenous pasty subsequently. Investigations also revealed that the fluidity and mould filling ability are improved by using air setting sand made with fine sand and alumina based mould coats and with increase in pouring time. Shrinkage Characteristics: Volume deficit and its distribution in to macrocavities, surface sinks, volumetric contraction and shrinkage porosity have been determined experimentally for cylindrical, cubical and rectangular shapes for Mg-Al alloys (Fig. 4) with increase in Aluminium content, the total volume deficit and macrocavities decrease and surface sink increases with no appreciable variation in internal porosity and volumetric contraction. With increase in pouring temperature, macrocavities decrease whereas the surface sinks and volumetric contraction increases with no appreciable variation in internal porosity. Macrocavities decrease with chilling power of the mould whereas the surface sinks and volumetric contraction increase with no appreciable variation on internal porosity [3]. Table 4 gives summary of recommendations favourable for reducing the defects.

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Process Parameter optimisation through Taguchi Technique Alloy additions, degree of superheat, fineness of sand used for moulding, moulding method and mould coats are the major process parameters which affect the casting characteristics. In order to optimise the process parameters to suit the shop floor conditions, Taguchi method of design of experiments is an effective procedure. Table 1 provides the details of major factors and their levels. Table 2 provides the optimum process parameters arrived at after carrying out analysis of variance and signal-noise ratio analysis. As far as shrinkage is concerned, the objective is to increase the macrocavity and decrease the shrinkage porosity. Taking this aspect alone into consideration, the desirable combination of factors are listed at Table 3 while increased Aluminium content upto 10 % is found to be advantageous from fluidity and mould filling ability points of view, Mg-2% Al was found to be ideal from shrinkage aspect. Green sand moulds and Zircon based coating are suitable for tackling shrinkage defects while air set mould and alumina coat are desirable from fluidity and mould filling ability points of view. Practical Significance of Experimental Findings During technology transfer phase of productionisation of shell castings, spongy shrinkage in the middle portion led to 80% rejection. This level was reduced to 20% by incorporating the following recommendations based on experimental study.

1. Operate at the lowest level of Aluminium content for AZ91C alloy. 2. Use of Zircon coating of green sand mould at localised areas. 3. Improved methoding practice to reduce pouring time.

Magnesium Research – Indian Scenario CECRI and NML, leading national laboratories have developed production technology for Magnesium metal through electrochemical and silicothermic technology respectively. Production units are facing problem of high cost of metal due to low volume of production. DMRL has developed and characterised Mg-Li alloys and ZM21 products and productionised through MIDHANI. Rapid prototype activities have been initiated at GTRE for development of magnesium alloy cast products. HAL has developed fluxless melting technique and Regional Research Laboratory is working on Magnesium matrix components. Extensive work has been carried out at ISRO on machining and protective treatments. Studies on process maps for rolled plates cut of cast ZM21 blocks have been carried out at IISc. IIT (Chennai), IIT (Mumbai) and RRL (Tiruvananthapuram) have developed methoding software for processing of Magnesium cast components. CECRI has carried out extensive studies on corrosion characteristics of Magnesium alloy cast products in various environments and have come out with recommendations for sequence of corrosion prevention procedures for cast products. HAL is the leading industry manufacturing magnesium

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alloy components in its divisions at Bangalore and at Koraput. Network of the industries, R&D labs and academic institutions has given a strategic advantage to India for entering global market. Spin off- Strategies for future Valued added products: Increase in usage of Magnesium alloy components will be as value added products for aerospace applications. This trend will be there for all components, requiring weight reduction and damping capacity. Spin off in the areas of medical for stretchers, light weight cots and wheel chairs and for light weight reflecting head gears for surgeons performing brain operations may be expected during the next 2 years. Machine tool plates and vibration tables are major business areas. The limited application as on now may be due to limited international players in this business. India is in a better position to cater to the needs of Asia and South east. Commercial Products: For automobile applications, Magnesium alloys have to compete with Aluminium alloys on cost aspects too. For heavy volume production like die castings, Magnesium alloys have an edge over Aluminium alloys due to the following reasons: - Less energy for melting with fluxless melting energy efficiency is still

increased due to reduction in melt losses. - Tool life of dies for Magnesium alloy die castings is 3.3 times more

than Aluminium alloy die castings resulting in one third reduction of cost per shot when die cost is amortised over the life of the tool.

The international trend shows an upward increase in demand pattern for magnesium die castings. Conclusions • Adapting Magnesium alloys at the design stage itself has better

advantages than substituting aluminium alloys with magnesium alloys at the product stage.

• Application oriented experimental research has been carried out on Mg-Al alloys on fluidity, mould filling ability and volume deficit and its distribution as macrocavities, surface sinks, volumetric contraction and internal porosity.

• Process parameters have been optimised through Taguchi techniques. Using the results it was possible to reduce rejection level from 80% to 20% by selective incorporations of recommendation in problem areas.

• Indian scenario of Magnesium research indicates that there are institutions with core knowledge on Magnesium components development and production. Network of industries, R&D labs and academic institutions provide strategic advantage to India to enter global market.

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• Spin off strategies for future are in two areas. The first is on development of value added products for aerospace and medical applications and the second is for cost effective alternative to Aluminium alloys for automobile and commercial applications.

References 1. Sundarrajan S, Ganapathy RS and Krishnadas Nair CG, Strategies

for Magnesium. A report by the National Materials Policy Project, Source Book on Magnesium Alloy Technology, Dec 1995, pp 1 – 14.

2. Sundarrajan S, Rangarao NTV and Roshan H Md and Ramachandran EG Evaluation of fluidity of alloys – a statistical approach, Proc of Institute of Indian Foundrymen, 33rd Annual Convention Vol-2, 1984 pp 109 – 114.

3. Sundarrajan S, Roshan H Md and Ramachandran EG Studies on shrinkage characteristics of Magnesium-Aluminium alloys, Transactions of the Indian Institute of Metals, Vol. 37, No. 4, August 1984, pp. 373 – 381.

Acknowledgements The author sincerely acknowledges the support and encouragement provided by many individuals and institutions. He gratefully remembers Late Prof. E.F. Emley, considered to be the father of Magnesium Technology, who technically guided him to enter the field and dedicates demanding requirements of research had come from Dr. V.K. Saraswat and Mr Prahlada, Programme Directors and currently Distinguished Scientists and Chief Controllers of DRDO. He records his thanks to Director, DRDL for his kind permission to present the paper.

E-mail address: s_sundarrajan@hotmail.com

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Table 1 Major Factors and their levels

Factors Level I Level II Level III Alloy Mg-Al4 Mg-Al7 Mg-Al 10 Degree of superheat 80°C 100°C 150°C Sand fineness No 25AFS 57 AFS 75 AFS Moulding Method Co2 Green Air set Mould coats Alumina Sulphur Graphite Table 2 Optimum Process Parameters (Fluidity and Mould filling ability) Alloy Mg – 10% Al Degree of superheat 150°C Sand fineness 75 AFS Moulding method Air set Mould coat Alumina base Table 3 Optimum Process Parameters (Increased macrocavity and reduced shrinkage porosity) Alloy Mg – 2% Al Degree of superheat 100°C Sand fineness 75 AFS Moulding method Green sand Mould coat Zircon base

Table 4 Summary of Recommendations favourable for reducing defects

FLUIDITY MOULD FILLING ABILITY

SHRINKAGE

Aluminium addition Beyond 5 % Up to 10 % Up to 2 % Moulding method Air setting Air setting Green sand [with

localised zircon sand in problem

areas] Mould coats Alumina

based Alumina based Graphite based

Pouring temperature [degree of superheat]

High (150°c)

High (150°c)

Low (50°c)

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Fig. 1 Magnesium Alloy Components

Fig. 2 Fluidity Test Setup

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Fig. 3 Mould Fillers Ability Test Set Up

Fig. 4 Shrinkage Test Set Up