Associate Professor Manoj Gupta is leading a team in Singapore that has developed a novel hybrid microwave-assisted sintering process to rapidly heat metallic materials. Here, he explains the appeal of magnesium and aluminium composite materials, and reveals the potential impact of this work in a number of sectors

What is your background in materials science and why is your current research centred on synthesising new magnesium (Mg) and aluminium (Al) composite materials?

My doctorate in materials science focused on processing and development of new lightweight materials. I have been interested in studying Al- and Mg-based materials and their composites for almost 30 years, as these are lightweight materials capable of saving fuel, reducing carbon dioxide emissions and creating a more sustainable environment. Between Al- and Mg-based materials, Mg is ~35 per cent lighter and has great potential in multiple engineering and biomedical applications.

Why are current reinforcing agents – ceramic materials – less than optimal for forming Al- and Mg-metal matrix composites?

At present, the main focus of researchers is to use ceramic reinforcements in Al and Mg composites, but they have their disadvantages. Recently, nanoscale ceramic reinforcements have shown potential in improving the overall mechanical response of Mg. This has been observed by our group, but only for certain types of reinforcements as there is no unified

theory that can predict the capability of any reinforcement to increase/deteriorate the properties of a metallic matrix.

Could you provide insight into amorphous alloys and the properties that result from using them instead of ceramics particles in Mg- or Al-metal matrix composites?

Using amorphous alloys allows for good wettability between metallic matrices and reinforcements – an efficient transfer of load from matrix to reinforcement leads to an increased probability of enhanced properties. Additionally, the high strength and elastic limit of amorphous reinforcements assists in improving the final composite material properties when subjected to stress. In the case of Mg, for example, the addition of nickel-niobium (Ni-Nb)-based amorphous reinforcement increases microhardness, compressive yield strength, ultimate compressive strength and ductility.

You have developed a microwave-assisted sintering technique to efficiently produce amorphous alloy composites. How does this process also influence the properties of the synthesised product?

As the hybrid microwave sintering process we have tailored is very time-efficient, it allows the microstructure of the metallic matrix to remain refined. It doesn’t allow significant matrix-reinforcement interaction, but does allow the amorphous reinforcement to stay amorphous in nature. All of these features assist in realising enhanced mechanical properties from a given system.

What methods do you use to characterise and assess the mechanical properties of the synthesised materials?

Our main focus and efforts concentrate on making materials that can one day find use in multiple engineering and biomedical applications. Accordingly, we characterise these materials for hardness, tensile properties, compressive properties,

tribological properties, high-temperature properties, dynamic properties, fatigue behaviour and corrosion. We characterise some of these properties with both local and international collaborators.

Can you shed light on the importance of the distribution of reinforcing particles in the strength of the metal matrix composite material?

The distribution of the reinforcing phase/particles is one of the key factors influencing the final properties of a composite material. Agglomerated particles are sites of stress concentration and voids that deteriorate the final mechanical properties of the materials such as strength and ductility. Hence, a uniform distribution of reinforcing particulates irrespective of length scale and type is extremely important to achieve good properties.

How do you compare the potential of amorphous alloy reinforcement and nanoceramic reinforcement in metal composite materials?

Amorphous alloy reinforcement has shown initial promise, however, nanoceramic particulates containing composites have also shown reasonable promise. Since the research conducted so far is limited to composites containing amorphous alloy reinforcement, it is a bit early to say which one will become a dominating force in the near future.

At which upcoming events are you expecting to disseminate your innovative microwave-assisted sintering technique?

For an academic researcher like me, upcoming events include disseminating results through conferences, publications and special issues in journals. In the past few years, I have given talks at both national and international levels and I hope these efforts will grab the attention of researchers soon. In addition, I also encourage my students to attend conferences and present our work in these areas.

Microwave-sintered metallic materials

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ASSOCIATE PRO

FESSOR M

ANO

J GU

PTA

Heating provided by microwaves and radiant heat from susceptors.

Amorphous alloy reinforcementA research group from the National University of Singapore is engaged in the discovery of high-performance metal composites that could have a number of applications in the materials science field and beyond

Amorphous alloys display a

number of desirable traits –

superior wettability, high elastic

limit, strength, etc. – leading

scientists to investigate their

capabilities in composite

reinforcement

MAGNESIUM (Mg) AND ALUMINIUM (Al) exhibit inferior properties in their purest forms which can often impact their commercial and practical applicability. For this reason, they are mostly used in their alloy form, enabling scientists to develop high-performance composites. As a result, the two materials have generated much interest, exhibiting application potential in a number of fields, including engineering, aerospace, transportation and biomedical science.

In this context, amorphous alloys display a number of desirable traits – superior wettability, high elastic limit, strength, etc. – leading scientists to investigate their capabilities as reinforcement. Amorphous alloys are stronger, harder and less corrosive than regular metal alloys; they can even be used in the formation of nanocomposite materials for a more robust product.

REINFORCING METALS

An amorphous alloy can be described as a solid metallic material with a disorganised atomic-scale structure. Whereas most metals are crystalline, and therefore have a highly ordered arrangement of atoms, amorphous metals are non-crystalline, hence they are often referred to as metallic glasses or bulk metallic glasses (BMGs).

Among other desirable qualities, these alloys display good strength and elastic limit, and for this reason they are replacing conventional ceramic reinforcements in metal. “The main issue with ceramic reinforcement is the less than optimal development of matrix-reinforcement interface characteristics that prevents the realisation of enhanced properties in many respects,” explains Dr Manoj Gupta, Associate Professor and former Head of the Materials Division of the Mechanical Engineering Department at the National University of Singapore (NUS).

NOVEL MATERIALS

Gupta’s team is investigating the incorporation of amorphous alloys in metallic matrices; namely, Mg-metal matrix composites (Mg-MMCs) and Al-metal matrix composites (Al-MMCs), and their impact in a range of fields.

In regard to the former, his team created Mg-Ni60Nb40 amorphous particle-reinforced composites and compared their properties with conventional Mg composites with ceramic/metallic reinforcements. Excitingly, this work highlighted improved mechanical properties, hardness and compressive strength. A similar investigation of novel amorphous alloy-enhanced Al-MMCs uncovered analogous enhancements in the properties of the composites.

SINTERING EFFICIENCY

These novel Mg- and Al-MMCs were created using an equally novel microwave-assisted two-directional rapid sintering technique, and subsequent hot extrusion. This method has been pioneered through a collaborative project Gupta is spearheading called ‘Using Energy Efficient Microwaves to Synthesise Energy Conserving High Performance Magnesium Composites’. The overall ambition of the project is to develop a cheaper, faster and more sustainable method of processing metallic materials in order to achieve energy-efficiency demands set by the International Energy Agency (IEA).

ASSOCIATE PROFESSOR MANOJ GUPTA

22 INTERNATIONAL INNOVATION

USING ENERGY EFFICIENT MICROWAVES TO SYNTHESIZE ENERGY CONSERVING HIGH PERFORMANCE MAGNESIUM (NANO)COMPOSITES

OBJECTIVE

To synthesise new lightweight materials (alloys as well as composites) primarily based on magnesium and aluminium using industrially adaptable, cost-efficient and energy-conserving novel processing methods such as microwaves-assisted synthesis, for the benefit of humankind.

COLLABORATOR’S COUNTRIES

USA • Hong Kong • Saudi Arabia • Qatar• India

FUNDING

Ministry of Education (MOE), Singapore

Qatar National Research Fund (QNRF), Qatar

CONTACT

Dr Manoj Gupta Associate Professor

Department of Mechanical Engineering National University of Singapore 9 Engineering Drive 1 Singapore 117576

T +65 981 633 49 E mpegm@nus.edu.sg

http://bit.ly/1l9iVk8

ASSOCIATE PROFESSOR MANOJ GUPTA has a PhD in Materials Science with a focus on the processing and development of new lightweight materials from the University of California at Irvine, USA. He has studied aluminium- and magnesium-based materials for almost 30 years, and published over 370 articles in peer-reviewed journals. Gupta was formerly Head of the Materials Group at the Department of Mechanical Engineering in the National University of Singapore.

The innovative technique is based around microwave heating – a method conventionally used to rapidly heat various materials such as food, ceramics, polymers, chemicals, biological tissues, etc.: “This process allows the microwaves to heat the billet from inside-out, while microwave susceptors heat the materials from outside-in simultaneously, which significantly reduces the overall heating time,” Gupta enthuses.

DEFYING CONVENTION

The NUS researchers’ novel technique is capable of quickly sintering metal powder compacts to high temperatures. They utilised Mg, Al and lead-free solder materials in both their pure and composite form, and found that they were able to successfully sinter them. In addition, the team discovered numerous benefits to using the microwave-based powder metallurgy approach. For example, it requires no holding time nor inert gas protection, making it particularly energy efficient and safe to use. Compared to conventional sintering methods, the finished products showed similar and/or better properties when the hybrid approach was used.

There are a number of applications of Gupta’s research concerning both microwave-assisted sintering and amorphous alloys: “The two have strong potential in other fields of materials science,” he comments. “In particular, the energy-saving capabilities of microwave sintering could be utilised to conserve energy and the environment without compromising the final properties of the materials.” In addition, the team hopes to explore their potential in such areas as waste remediation and metal melting.

INTERNATIONAL COLLABORATION

Gupta has forged research collaborations with groups from the US, India, Hong Kong, Saudi Arabia and Qatar, dramatically increasing the impact of the team’s work: “International

collaborations have allowed us to characterise the materials we are using in many different ways that are beyond our main expertise,” he adds.

The team’s collaboration with researchers in the US has enabled them to successfully characterise the fatigue properties of their materials; with their Indian collaborators, work is underway to examine the tribological behaviour of the materials; in Hong Kong, partners are studying high-temperature behaviour; while Qatari colleagues are assisting the project financially so that new materials can be developed.

FUTURE PLANS

Much still needs to be completed in the project before Mg- and Al-MMCs can be used on a large scale, as Gupta recognises: “We need to scale up our lab facilities to the industrial level, and by doing so bridge the gap between lab and industry results, and build up trust with engineers to better utilise these new materials if they clearly exhibit superior properties”.

Furthermore, Gupta hopes to build on his microwave-assisted sintering technique in the coming years: “At the lab scale, our microwave sintering setup has worked effectively for almost 10 years and it costs us only US $1,000 per year to run and maintain it,” he enthuses. “And I am already aware of many industries building and marketing microwave furnaces.” Looking towards the future, Gupta and his team hope they can provide a cheaper, sustainable, energy-efficient product so that industry can benefit from its multitudinous applications.

Gupta and his NUS colleagues are working closely with industry to ensure their goals are met, and by providing consultancy and training opportunities to their students, they are ensuring the next generation of materials science researchers are fully equipped with both academic and industrial knowledge.

MICROWAVE VERSUS CONVENTIONAL HEATING

Shorter processing time (80-87.5 per cent reduction)

Rapid heating rate (>>10 oC/min)

More uniform temperature profile with the use of two-directional heating

Minimal coarsening in microstructure

Improved hardness and tensile properties

Savings in cost and energy

INTELLIGENCE

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