Surface technologies show the way for magnesium

Materials World magazine
,
1 Dec 2016

Steve Montisci explains how the plasma electrolytic oxidation process developed by Keronite could benefit the increased use of magnesium in the defence sector.

A strong, lightweight and easily machined material, magnesium alloy holds much promise as an alternative to heavier metals such as aluminium. Whether it’s reducing the weight of aircraft, high performance cars, or portable equipment used by the military, weight reduction is at the top of many industry agendas. 

Magnesium has the lowest density of all structural alloys, making it 33% and 75% lighter than aluminium and steel respectively. Despite the lower density, magnesium alloys are both stiffer and stronger weight-for-weight than aluminium. There are many other advantages to using magnesium, such as its high vibration damping capacity, making it an ideal material for many high-speed applications on reciprocating machinery. It is also 100% recyclable using a fraction of the energy required to produce the primary material. 

There is the perception that magnesium corrodes easily. This was true of older, legacy alloys, but things have changed. The development of more stable alloys, together with advances in surface technologies, means that magnesium is becoming a viable option, particularly for applications where metallic components are preferred for their strength, ductility, and specific thermal or electrical properties. Metallic components and assemblies are also easy to fabricate. 

A range of industries are already taking advantage of the weight savings offered by magnesium, including aerospace, automotive, consumer electronics, biomaterials and, in particular, defence – where magnesium can readily be used to reduce the weight of man-portable equipment, such as night vision sensors, weapons systems and parts, and communications equipment. 

Magnesium in defence

Ruggedised computers, which are specifically designed to operate reliably in harsh usage environments – including strong vibrations, extreme temperatures and wet or dusty conditions – use magnesium for their casings and chassis structures for light-weighting, durability, and to assist with thermal management. Interestingly, this material technology has found its way into many popular consumer electronic devices, such as notebooks, tablets, phones and cameras. Many systems including night vision and target acquisition are weapons-mounted, so keeping weight down is important, not just in terms of portability, but also to optimise the balance and feel of the weapon itself. The SAKER Fused Weapon Sight (FWS) from UK-based Qioptiq, uses magnesium in its construction to achieve a fully assembled weight of just 890g. One system that typifies a fully integrated digital battlefield system is the French Army’s ‘FELIN’. This is a digital integrated equipment suite, designed to enhance dismounted warfighters' capabilities in terms of firing precision, day and night combat, intelligence, and individual and collective self-protection. French defence company Sagem Défense Sécurité, the primary equipment contractors, also make long-and medium-range binocular systems (weighing around 2.5kg each) that employ lightweight materials to ensure that a large number of features can be crammed-in – including a third generation infrared channel (3–5µm), an eye-safe laser rangefinder, a magnetic compass, a laser pointer, a colour video channel and a GPS.

Light-weighting

In the military arena, particularly in the USA, you’ll often come across the acronym ‘SWaP’, which stands for size, weight, and power. This is an initiative to reduce the size, weight, and power consumption of electronic components and systems. Magnesium can play an important role here. 

Magnesium can even be considered for armour plate. Having been the subject of intensive US Army Research Laboratory (ARL) studies over the past few years, magnesium plate has been shown to exhibit similar performance to aluminium alloys in a number of ballistic trials. It can also be used as part of a composite system. One example is REL Inc's magnesium-encapsulated squeeze cast ceramic panel that was developed for the ramp door of the US Stryker warfighting vehicle and is designed to be lightweight as well as being capable of surviving multiple hits. Away from the defence arena, Magnesium Elektron, UK, has recently shown that magnesium alloys can meet tough flammability requirements. This has driven a change in the wording of SAE AS8049 – the aerospace safety standard – that will allow magnesium components into commercial cabin interiors. But this is only half of the story. 

Corrosion is the greatest enemy of magnesium, which is where surface treatments come in. As a chemically active metal, magnesium can be very challenging to protect. Plasma electrolytic oxidation (PEO) imparts a highly scratch- and corrosion-resistant ceramic coating to light metal alloys, namely aluminium, magnesium and titanium. This is a rapidly developing sector in surface engineering and the opportunities it is creating to increase the use of magnesium are delivering performance improvements without adding significant weight in the process.

The PEO process involves the use of much higher voltages than traditional anodising and the electrolyte usually consists of low concentration phosphate, aluminate or silicate containing solutions. It’s a clean, environmentally friendly technology. 

As a result, the process has generated significant interest in offering an improved surface treatment for magnesium, aluminium and titanium alloys, and as a replacement for conventional acid based processes/conversion treatments (including acid based anodising processes) that contain Cr(VI) or other environmentally hazardous substances, such as heavy metals.

The process results in the formation of a hard, inert, and electrochemically neutral ceramic layer that offers protection to the base alloy in terms of corrosion and wear, with additional functional characteristics including thermo-optical, dielectric, thermal barrier and friction modification. It can also be used as a pre-treatment for topcoat paints and other metals/ceramics to create composite and duplex coatings. 

The process can be honed to offer bespoke solutions for different applications, by making adjustments to a range of controllable parameters. The coating can be engineered to offer multi-functional performance on different parts of the same component – where one part of the component might need increased wear resistance, another could need better thermal or aesthetic qualities, all of which can be delivered in the same treatment process. 

The coating itself is derived from the original base metal, which means that the process doesn’t bulk out the component – a feature of many traditional coatings that ultimately undermines the light-weighting effort. Being integral to the substrate also means that it has excellent adherence and will not delaminate or peel away – unlike some over-coatings. It also removes the need for primers, making the potential weight savings significant. 

In practice for high-performance industries such as the aerospace and defence sectors, where demand is rising for advanced higher performance, high-temperature magnesium alloys that are also corrosion-resistant and ignition-resistant, high-performance magnesium alloys can now offer component designers high-temperature characteristics, pressure tightness and the ability to produce complex shapes by casting, machining, extruding or forging. These alloys enable engines and power transmission systems to run safely at higher temperatures without gear misalignment or the need to seal the component against oil seepage.

In addition, the recent change to AS8049 opens up a raft of potential opportunities for aircraft and component designers due to the large volumes and repeatability of components required. For example, we see an opportunity to use magnesium in aircraft seating that could offer potential weight savings of nearly 20% per part. With hundreds of seats on a typical commercial aircraft, the overall savings would be huge, making a notable difference to the aircraft’s environmental impact, as well as its running costs. 

Similarly, commercial aircraft use a great deal of paint, equating to around one tonne per aircraft. Reducing the need for this protective paintwork by adopting PEO-coated magnesium components could offer further significant weight reductions, as well as reduced maintenance requirements. 

It’s an exciting time for the industry. PEO is the fastest growing advanced surface technology for light alloys during the last decade and will play a key role in enabling light alloys such as magnesium, aluminium and titanium to be used in extreme and demanding applications that were previously deemed to be impossible.

Steve Montisci is Business Development Manager at Keronite, UK. He has a broad range of industry experience, working in commercial roles across the aerospace, automotive, defence and medical technologies industries over more than 20 years. www.keronite.com