A Physical Approach to Grain Refinement of Wrought Mg Alloys via Solidification Control

Human society is highly dependent on the Earth's climate, as climate patterns largely determine whether we will have enough food and fresh water. The gradual increase in the global temperature, due primarily to the increased amount of an undesirable gas, CO2, in the atmosphere, can cause significant climate changes. Road transport activities account for about 24% of the total man-made CO2 released to the atmosphere, and passenger cars are responsible for about 60% of these emissions. It is therefore an important issue to reduce the CO2 emissions from passenger cars in order to prevent the Earth's climate from deteriorating. One of the most efficient and easiest ways to do this, is to reduce the weight of a car so that the car will burn less petrol or diesel. Magnesium is a very light metal, in fact, some magnesium materials can be made lighter than water. On the other hand, they are still strong enough for making most of the parts used in a car. Therefore, they are very attractive to car manufacturers.Just like a sand castle that is made of many grains of sand, magnesium materials are composed of many small grains as well. The grain size in a magnesium material plays a very important role in determining whether the material is ductile or not. In general, the smaller the grain size, the more ductile the magnesium material will be. It is thus highly beneficial for magnesium materials to have a very small grain size, so that we can readily manufacture them into different shapes, such as sheet, tubes, bars, rods, etc. These forms of magnesium products are all very useful for making car parts and parts used in toys, bicycles, computers, mobile phones, televisions, etc. Unfortunately, large, thick magnesium materials normally have a coarse grain size, as a result, they are not ductile enough. Therefore, one has to use a very slow manufacturing process to make magnesium sheet, tubes, bars, rods, etc. This makes them very expensive and not many customers including most car manufacturers, are willing to use them in a large quantity. It is therefore crucial to reduce their grain size. Similar to the process of water becoming ice, solid magnesium materials start off as liquid magnesium. The change from liquid to solid is called solidification, which determines the grain size of a magnesium material. By effectively controlling the solidification process one can obtain a very fine grain size. In the past 65 years, there have been many efforts towards controlling the solidification process of magnesium materials. Although there have been some positive developments, the resultant grain size is still not small enough. In this programme, we propose a unique approach, designated 'twin-screw melt shearing'; it can effectively control the solidification of a magnesium material. The key point of this approach is to ensure that as many small grains as possible survive in the liquid during solidification. This is done by rapidly lowering the bulk liquid temperature to below a critical value; in doing so, the process will give a very fine and uniform grain size. Preliminary experiments have given very encouraging and exciting results, suggesting that the concept is feasible. Therefore it is hoped that further study into this new solidification control process will develop a hugely beneficial processing system, which will be able to deliver a fine grain size in large, thick magnesium products. It is further anticipated that the new technology will also be applicable to the solidification of other materials, such as aluminium and titanium.The anticipated results from this study will be both environmentally and economically beneficial to the global community.