Melting key to fast structures

Tom Shelley reports on the latest advances in the rapid manufacturing of finished parts of extraordinary intricacy Selective laser melting as opposed to selective laser sintering enables the manufacture of dense metal parts of great intricacy and complexity out of stainless steel, titanium, and other difficult to work with materials Maximum load to weight and stiffness to weight performances can be achieved on sub millimetre scales or mechanical behaviour tailored in different parts of the same component by grading the structure. Combined with other new techniques, such as rotating work surfaces as opposed to traversing surfaces, it is possible to make complex products using compact equipment - perhaps only large suitcase size in some instances. Parts being experimentally manufactured range from micro heat exchangers for new engines, through parts for aerospace impossible to fabricate any other way to pill medicines. Selective Laser Melting is a technique invented and pioneered in the Engineering Department of the University of Liverpool, starting with work on 316 stainless steel powder in 1998. Like Selective Laser Sintering, it is based on heating layers of metal powder with a laser beam, but at sufficient temperatures to fully melt the powder particles. The research group is led by Dr Chris Sutcliffe and assisted by, among others, experimental officer Lawrence Bailey. During a recent visit, these gentlemen showed Eureka the original apparatus with which the technique was invented, based on a 100W YAG laser normally sold for laser marking. This has since been supplemented by an 'MCP Realizer SLM' built by the German company Fockle und Schwartz, which invented the SLM process independently and at about the same time. Mr Bailey told Eureka that the technique was, "Quite difficult initially", because the melted metal particles tended to "Ball up" under the effects of surface tension. The problem was eventually be overcome by a combination of "Right powder layer thickness", and "Right scan strategy." The latest innovation to be achieved by the group is, 'Spiral growth manufacturing'. In this technique, the working table is rotated and descends down the threads of a lead screw as it is rotated by a toothed belt acting on a pulley. Because the screw thread is a little coarse, the lead screw is mounted in a screw thread in a second pulley also driven by a toothed belt. If both pulleys are rotated at the same speed, the net effect is that the screw is driven upwards by the same amount that the table descends downwards, so there is no net motion. In order to lower the table in such a way that fine control can be achieved, the pulleys are driven at slightly different speeds. The spiral growth manufacturing technique is being developed both for SLM processing of metals and the rapid manufacture of plastic parts using a process based on the Xaar inkjet method for printing of binders. It is a version of the latter with up to four powder hoppers that is being developed for the manufacture of pills. It is anticipated that a machine equipped with a rotating table 1m across can be used to produce 400,000 pills from a mixture of ingredients. The metal SLM process is also being used to experimentally produce parts in batches of a lot more than 1. Batches of 30 test pieces are already being produced routinely and Mr Bailey comments that there would be no problem to increase the quantities produced considerably. Typical structural test pieces shown to Eureka included 10mm diameter lattice structure cylinders capable of supporting 1.3 tonnes. Dr Sutcliffe said that each batch took about 15 minutes to make. There is no problem is adding to complexity - parts can be made to be stiffer in one mode of deformation as opposed to another, or stiffer and softer in different areas. Properties are far superior to metal foams, already used in some sandwich constructions. Holding a three dimensional girder of pin thickness struts of considerable complexity, Dr Sutcliffe compared it with a metal foam cored part which he described as having, "Completely uncontrolled geometry whatever the makers tell you." He then added that, "The more complex the component, the quicker and cheaper it is to make by SLM relative to other technologies." Test pieces made for Ph.D research projects include a two dimensional aerofoil section, suitable for a light weight UAV, with a high complex internal structure optimised to achieve desired mechanical properties at minimal weight. While we were present, the MCP machine was being used to manufacture prototype cross flow heat exchangers with 0.8mm hydraulic diameter channels. One of the applications is apparently the regenerators in Stirling engines made by Sustainable Energy Systems. Finer channels following truly tortuous paths could apparently be made just as easily if desired. The heat exchangers are tested using water, although they are suitable for use with any fluid or gas. One idea tried, so far without much success, is forming a construction combining tool steel for plastic injection moulding with copper elements to assist heat removal. Dr Sutcliffe said that the melting temperatures were in this instance too far apart, but the idea should still work if the temperatures of the components were closer together. The next intended stage in machine development, we are told, will be to develop a 0.5m x 0.5m x 0.5m working volume for the SLM machine, reduce spot size, and possibly, double power output to 200W. Dr Sutcliffe also believes it will be necessary to contain all working operations within an inert gas environment as the process develops into volume production. There are obvious hazards to be associated with handling large quantities of titanium and aluminium powder in air. The powders are supplied by Sandvik Osprey. Dr Bob Howells, director of the company had previously told Eureka that trends in both SLS and SLM development are requiring the manufacture of finer and finer sized powders. Goals being pursued elsewhere include the powder based manufacture of filters capable of removing bacteria from water. It is evident that a technology originally developed to produce single prototypes is being transformed into one also amenable to large scale mass production. The University of Liverpool Manufacturing Science and Engineering Research Centre Dr Chris Sutcliffe Eureka says: These techniques can no long be referred to as either 'rapid prototyping' or 'short run manufacturing' since they are clearly entering the mass manufacturing arena albeit for making very special products Pointers * Selective laser melting as opposed to sintering enables the manufacture of dense metal parts of great intricacy and complexity * Mechanical properties can be optimally engineered down to sub millimetre scales * Combined use of new rotating work surface cells, it is possibly to greatly reduce size, render possible thedevelopment of machines of large suitcase sized that might be used in space. * Parts being experimentally manufactured include micro heat exchangers, aerospace parts, and pills