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.
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.
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.
People |
ORCID iD |
| Christopher Gourlay (Principal Investigator / Fellow) |
Publications
Belyakov S
(2017)
Recommended values for the ßSn solidus line in Sn-Bi alloys
in Thermochimica Acta
Belyakov S
(2015)
The Influence of Cu on Metastable NiSn4 in Sn-3.5Ag-xCu/ENIG Joints
in Journal of Electronic Materials
Belyakov S
(2017)
Influence of bismuth on the solidification of Sn-0.7Cu-0.05Ni-xBi/Cu joints
in Journal of Alloys and Compounds
Belyakov S
(2015)
Metastable eutectic in Pb-free joints between Sn-3.5Ag and Ni-based substrates
in Materials Letters
Belyakov S
(2014)
Heterogeneous nucleation of ßSn on NiSn4, PdSn4 and PtSn4
in Acta Materialia
Belyakov S
(2018)
Precipitation and coarsening of bismuth plates in Sn-Ag-Cu-Bi and Sn-Cu-Ni-Bi solder joints
in Journal of Materials Science: Materials in Electronics
Cui Y
(2020)
Solidification orientation relationships between Al3Ti and TiB2
in Acta Materialia
Gourlay C
(2014)
Synchrotron Radiography Studies of Shear-Induced Dilation in Semisolid Al Alloys and Steels
in JOM
Gourlay C
(2015)
Nucleation and Growth of Tin in Pb-Free Solder Joints
in JOM
Gourlay C
(2018)
Advances in Electronic Interconnection Materials
in JOM
Gu T
(2018)
Evaluating Creep Deformation in Controlled Microstructures of Sn-3Ag-0.5Cu Solder
in Journal of Electronic Materials
Gu T
(2020)
In-situ electron backscatter diffraction of thermal cycling in a single grain Cu/Sn-3Ag-0.5Cu/Cu solder joint
in Scripta Materialia
Hou N
(2018)
Competition between stable and metastable eutectic growth in Sn-Ni alloys
in Acta Materialia
Kareh K
(2014)
Pore behaviour during semi-solid alloy compression: Insights into defect creation under pressure
in Scripta Materialia
Kareh K
(2017)
Dilatancy in semi-solid steels at high solid fraction
in Acta Materialia
Kareh KM
(2014)
Revealing the micromechanisms behind semi-solid metal deformation with time-resolved X-ray tomography.
in Nature communications
Li Z
(2017)
The Influence of Primary Cu6Sn5 Size on the Shear Impact Properties of Sn-Cu/Cu BGA Joints
in Journal of Electronic Materials
Ma Z
(2017)
Nucleation, grain orientations, and microstructure of Sn-3Ag-0.5Cu soldered on cobalt substrates
in Journal of Alloys and Compounds
Ma Z
(2017)
Harnessing heterogeneous nucleation to control tin orientations in electronic interconnections
in Nature Communications
Ma Z
(2018)
Nucleation and twinning in tin droplet solidification on single crystal intermetallic compounds
in Acta Materialia
Ma Z
(2019)
Mechanisms of beta-Sn nucleation and microstructure evolution in Sn-Ag-Cu solders containing titanium
in Journal of Alloys and Compounds
Ma Z
(2016)
Effects of cobalt on the nucleation and grain refinement of Sn-3Ag-0.5Cu solders
in Journal of Alloys and Compounds
Mohd Salleh M
(2016)
Suppression of Cu 6 Sn 5 in TiO 2 reinforced solder joints after multiple reflow cycles
in Materials & Design
Mohd Salleh M
(2015)
Effect of Ni on the Formation and Growth of Primary Cu6Sn5 Intermetallics in Sn-0.7 wt.%Cu Solder Pastes on Cu Substrates During the Soldering Process
in Journal of Electronic Materials
Morita S
(2015)
Application of a macroscopic model to predict the band segregation induced by shear deformation of semisolid
in IOP Conference Series: Materials Science and Engineering
Nagira T
(2014)
In Situ Observation of Deformation in Semi-solid Fe-C Alloys at High Shear Rate
in Metallurgical and Materials Transactions A
Nagira T
(2015)
Localization of shear strain and shear band formation induced by deformation in semi-solid Al-Cu alloys
in IOP Conference Series: Materials Science and Engineering
Nogita K
(2017)
Reply to 'Comments on "Evidence of the hydrogen release mechanism in bulk MgH2"'.
in Scientific reports
Nogita K
(2015)
Evidence of the hydrogen release mechanism in bulk MgH2.
in Scientific reports
Ollivier M
(2017)
Reaction-induced surface reconstruction of silver in contact with zirconium
in Journal of Alloys and Compounds
Ollivier M
(2016)
Thermal etching of silver: Influence of rolling defects
in Materials Characterization
Ollivier M
(2015)
Etch pitting and subsurface pore growth during the thermal etching of silver
in Philosophical Magazine Letters
Pandee P
(2018)
AlSi2Sc2 intermetallic formation in Al-7Si-0.3Mg-xSc alloys and their effects on as-cast properties
in Journal of Alloys and Compounds
Pandee P
(2014)
Eutectic Morphology of Al-7Si-0.3Mg Alloys with Scandium Additions
in Metallurgical and Materials Transactions A
Peng L
(2021)
Al2MgC2 and AlFe3C formation in AZ91 Mg alloy melted in Fe-C crucibles
in Journal of Alloys and Compounds
Salleh MA
(2017)
In situ imaging of microstructure formation in electronic interconnections.
in Scientific reports
Shang H
(2017)
Grain refinement of electronic solders: The potential of combining solute with nucleant particles
in Journal of Alloys and Compounds
Su T
(2019)
Semi-solid deformation of Al-Cu alloys: A quantitative comparison between real-time imaging and coupled LBM-DEM simulations
in Acta Materialia
Su T
(2020)
Rheological transitions in semi-solid alloys: In-situ imaging and LBM-DEM simulations
in Acta Materialia
Xian J
(2015)
Controlling Bulk Cu6Sn5 Nucleation in Sn0.7Cu/Cu Joints with Al Micro-alloying
in Journal of Electronic Materials
Xian J
(2017)
Nucleation of tin on the Cu6Sn5 layer in electronic interconnections
in Acta Materialia
Xian J
(2018)
Influence of Ni on the refinement and twinning of primary Cu6Sn5 in Sn-0.7Cu-0.05Ni
in Intermetallics
Xian J
(2017)
Cu6Sn5 crystal growth mechanisms during solidification of electronic interconnections
in Acta Materialia
Xian J
(2015)
Heterogeneous nucleation of Cu6Sn5 in Sn-Cu-Al solders
in Journal of Alloys and Compounds
Xian J
(2017)
Anisotropic thermal expansion of Ni 3 Sn 4 , Ag 3 Sn, Cu 3 Sn, Cu 6 Sn 5 and ßSn
in Intermetallics
Zeng G
(2017)
Magnesium Technology 2017
Zeng G
(2017)
Magnesium Technology 2017
Zeng G
(2018)
Nucleation and growth crystallography of Al8Mn5 on B2-Al(Mn,Fe) in AZ91 magnesium alloys
in Acta Materialia
| 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. Research on nucleation in solder joints has led to a patent WO2019009427 on new nucleant particles that can be used to control tin grain orientations in electronic systems. Both patents continue to be assessed by industrial partners. |
| First Year Of Impact | 2013 |
| Sector | Electronics |
| Impact Types | Economic |
| Title | ??????????? |
| Description | Provided are a solder joint in which ßSn grains are oriented in a specific desired direction, the ßSn grains having a desired structure, and a bonding method relating to the solder joint. Provided is a solder joint in which at least two copper substrates (2) are bonded to each other using a lead-free solder alloy including Sn. A solder ball (1) relating to the lead-free solder alloy comprises: one or a plurality of nucleation grains (4); and a single grain ßSn of which a [001] direction is parallel with a facet plane of the nucleation grains (4), and which is crystal-oriented in a specific direction with respect to the copper substrates (2). |
| IP Reference | WO2019009427 |
| Protection | Patent granted |
| Year Protection Granted | 2019 |
| Licensed | No |
| Impact | This discovery has been patent protected and we are working with our industrial partner to implement it industrially. |