Made to Measure…Cast Iron

Who are you?: Dr Ron Logan

What is your role?: Water Consultant at Mouchel. Currently on secondment to Thames Water.

What was your PhD about?: Studying the graphitic corrosion of grey cast iron in the buried environment.

I beg your pardon?: Graphitic corrosion is an unpredictable and highly localised form of corrosion specific to grey cast iron, whereby the ferritic component of the cast iron alloy goes into solution leaving behind a soft residue of the original graphite flakes interspersed with iron oxides and other insoluble products.

Why?: Graphitic corrosion on the external surface of the pipes is the principal underlying reason for a pipe to fail and burst in service. Of the 20,000 km of the pipe network that delivers 2.3 billion litres of water a day to London, 44% is constructed from grey cast iron, around half of which have been in the ground for over 100 years. It is therefore imperative to understand the rates and mechanisms by which these pipes are deteriorating as the cast iron network continues to serve London.

And?: The traditional view of graphitic corrosion is that it is a process of dissolution and transport of the iron matrix with the graphite flake structure left behind, undisturbed. My recent doctoral studies showed that this is an incomplete picture. Scanning electron microscopy, along with energy dispersive and wavelength dispersive X-ray spectroscopy (EDS & WDS), was used to characterise graphitic corrosion. The graphite flakes, are in fact deteriorating as part of the corrosion process. It is proposed that the interface between the metal matrix and the continuous graphite flake network acts as transport network for soluble ions. EDS and WDS investigations confirm the presence of chlorides at the corrosion interface of graphitic pits on the external surface of the trunk main.

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Figure 1 : Digital Photomicrograph via Scanning Electron Microscopy: Grey scale image of a cluster of graphite flakes in an iron matrix, with an elemental analysis showing the presence and concentration of Cl- ions, highlighted in red.

So what?: These investigations show that chlorides in the soil are likely to be the major driver of graphitic corrosion. The identification of chlorides as a risk factor has an impact on the overall asset management of the network.

Final Thought: It is not just material scientists and water network engineers who are trying understand how cast iron corrodes. I found that the archaeologists are too, which is helpful as London’s cast iron pipe network can be thought of as an antique that happens to remain in service.

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Made to Measure…Silicon Carbide Monofilaments

Who are you? Michael Rix

What is your role? EngD Research Engineer

What is your work about? Silicon Carbide Monofilaments for the Reinforcement of Titanium Metal Matrix Composites

I beg your pardon? Silicon carbide monofilaments are continuous fibres that are about as thick as a human hair and several kilometres in length. They are extremely strong, even at high temperatures, but like all ceramics they are brittle and therefore difficult to use in structural applications. Reinforcing titanium with these monofilaments takes advantage of the ceramic’s properties to produce a composite that is both stronger and lighter than monolithic titanium. Figure 1 shows a cross-sectioned composite with the fibres in a hexagonal array. My doctoral research with the University of Surrey and TISICS has been focussed on the development and characterisation of the monofilaments.

 

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Figure 1 – A secondary electron image of a metal matrix composite panel.  Note the bright dots in the centre of the fibres, which are the tungsten wire precursor on which the SiC is deposited by chemcial vapour deposition (CVD).

Why? Lightweight, high strength materials are desirable in many industries. In aerospace in particular reducing weight is always a priority for improving efficiency. Ceramics theoretically have extremely high specific strength but in practise they often fail to achieve this due to the wide range of defects that occur during their production. They are also difficult to incorporate into complex structures due to their brittle nature. The silicon carbide monofilaments produced at TISICS have a very narrow strength distribution as a result of a carefully controlled chemical vapour deposition (CVD) process. While the monofilaments are still brittle it is possible to design composites to take advantage of their strength while protecting them from damage by exploiting the toughness of the metal matrix.

And? My research looks at improving the CVD process by reducing its complexity and optimising the rate of production. The simplest metal matrix component requires several kilometres of monofilament—as much as possible of which should be made in a single length with no defects. I am also investigating the microstructure of the monofilament coating, which is important to the performance in composite.

So what? When the monofilaments are produced correctly, the resulting metal matrix composite exhibits double the strength of the equivalent titanium alloy with a reduction in density of up to 40%. Additionally, because it is possible to change the volume fraction of monofilaments within the matrix the properties of the composite can be tailored to specific design requirements.

Final Thought: Made to measure metal matrix composites using these silicon carbide monofilaments exploit the best properties of ceramics and metals to produce high performance composites for jet engines, air frames, landing-gear and pressure vessels.

Made to Measure…Aluminium

Who are you?: Emma Ryan
What is your role?: EngD Researcher
What?: Wire Arc Additive Manufacture of Aerospace Aluminium Alloys

WAAM

Figure 1: A WAAM fabrication cell. Showing the robot arm, the welding torch head on the end of the arm and what is, in this case, a rotatable platen for deposition.

I beg your pardon?: Additive manufacture refers to a process of depositing a material to make objects from a 3D computer model, usually layer upon layer. It is different to many conventional methods as it is not subtractive. There are many different types of additive manufacture such as 3-D printing (binder and material jetting techniques), laser sintering (powder-bed fusion techniques) and wire arc additive manufacture (directed energy deposition techniques).  The focus of my project is on wire arc additive manufacture (WAAM). WAAM is a process where a welding torch is used to produce an arc: wire is fed through it so that weld beads are deposited onto a substrate. Subsequent layers are deposited in order to form a near-net shape part.
OK, Why?: WAAM has a high ‘buy-to-fly’ ratio, or in other words, little material is wasted. This is an advantage common to all additive processes. Unlike some other processes, the deposition rates are high. Due to the near-net shape produced, both material waste and lead-time are reduced and considerable cost reductions can be made.
And?: A lot of work has been carried out on high strength aerospace aluminium because of its current use for conventionally manufactured parts built by Lockheed Martin, particularly forgings (although I can’t say too much about the parts themselves!) and its ready availability (you can buy aluminium welding wire ‘off the shelf’). Current use of aluminium for conventional parts is due to various desirable properties including low density, high specific strength, reasonable corrosion resistance and relatively low cost.
So what?: There are challenges involved with making WAAM applicable for industrial purposes. Studying the effect of variables of the WAAM process is required in order to determine critical variables to control and monitor for certification purposes. Repeatability of the process needs to be ensured for certification purposes. Significant porosity is currently found in WAAM parts which needs to be reduced to improve mechanical properties and reduce the risk of failure.  Currently, I am analysing the influence of wire quality on the stability of the WAAM build and the resulting mechanical properties in the part. It is known that the wire quality can affect WAAM (the stability, repeatability and mechanical properties of the part) and cause porosity. My work is contributing to the goal of knowing how good the materials properties will be, and just as importantly, what steps need to be taken to make these properties reproducible.

Final Thought: A better understanding of the influence of wire quality on WAAM can provide ‘made to measure’ aluminium components.