Engineering Fellowships for Growth: Solidification Processing of Alloys for Sustainable Manufacturing

Lead Research Organisation: Imperial College London
Department Name: Materials

Abstract

We rely on metallic objects every day, from bicycles and bridges to the solder joints in our electronics. In each case, a key step in manufacturing is the solidification of liquid alloy, and it is through controlling solidification that we can control grain structure and defects. Solidification is at the heart of current challenges facing the UK: steel and aluminium production contributes more than 10% to global industrial CO2 emissions, and new solder technologies are required to enable the manufacturing of smaller, more powerful portable electronics. In all these industries, advances will involve controlling the solidification microstructure and controlling solidification defects.

Key to the development of grain structure in solder joints and structural castings are the earliest stages of solidification when the number-density of grains is determined by the number density of nucleation events. The project will use new microscopy techniques which combine focussing an ion beam to micro-machine into the centre of crystals and find nucleant particles with electron diffraction to understand how the particles catalyse nucleation. With this information, new ways to control nucleation will be explored.

After nucleation, the semi-solid grain structure goes on to significantly affect the formation of defects in castings and solder joints. Part of tackling this challenge is to develop a deeper understanding of how and why casting defects form. It is known that the origin of semi-solid cracking is the stresses and strains that develop during solidification but, to understand the details, we need to observe and measure how numerous solidifying crystals respond to loads during solidification. Metals and alloys are opaque to visible light and their inner structure is therefore hidden from our eyes. By pouring liquid alloy, we can see that they have a low viscosity and that the viscosity increases considerably as alloys solidify, but we cannot see or measure what structural changes are causing these changing flow properties. X-rays can be transmitted through metals, offering the potential to observe the development of microstructure, but it is only in the last decade that X ray sources have become available with sufficient flux and coherence to allow real-time imaging of crystal growth in alloys. This was an enormous step forward as it became possible to test solidification theories developed in 'post-mortem' studies using real metallic samples.

This project will extend these synchrotron techniques to observe and measure the solidification of intermetallic grains in solder joints, and to study how deformation of the semi-solid grain structure leads to casting defect formation. We aim to observe and measure for the first time where intermetallics nucleate in solder joints and how they grow during solder reactions. This will give us insights that we can use to engineer solder joint microstructures and tackle the final frontiers in the transition to Pb-free soldering such as a replacement for high-Pb solder for use at T>180C.

Similar techniques will be applied to imaging the formation of inter-columnar cracking in experiments analogous to the continuous casting of steel, a process used to produce more than one billion tonnes of steel annually. An exciting aspect of this part of the research is that much about semi-solid alloy deformation is unknown: How is force transmitted from crystal to crystal? What happens when two crystals are pushed into one another? Do they bend? Do they fragment? Do they behave as rigid bodies? Why do strain instabilities develop? Where do cracks begin and how fast do they grow? These questions can only be fully answered with in-situ observations of deformation at the scale of the microstructure. We have begun to address these questions in pilot studies and now we aim to expand this to crack movement in the mush.

Planned Impact

Knowledge impact:
In metal casting, modellers of any casting process where significant mush deformation occurs would benefit from knowledge of the fundamental mechanisms that underpin defect formation phenomena. In soldering, knowledge of how beta-Sn and intermetallics nucleate and grow is important for process modelling and for the development of new solders.

Furthermore, improved knowledge of heterogeneous nucleation mechanisms is important to a wide variety of areas beyong metals manufacturing from nucleating ice in clouds to make rain to preventing the nucleation of ice in biological applications to using phase transformations as electronic switches.

Economic impact:
Metal manufacturing is of strategic importance to the UK as identified by industrial and government agencies. An improved ability to understand and predict grain structure and casting defects would impact on major UK exporters (TATA steel, Rolls Royce, Jaguar-Land Rover etc.) and their supply chains, helping them to compete into the future. The development of second generation Pb-free solders, new inoculants for solders and replacements for high-Pb solders could be a significant boost to the remaining UK solder manufacturers (e.g. DKL the UK partner to Nihon Superior) and is well suited to the formation of start-ups as the UK looks to rebalance the economy.

Societal impact:
The search for replacements for high-Pb solders is one of the final frontiers in the transition to Pb-free soldering. It can be argued that sustainable solder materials are a UK international environmental obligation as much of our e-waste still finds its way to Africa and India where the Pb causes most harm.

Advances that expand the use of recycled Al alloys and decrease scrap rates in the continuous casting of steel have the potential to reduce the energy use and CO2 emissions of the UK. Since steel and aluminium production contributes more than 10% to global industrial CO2 emissions, this could contribute to the UK meeting its emissions targets while improving the sustainability of alloy processing in the UK.

Publications

10 25 50
 
Description (1) A campaign has been undertaken to develop new knowledge and understanding on the nucleation of tin during the solidification of electronic interconnections. Various heterogeneous nucleants have been identified to control tin nucleation. It has been shown that it is possible to grain refine solder joints by combining these nucleant with solute. These nucleants have also been employed as seed crystals in ball grid array (BGA) solder joints to control tin orientations.


(2) A variety of characterisation techniques have been developed and combined to study intermetallic crystal (IMC) growth. These include (i) selective dissolution methods to release IMC crystals for 3D studies,(ii) combined EBSD-FIB tomography of the crystallography of growth, and (iii) synchrotron radiography to quantify growth dynamics. With these approaches, the development of intermetallics in electronic soldering and structural castings are better understood and, in some cases, can be controlled. Specific examples include understanding the formation of metastable NiSn4 in solder joints, refining the size of Cu6Sn5 crystals in solder joints, controlling the distribution of impurity-Fe in B2 and Al8Mn5 crystals in Mg castings, and understanding orientation relationships between Al3Ti and TiB2 in aluminium castings.


(3) New insights have been gained into the behaviour of solidifying alloys under load (e.g. as occurs in the continuous casting of steels and high pressure die casting of Al and Mg alloys). It has been shown that shearing alloys as they solidify can lead to shear-induced dilation which opens liquid-filled fissures and can cause cracking. Synchrotron X-ray imaging studies have provided direct proof of mechanisms and enabled grain level quantification of behaviour. These X-ray studies have been combined with simulations of two-phase granular flow using the lattice Boltmann method - discrete element method (LBM-DEM) to build a quantitative understanding of semi-solid deformation in alloys. These approaches have built an improved understanding of rheology in casting processes such as high pressure die casting and the continuous casting of steel.
Exploitation Route The new knowledge on heterogeneous nucleation mechanisms of tin in solder joints could be taken forward to develop methods to control the number of tin grains in each joint and/or the orientation of the tin grains in solder joints produced during electronics manufacturing.

The measurement of deformation mechanisms in semi-solid steels could for the basis of physically-based models of semi-solid deformation during the continuous casting process which could help in the quest to reduce casting defects during steel manufacturing.
Sectors Aerospace, Defence and Marine,Electronics,Manufacturing, including Industrial Biotechology,Transport

URL http://www.imperial.ac.uk/people/c.gourlay/research.html
 
Description Research on phase transformations in Cu6Sn5 has led to a patent: WO2013002112 A1 on reducing the strains or cracks caused by a volume change in the Cu6Sn5 layer in electronic interconnections. Ongoing industrial trials are assessing the feasibility of incorporating this method into the soldering process. Research on nucleation in solder joints has led to new nucleants that can be used to control tin grain orientations in electronic systems. This research has been filed as a patent: Japanese patent 2017-133073.
First Year Of Impact 2013
Sector Electronics
Impact Types Economic