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Rheocasting of aluminium alloys: Process and components characteristics
Jönköping University, School of Engineering, JTH, Materials and Manufacturing. Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.ORCID iD: 0000-0001-6755-2123
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Semi-Solid Metal (SSM) casting is a promising technology offering an opportunity to manufacture net-shape, complex geometry metal components in a single operation. However, the absence of foundry guidelines and limited design data for SSM casting makes it challenging to predict the performance of both process and components. The objective of this research was to develop and offer new solutions to material processing-related issues in the electronics industry. By investigating the opportunities afforded by the recently developed RheoMetalTM rheocasting process, a better understanding of the critical factors needed for an effective manufacturing process and optimised component characteristics was achieved.

A study of the evolution of microstructure at different stages of the RheoMetalTM process demonstrated the influence of multistage solidification on the microstructural characteristics of the rheocast components. The microstructure of a slurry consists of the solute-lean and coarse globular α-Al particles with a uniform distribution of alloying elements, suspended in the liquid matrix. Additional solute-rich α-Al particles were identified as being a consequence of discrete nucleation events taking place after the initial slurry production. In the final components, macrosegregation was observed in the form of variations in the ratio of solute-lean coarse globular α-Al particles and solute-rich fine α-Al particles in both longitudinal and transverse directions.

The relation between microstructural characteristics and material properties was established by determination of the local properties of a rheocast component. The fracture of a rheocast telecom component was strongly affected by microstructural inhomogeneity. In particular, macrosegregation in the form of liquid surface segregation bands and sub-surface pore bands strongly affected the fracture behaviour. Thermal conductivity measurements revealed that regions of the component with a high amount of solute-lean globular α-Al particles showed higher thermal conductivity. The effect of the local variation in thermal conductivity on the thermal performance of a large rheocast heatsink was evaluated by simulation. The results clearly show the importance of considering material inhomogeneity when creating a robust component design.

Place, publisher, year, edition, pages
Jönköping: Jönköping University, School of Engineering , 2016. , 45 p.
Keyword [en]
Rheocasting, aluminium alloy, RheoMetal(TM) process, microstructural characteristics, component properties and performance, robust design process
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:hj:diva-31428ISBN: 978-91-87289-16-3OAI: oai:DiVA.org:hj-31428DiVA: diva2:954428
Public defence
2016-09-23, E1405, School of Engineering, Jönköping, 10:00 (English)
Opponent
Supervisors
Funder
Knowledge Foundation
Available from: 2016-08-22 Created: 2016-08-22 Last updated: 2016-08-22Bibliographically approved
List of papers
1. Effect of superheat on melting rate of EEM of Al alloys during stirring using the RheoMetal process
Open this publication in new window or tab >>Effect of superheat on melting rate of EEM of Al alloys during stirring using the RheoMetal process
2013 (English)In: Solid State Phenomena, ISSN 1012-0394, E-ISSN 1662-9779, Vol. 192-193, 392-397 p.Article in journal (Refereed) Published
Abstract [en]

The RheoMetal process (previously called the Rapid S- and RSF- process) is a novelmethod to produce cost effective, high quality, semisolid slurries for component casting. TheRheoMetal process uses an Enthalpy Exchange Material (EEM) as cooling agent to absorb heat andproduce a slurry. Critical process parameters to create a slurry by robust melting of the EEM arealloy content, stirring speed, EEM to melt ratio, EEM temperature, EEM microstructuralcharacteristics and melt superheat.In this paper, the melting sequence and melting rate of the EEM was studied experimentally. Theeffect of EEM composition, as well as superheat, on evolution of shape and dimension of the EEMduring stirring was investigated. Initial material freezing onto the EEM was observed, followed by astationary phase with subsequent gradual melting of the EEM. It was shown that the characteristicsof freeze-on layer were strongly correlated to melt superheat, EEM temperature, as well as materialcomposition, hence also has significant influence on the melting sequence.

Keyword
Semi-Solid casting, melting, non-dendritic slurry, RheoMetal Process, Experimental Design, Rapid S, RSF
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-19514 (URN)10.4028/www.scientific.net/SSP.192-193.392 (DOI)000315930600060 ()2-s2.0-84870797417 (ScopusID)
Projects
RheoCom
Available from: 2012-09-27 Created: 2012-09-25 Last updated: 2016-08-22Bibliographically approved
2. Solidification and re-melting phenomena during the slurry preparation stage using the RheoMetalTM process
Open this publication in new window or tab >>Solidification and re-melting phenomena during the slurry preparation stage using the RheoMetalTM process
2016 (English)Report (Other academic)
Abstract [en]

The melting sequence of the Enthalpy Exchange Material (EEM) and formation of slurry in the RheoMetalTM process was investigated. The EEM was extracted, together with a portion of the slurry at different times before complete melting, and quenched. The EEM initially increased in size due to melt freezing onto its surface, forming a freeze-on layer. The initial growth of this layer was followed by a constant diameter of the EEM and thereafter subsequent melting. Microstructural characterization of the size and morphology of different phases in the EEM and the freeze-on layer was made. Dendritic equiaxed grains and eutectic regions containing Si particles and Cu-bearing particles were observed in the as-cast EEM. The freeze-on layer consisted of dendritic aluminum slightly tilted by about 30° toward the upstream direction, caused by the rotation of the EEM. Energy Dispersion Spectroscopy analysis showed that the freeze-on layer had a composition corresponding to a higher melting point than the EEM.

Microstructural investigation of the EEM showed that the temperature rapidly increased to 495 ºC, causing incipient melting of Al2Cu and Al5Mg8Si6Cu2 phases in grain boundary regions. Following the incipient melting, the temperature in the EEM increased further and binary Al-Si eutectic started to melt to form a region of a fully developed coherent mushy state. Experimental results and a thermal model indicated that as the dendrites spheroidized and the interface at the EEM/freeze-on layer reached a mushy state with 25% solid fraction, coherency was lost and disintegration of the freeze-on layer took place. Subsequently, in the absence of the shielding effect from the freeze-on Layer, the EEM disintegrates at a higher solid fraction, estimated to be 50%. The fast and complex slurry generation in the RheoMetalTM process is a hybrid process with both rheocasting and thixocasting elements in the process.

Place, publisher, year, edition, pages
Jönköping: Jönköping University, School of Engineering, 2016. 21 p.
Series
JTH research report, ISSN 1404-0018 ; 5
Keyword
Rheocasting, RheoMetal(TM) process, Slurry fabrication, Solidification, Melting, Heat Transfer Model
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-31430 (URN)
Available from: 2016-08-22 Created: 2016-08-22 Last updated: 2016-08-22Bibliographically approved
3. Solidification sequence and evolution of microstructure during rheocasting of four Al-Si-Mg-Fe alloys with Low Si content
Open this publication in new window or tab >>Solidification sequence and evolution of microstructure during rheocasting of four Al-Si-Mg-Fe alloys with Low Si content
2016 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 47, no 3, 1215-1228 p.Article in journal (Refereed) Published
Abstract [en]

Four Al-Si-Mg-Fe alloys with Si contents varying from 1.6 to 4.5 wt pct were rheocast, using the RheoMetal™ process to prepare slurry and cast in a vertical high-pressure die casting machine. Particle size and Si concentration in the α-Al particles in the slurry and in the as-rheocast component were investigated. A uniform distribution of Si in the globular α 1-Al particles was achieved in the slurry. In the rheocast samples, measurement of the α 1-Al particles showed that these particles did not increase significantly in size during pouring and secondary solidification. The two additional α-Al particles types, α 2-Al particles and α 3-Al particles, were identified as being a result of two discrete nucleation events taking place after slurry production. The Si concentration in the α 2-Al and α 3-Al particles indicated that the larger α 2-Al particles precipitated before the α 3-Al particles. In addition, in the as-rheocast condition, the Si distribution inside the α 1-Al particles showed three distinct zones; an unaffected zone, a transition zone, and in some cases the start of a dendritic/cellular zone. The phenomenon of dendritic growth of globular α 1-Al particles during secondary solidification occurred concomitantly with the final eutectic reaction and increased with increasing amount of the Al-Si eutectic phase.

National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-26312 (URN)10.1007/s11661-015-3290-9 (DOI)2-s2.0-84951985830 (ScopusID)
Note

Submitted (and included in licentiate thesis) under the name: Solidification sequence and evolution of microstructure during rheocasting of Al-Si-Mg-Fe alloys with low Si content

Available from: 2015-04-08 Created: 2015-04-08 Last updated: 2016-08-22Bibliographically approved
4. The effect of microstructural inhomogeneity on thermal diffusivity in a rheocast component
Open this publication in new window or tab >>The effect of microstructural inhomogeneity on thermal diffusivity in a rheocast component
2016 (English)Article in journal (Refereed) In press
Abstract [en]

The relationship between microstructural characteristics and thermal diffusivity of a rheocast component was investigated for two Al-Si-Mg-Fe alloys with low Si contents. The microstructural investigation of the components clearly depicted the formation of coarse solute-lean globular α1-Al particles during the slurry fabrication process and fine solute-rich α2-Al and α3-Al particles during the secondary solidification in the die cavity. The microstructural characterization was quantified based on the amount of α1-Al particles in different locations of the component. The result clearly revealed a presence of both longitudinal and transverse macrosegregation of solute-lean α1-Al particles in the rheocast component. The study of thermal diffusivity and hardness revealed that the regions of the component with a high fraction of α1-Al particles had a higher thermal diffusivity but a lower hardness. Silicon in the solid solution was observed to be a critical factor in reducing the thermal diffusivity. The comparison between the effect of longitudinal and transverse segregation on thermal diffusivity showed that the transverse segregation had a stronger impact.

Place, publisher, year, edition, pages
Milano: Associazione Italiana di Metallurgia, 2016
Keyword
SSM casting, Aluminium Alloys, Inhomogeneity, Microstructure, Hardness, Thermal Diffusivity
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-30581 (URN)
Conference
High Tech Die Casting 2016, Venice, 22-23 June, 2016.
Projects
CompCastRheoCom
Funder
Knowledge Foundation, 20100280
Available from: 2016-06-16 Created: 2016-06-16 Last updated: 2016-08-22
5. Influence of Microstructural Inhomogeneity on Fracture Behaviour in SSM-HPDC Al-Si-Cu-Fe Component with Low Si Content
Open this publication in new window or tab >>Influence of Microstructural Inhomogeneity on Fracture Behaviour in SSM-HPDC Al-Si-Cu-Fe Component with Low Si Content
2015 (English)In: Solid State Phenomena, ISSN 1012-0394, E-ISSN 1662-9779, Vol. 217-218, 67-74 p.Article in journal (Refereed) Published
Abstract [en]

In the current paper, a low-Si aluminium alloy (1.4-2.4% Si) was used to fabricate acomplex shape telecom component using Semi-Solid High-Pressure Die Cast (SSM-HPDC),process. Microstructure and fracture characteristics were investigated. The cast material exhibitedmicrostructural inhomogeneity, in particular macrosegregation in the form of liquid surfacesegregation bands in addition to sub-surface pore bands and gross centre porosity. Tensilespecimens were taken from the cast components. Elongation and microstructural inhomogeneitywere investigated and correlated. Fracture surfaces of the tensile specimen were examined underscanning electron microscope (SEM). The study showed that both near surface liquid segregationbands and subsurface porosity strongly affected the fracture behaviour. Dominant for loss ofductility was gross centre porosity. The centre porosity was found to be a combination of trappedgas and insufficient, irregular feeding patterns.

Place, publisher, year, edition, pages
Switzerland: Trans Tech Publications Inc., 2015
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-24393 (URN)10.4028/www.scientific.net/SSP.217-218.67 (DOI)2-s2.0-84914151681 (ScopusID)
Projects
RheoCom
Funder
Knowledge Foundation
Available from: 2014-08-21 Created: 2014-08-21 Last updated: 2016-08-22Bibliographically approved
6. Mechanical And Thermal Properties Of Rheocast Telecom Component Using Low Silicon Aluminium Alloy In As-Cast And Heat-Treated Conditions
Open this publication in new window or tab >>Mechanical And Thermal Properties Of Rheocast Telecom Component Using Low Silicon Aluminium Alloy In As-Cast And Heat-Treated Conditions
2015 (English)In: Light Metals 2015 / [ed] Margaret Hyland, Hoboken, NJ: John Wiley & Sons, 2015Conference paper (Refereed)
Abstract [en]

The growing demand for increasingly more cost and energy effective electronics components is a challenge for the manufacturing industry. To achieve higher thermal conductivity in telecom components, an aluminum alloy with a composition of Al-2Si-0.8Cu-0.8Fe-0.3Mn was created for rheocasting. Yield strength and thermal conductivity of the material were investigated in the as cast, T5 and T6 heat-treated conditions. The results showed that in the as-cast condition thermal conductivity of 168 W/mK and yield strength of 67 MPa was achieved at room temperature. A T5 treatment at 200°C and 250°C increased thermal conductivity to 174 W/mK and 182 W/mK, respectively, while only a slight increase in yield strength was observed. Moreover, a T6 treatment resulted in similar thermal conductivity as the T5 treatment at 250°C with no significant improvement in yield strength. Therefore, the T5 treatment at 250°C was suggested as an optimum condition for the current alloy composition.

Place, publisher, year, edition, pages
Hoboken, NJ: John Wiley & Sons, 2015
Keyword
SSM HPDC, Thin-wall component, RheoMetal process, Thermal conductivity, Heat treating process
National Category
Engineering and Technology
Identifiers
urn:nbn:se:hj:diva-24859 (URN)10.1002/9781119093435.ch37 (DOI)9781119082446 (Print) (ISBN)9781119093435 (Online) (ISBN)
Conference
TMS 2015 144th Annual Meeting & Exhibition
Funder
Knowledge Foundation
Available from: 2014-10-08 Created: 2014-10-08 Last updated: 2016-08-22Bibliographically approved
7. Influence of microstructure and heat treatment on thermal conductivity of rheocast and liquid die cast Al-6Si-2Cu-Zn alloy
Open this publication in new window or tab >>Influence of microstructure and heat treatment on thermal conductivity of rheocast and liquid die cast Al-6Si-2Cu-Zn alloy
2016 (English)In: International Journal of Cast Metals Research, ISSN 1364-0461, E-ISSN 1743-1336, Vol. 29, no 4, 202-213 p.Article in journal (Refereed) Published
Abstract [en]

Thermal conductivity of a rheocast component made from Stenal Rheo1 (Al-6Si-2Cu-Zn) alloy was investigated in as-cast, T5 and T6 conditions. The thermal conductivity measurement in different locations showed variation of this property throughout the rheocast component. The results of microstructural investigation revealed that the ratio of solute-lean α1-Al particles formed during slurry preparation to fine solute-rich α2-Al particles formed during secondary solidification had significant influence on thermal conductivity. The reduced amount of solutes in the α1-Al particles was determined as the root cause of higher thermal conductivity. A linear relation between the fraction of precipitates and the increase in thermal conductivity was obtained and silicon in solid solution is shown to have a dominant influence. As silicon was precipitated during the heat treatment, thermal conductivity increased. For an optimal combination of thermal and mechanical properties, it is therefore important to use an ageing temperature above the temperature of Si precipitation.

Keyword
Aluminium alloy, High-pressure die casting, Microstructure characteristics, Rheocasting, Thermal conductivity, Aluminum, Aluminum alloys, Copper alloys, Die casting, Heat treatment, Microstructure, Silicon, Thermal conductivity of solids, Zinc, Ageing temperature, High pressure die casting, Microstructural investigation, Optimal combination, Si precipitations, Thermal and mechanical properties, Thermal conductivity measurements
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-31227 (URN)10.1080/13640461.2015.1125990 (DOI)000378146800003 ()2-s2.0-84978732257 (ScopusID)
Available from: 2016-08-11 Created: 2016-08-11 Last updated: 2016-08-22Bibliographically approved
8. Effect of Material Inhomogeneity on Thermal Performance of a Rheocast Aluminum Heatsink for Electronics Cooling
Open this publication in new window or tab >>Effect of Material Inhomogeneity on Thermal Performance of a Rheocast Aluminum Heatsink for Electronics Cooling
2016 (English)In: Journal of materials engineering and performance (Print), ISSN 1059-9495, E-ISSN 1544-1024, Vol. 25, no 6, 2116-2127 p.Article in journal (Refereed) Epub ahead of print
Abstract [en]

The relation between microstructural inhomogeneity and thermal conductivity of a rheocast componentmanufactured from two different aluminum alloys was investigated. The formation of two different primarya-Al particles was observed and related to multistage solidification process during slurry preparationand die cavity filling process. The microstructural inhomogeneity of the component was quantified as thefraction of a1-Al particles in the primary Al phase. A high fraction of coarse solute-lean a1-Al particles inthe primary Al phase caused a higher thermal conductivity of the component in the near-to-gate region. Avariation in thermal conductivity through the rheocast component of 10% was discovered. The effect of aninhomogeneous temperature-dependent thermal conductivity on the thermal performance of a largerheocast heatsink for electronics cooling in an operation environment was studied by means of simulation.Design guidelines were developed to account for the thermal performance of heatsinks with inhomogeneousthermal conductivity, as caused by the rheocasting process. Under the modeling assumptions, the simulationresults showed over 2.5% improvement in heatsink thermal resistance when the higher conductivity nearto-gate region was located at the top of the heatsink. Assuming homogeneous thermo-physical properties ina rheocast heatsink may lead to greater than 3.5% error in the estimation of maximum thermal resistanceof the heatsink. The variation in thermal conductivity within a large rheocast heatsink was found to beimportant for obtaining of a robust component design.

Place, publisher, year, edition, pages
Springer, 2016
Keyword
computer simulation, heatsink, microstructural inhomogeneity, rheocasting, thermal conductivity, thermal management
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-29918 (URN)10.1007/s11665-016-2102-8 (DOI)
Projects
CompCast
Funder
Knowledge Foundation, 20100203
Available from: 2016-05-16 Created: 2016-05-16 Last updated: 2016-08-22Bibliographically approved

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