Professor John Campbell, OBE provides an insight into the innovative ideas underpinning his endeavours to create casting processes that could revolutionise the world of metallurgy and engineering
As an introduction to your high-profile background, could you offer a brief overview of the Cosworth Casting Process you developed in 1978-85? How has the process improved the reliability of Cosworth engines, most notably in Formula 1?
The process I set up for Cosworth made it the first casting plant in the world to rely completely on an electromagnetic pump to fill aluminium alloy castings in a production foundry. The melt was allowed to stand for some hours to permit oxides to sink or float, after which it was transferred uphill into moulds. The absence of entrained oxides (only much later identified as oxide bifilms) transformed the properties of the castings. 50 per cent of traditionally poured cylinder heads never made their way into a racing car because they failed on the test bed from thermal fatigue. Cracking occurred between the exhaust
ports, whereas castings from the new process enjoyed zero failures.
What are Griffith cracks and, more specifically, bifilms, and how do they form during metal casting processes? Are there any other hypotheses for how they form?
Griffith cracks are small, crack-like defects predicted to be present in most metals. All theories of metal fracture assume they exist because failure by cracking cannot occur without a ‘starter’ crack. My proposition is that bifilms are the Griffith cracks. Whereas conventional metallurgical thinking has identified inclusions and other phases and interfaces as sources of such crack initiators, the fact that all these features either nucleate and grow on or become attached to bifilms means that in most cases – if not every case – metal failures actually occur from bifilms. (Without the bifilm many inclusions associated with cracks would not have formed and many boundaries and interfaces would not occur.)
You have proposed two solutions to the defect problem, involving a fairly conventional gravity pouring system and a counter-gravity approach. How does your first approach substantially reduce turbulence, and thus bifilm content?
I see the improvement of gravity pouring systems as an interim step towards the reduction of bifilm content in cast materials. It is far from perfect, but it reduces defect concentrations by at least a factor of 10. It works by designing filling channels in which air can be excluded completely, so the metal completely fills the channels, and the melt
enters the mould cavity at a speed below 0.5 ms−1. This speed is a critical threshold above which the surface oxide becomes entrained into the bulk of the melt by surface turbulence such as jetting, fountaining and waves. When filling below the critical velocity, the mode of filling is tranquil and no entrainment of its surface can occur.
Do you have close collaborations with particular experts in either industry or academia?
It is a pleasure to acknowledge a few good friends in both the academic and industrial worlds. Professor Murat Tiryakioglu, Head of Engineering at North Florida University, USA, is a world expert on statistics. He and I have collaborated on many scientific papers concerning the reliability of castings and failure rates in cast products by fatigue, using the bifilm approach as the underpinning of the phenomena.
In the industrial world, I collaborate with Bob Puhakka, a Canadian. He has proven my filling system design concepts in both steel and aluminium castings, revolutionising his foundries in the process.
I am also working with good friends in a steel foundry in Sheffield, UK, making valves and fittings up to several tonnes in weight for the petrochemical and North Sea oil markets. In North Yorkshire, we are making investment castings by another revolutionary technique for the aerospace and electronic industries. A further project in Norway involves an exciting new development casting bronze propellers for ships using counter-gravity.
Engineering excellenceIt is increasingly evident that defective metals are the result of inadequate casting processes. Research from the University of Birmingham, UK, is looking to change prevailing attitudes completely to encourage industry and academia to see that the solution is already at hand
FROM THE CRUCIBLE to the mould, the ancient art of casting liquid metal into desirable tools and trinkets offers ample opportunity for the birth of defects. Issues with gas porosity, shrinkage, pouring and cooling processes, to name a few, can all result in substandard metal products, in which small defects can range from simple aesthetic concerns to more critical issues of structural integrity and human safety.
DRAWING IN THE DEFECTS
Since Alan Arnold Griffith’s theories of crack propagation revolutionised engineering in the 1920s, the study of fracture mechanics in metallurgy has modelled itself around the notion that metals fail prematurely due to the existence of tiny flaws. While it is widely accepted that under external stress ‘Griffith cracks’ will cause a material to fail long before it would otherwise be expected, the question of
their origin is a more divisive issue. For Professor John Campbell, OBE, the answer is clear.
Campbell is proposing a complete turnaround in current thinking about crack formation. It is generally assumed that inclusions in metals act as stress raisers and nucleate cracks. Instead, Campbell proposes that the cracks are not nucleated but pre-exist from the casting process. During casting, turbulence causes the surface oxide on the liquid metal to submerge as a double film (bifilm) whose dry top surfaces oppose each other. Since they cannot bond together, a narrow space which acts as a crack forms between the two halves of the bifilm; a weakness that is sealed into the cast metal, but normally difficult to see as a result of its extreme thinness. The inclusions which form in the liquid state appear to always precipitate on the bifilms as a favoured substrate. Thus, the inclusion forms on the ‘crack’ – not vice versa.
WEALTH OF KNOWLEDGE
As Emeritus Professor of Casting Technology at the University of Birmingham’s School of Metallurgy and Materials, Campbell’s prestigious career and impact on casting science is evidenced by the numerous initialled qualifications that follow his name, not least among them his being a Fellow of the Royal Academy of Engineers and recipient of an Order of the British Empire in 1993.
Just as Griffith’s theories on crack propagation helped subsequent generations more clearly understand the principles of material failure, Campbell believes that by acknowledging
The UK’s Institute of Cast Metal Engineers awards the John Campbell Medal every year in your honour to researchers who have made significant progress in the casting industry. Have you been impressed with some of the winners?
The creation of the award was a complete surprise. I do not choose the winners; they are chosen by the Technical Committee of the Institute of Cast Metal Engineers (ICME). I am permitted to suggest candidates. So far, of several candidates I have suggested, Professor Murat Tiryakioglu and Dr Philip Ramsell have been selected. Nominations remain on the shortlist for three years, so my remaining, as yet confidential, recommendations might yet be adopted.
In the fast lane
The 3.0 litre Cosworth DFV (Dual Four Valve) V8 was the most successful engine in Formula 1 history, racking up 155 victories over the various generations of the engine from 1967-83. Its first victory came at the hands of Jim Clark at the 1967 Dutch Grand Prix fitted to a Lotus 49, and its last in Detroit in 1983, powering a Tyrrell driven by Michele Alboreto.
A Ford Cosworth DFV engine at the National Motor Museum, Beaulieu, UK.
WWW.RESEARCHMEDIA.EU 45
PROFESSOR JOHN CAMPBELL, OBE
DEFECT-FREE METALS
OBJECTIVES
To help the casting industry, with a view to benefi t metallurgy and engineering with revolutionary improved metals.
CONTACT
Professor John Campbell, OBE
Campbell Technology Ltd6 Old Market CourtLedburyHR8 2GEUK
PROFESSOR JOHN CAMPBELL, OBE is a physicist who became a devoted teacher and promoter of good casting technology. His Cosworth Casting Process developed in 1978-85 is still the world leader in productivity and quality for cylinder blocks. As Professor of Casting Technology at the University of Birmingham, Campbell’s Complete Castings Handbook, published by Butterworth-Heinemann, has become the defi nitive textbook.
the existence of bifi lms, the engineering world would be poised for a revolution that could produce metal casts en masse free from damaging defects.
There are already methods that eliminate defects from the casting process. The problem is that only a small number of people in the industry are aware of them. According to Campbell, a large part of the problem appears in the chasm between academia and industry. “Metallurgists suffer the disadvantage of a classical metallurgical education
that emphasises the physical metallurgy of metals. Process metallurgy has been eliminated from many teaching programmes because it is seen as ‘smoke-stack’ and too industrial – not suffi ciently respectable to be included in modern university or college teaching,” Campbell explains. “Thus, the profession overlooks and fails to understand the elementary fact that metals inherit the problems introduced in their manufacture.”
FOUNDRY FINDINGS
During the casting process, there are plenty of chances to damage the liquid metal. Poured from a typical height of 1 m from a furnace into a crucible or ladle, the melt can reach a speed of 4.5 ms−1 – well above the critical threshold at which turbulence smashes surface oxides into the liquid. Poor fi lling system designs inside the mould compound the problem. The whole process often results in an abundance of costly scrap castings.
To counteract this scenario, Campbell spends a signifi cant proportion of his time introducing his naturally pressurised fi lling system to foundries. Rather than eradicating defects entirely with a complete system overhaul, the design complements standard gravity pouring systems by dramatically reducing turbulence and resultant bifi lm formation. The benefi ts for the user are most strikingly illustrated by the case of the Deloro Stellite Foundry in Swindon, UK, when it converted to a naturally pressurised fi lling system in the 1970s. Over a period of 18 months, the foundry’s production of around 80 per cent scrap castings plummeted to zero while its profi ts skyrocketed. This example is not a rarity, enthuses Campbell: “The reduction of scrap means that very often foundries become profi table for the fi rst time”.
INDUSTRIAL EVIDENCE
Another innovative method designed by Campbell, counter-gravity offers enhanced quality meaning less time and money is wasted melting defective scrap castings back down for recasting. The counter-gravity process pressurises the liquid metal by gas or pump, causing it to ascend via a riser tube into the
mould. Its velocity of rise can be controlled below the critical entrainment speed so that the liquid is free from damage by the time it settles in the mould, resulting in a casting that can be defect free. Additionally, properties like ductility are improved at least two-fold in the fi nal product.
This casting method is already used widely in North America, China, Korea and the UK in the form of Campbell’s Cosworth Casting Process. However, the majority of the companies that have adopted counter-gravity casting have not implemented the design fully. This is perhaps due to the method’s remarkable ability to outperform other modes even when it is not exploited to its full potential, leaving customers prematurely satisfi ed with products that could actually be far better.
If employed in full, there is no reason why Campbell’s bifi lms, the Griffi th’s cracks, cannot be largely excluded from cast metals, making defect-free metals the norm, eradicating expensive waste and leading to fewer cases of human lives being lost from material failure. Until the academic world begins to bring these ideas into the fold of metallurgical education, it is of some satisfaction to Campbell that at least the casting industry has the energy and enthusiasm to implement common-sense requirements, one foundry at a time.
A SHOW OF STRENGTH
In addition to visiting foundries, running training courses and workshops, and trying to spread his message far and wide, Campbell is still avidly conducting research into bifi lms themselves, as shown in his most recent collaboration investigating bifi lms in steels and nickel-based alloys with researchers from Sheffi eld Hallam University. “The initial results have been sobering: some steels are so full of oxides that on a fracture surface it is diffi cult to fi nd any metal,” Campbell mourns. “However, when we get melt conditions right we can fi nd metal, and moreover, it is no longer apparently brittle but exhibits signifi cant ductility.”
In efforts to break into the marine stainless steel casting market, Campbell is also pursuing the potential benefi ts of quiescent melting and fi lling with Norwegian bronze foundry Oshaug Metall. And he is working with foundry installation expert Jack Frost to demonstrate how counter-gravity casting can benefi t the properties of seafaring products by building defect-free 5 tonne propeller blades for ships, vertically. “This is a revolutionary world fi rst for such a tall product,” Campbell enthuses.
With such improvements in quality, and resultant reduction in waste and increase in profi ts for industry, it should not be long before all foundries adopt Campbell’s ingenious methods. When this happens, academia will be forced to acknowledge the bifi lm concept, turning decades of prevailing thought on its head.
A modern straight-six engine block for a passenger car, integrating the crankcase and all cylinders. The sand casting process permits the rigid closed top deck design, in contrast to most high-presure diecasting processes.
