ASM Online Member Community

Expand all | Collapse all

Segregation during solidification

  • 1.  Segregation during solidification

    West Michigan Admin
    Posted 11-30-2020 11:36
    Greetings!

    I am looking for help understanding some segregation phenomena, specifically in aluminum alloys, although I can see the topic being relevant to other alloy systems. While it would be nice to have the answers through a discussion here, I would also be interested in references to articles / papers that I could acquire and review.

    I was recently reading an article on the influence of mold design (for OES cast buttons) on the results from OES testing of said buttons (Fortier and Trembley 1984). In it, they state that "For elements forming eutectics with aluminium (like Fe, Si, Mg, etc.), there is usually an enrichment in the zones which are cooled first, that is to say, toward the walls of the sample. For peritectic elements (like Ti), the phenomena is inverse, the enrichment is found toward the zone which is cooled last."

    I am looking to understand this better. I remember from my metallurgy courses, when a hypoeutectic material is solidified (non-equilibrium), the first material to solidify is more "pure" while the remaining liquid becomes more highly alloyed. This (to my mind) contradicts the phenomenon that eutectic elements solidify first. Which means I need to expand my knowledge, because it does happen.

    Thank you, in advance, for your help!

    David Betz

    ------------------------------
    David Betz
    Sr. Laboratory Engineer
    Hydro Aluminum Metals, USA
    dbetzasm@gmail.com
    ------------------------------


  • 2.  RE: Segregation during solidification

    Posted 12-01-2020 10:59
    Hello Mr. Betz,

    An initial paper that you could consider is referenced below:

    Sigworth, G.K. Fundamentals of Solidification in Aluminum Castings. Inter Metalcast 8, 7–20 (2014). https://doi.org/10.1007/BF03355567

    It can be accessed for free via ResearchGate:

    https://www.researchgate.net/profile/Geoffrey_Sigworth/publication/267343601_Fundamentals_of_Solidification_in_Aluminum_Castings/links/5dc4c5c3a6fdcc2d2ffb7485/Fundamentals-of-Solidification-in-Aluminum-Castings.pdf

    Once those fundamentals are familiar once again, you could explore subtopics within metallic solidification and segregation by checking out Chapter 14 "Solidification of Metals" of the book Physical Metallurgy Principles by Abbaschian et al.

    In the mean time, what level of technical detail are you looking for beyond what you have mentioned above? I'd be happy to facilitate a conversation with references if you'd like to continue communicating.

    Best,
    Bryer C. Sousa, WPI

    ------------------------------
    Bryer Sousa
    PhD Candidate
    Worcester Polytechnic Institute
    Worcester MA
    (978) 518-1358
    ------------------------------



  • 3.  RE: Segregation during solidification

    West Michigan Admin
    Posted 12-02-2020 08:53
    Bryer,

    Thank you for the response! I downloaded the article and found it quite interesting. A lot I already knew, but there were a number of ideas and perspectives I hadn't considered before. I used to have a copy of the Reed-Hill book lying around - I should see if I still have it. 

    As for the technical level I am looking for.... I probably don't need answers on too technical a level. A lot of high level math and thermodynamics discussions would probably be way above my interest level. But I can definitely handle more than an introduction. I am interested in a conceptual understanding of why this type of macro segregation occurs - what drives it. Then I can leverage that understanding into teaching others.

    At our lab, we have the pleasure of working with a steady stream of interns. And at our production facilities we have a lot of non-metallurgists operating the equipment. These people are, of course, curious as to "why" something happens or "why" something needs to be done in a particular way. And a lot of the people "in charge" have come to accept that things are the way they are without knowing why. Employee turnover, in some cases, has turned tribal knowledge into operating by inertia. I wish to correct that by "re-learning" the fundamentals that have become accepted without knowledge and sharing what I have found out.

    The OES question is more of a curiosity for me than a need-to-know. But as I help to oversee OES operations, explaining why the cast button has to be machined to a certain depth invariably invites the question as to why the variation in chemistry occurs in the first place.

    Of more particular interest is seeing if there is a connection to the semi-continuous casting of aluminum billet, where alloying elements segregate to the billet surface in a "skin."  I have still not had a good explanation of why this segregation occurs. It has become so accepted as true that nobody seems to care why anymore. On, I am sure someone knows and cares and it is probably easier to find than I give credit, but I am hoping for a signpost or two to get me started.
     
    Thanks!

    ------------------------------
    David Betz
    Sr. Laboratory Engineer
    Hydro Aluminum Metals, USA
    dbetzasm@gmail.com
    ------------------------------



  • 4.  RE: Segregation during solidification

    Posted 12-03-2020 02:40
    David,
    With respect to the analysis work and tribal knowledge, that is a problem at any institution, especially with high turnover.  Once the way to do something has been filtered by one or two generations of learners/operators, the original path of the knowledge can become obscured quickly.  I had a portable arc-spark OES where the operators, who had only adequate metallurgical knowledge, attempted to not only pass along the operation training (without the original training materials) but then to give the instrument to someone to operate without assistance in the initial use!  In such a situation you have to have a solid reference for operation, not detailing just a "recipe" for which button to push but a minimum level of the principles involved, and hopefully a trainer able to give direction and feedback.  Any quality control program will quickly break down without such.

    For predicting which way the local composition will skew during solidification, it is complicated and I don't pretend to have the experience.  But your mention of the difference between a eutectic and peritectic points up the fact that to understand which phases are forming, you need to know the general layout of the phase diagram, influencing the local composition/initial solidification.  In a simple eutectic the phases coming out are the near-pure elements, but once you have intermediate ordered compounds or intermetallics and more than a binary system then the next line over may well be a peritectic or intermetallic, and your solidification may be throwing more of that next compound/mixture out rather than the near-pure 0 and 100% lines.  This allows the remaining melt to skew in the opposite direction.  

    As far as the button production, the quicker the solidification the less segregation will occur.  At the opposite end of the time spectrum, for some systems equilibrium cooling may reduce the need for calibrated preparation, though that is rarely feasible in a commercial production environment.  But taking a button and grinding it a certain amount obviously depends on an experience base that has shown in the past that this amount of grinding produces a reasonably representative answer for that given alloy system.  And that is an approximation, one that may vary with the alloy system and even with relatively minor variations within one.  It's possibly worth taking a button and polishing it in cross section, and running SEM / EDS or microprobe on it to get a real handle on the composition vs. depth to validate your process(es).  Normal EDS will handle a lot of elements but you may need an instrument with one of the micro-XRF setups on it to cover all of the elements with higher precision/lower error.





  • 5.  RE: Segregation during solidification

    Santa Clara Valley Admin
    Posted 12-01-2020 12:54
    The Metals Handbook on Casting, Volume 15, 9th(?) Edition has a whole section on "Fundamentals of Growth". There are several short articles discussing the various types of cast structures and the principles of their formation.  ASM published "Solidification" in 1971 based on an ASM seminar.  There are two articles in it by some of the outstanding experts in the subject.

    ------------------------------
    [Dave] Himmelblau
    [Retired Materials & Processes Engineer]
    Mountain View CA
    (650) 968-1121
    CharlesRetired
    ------------------------------



  • 6.  RE: Segregation during solidification

    Pune Admin
    Posted 12-02-2020 02:03
    It's great that you are interested in a better understanding of this phenomenon.
    You should refer to the handbook of phase diagrams for specific alloy systems.
    Alloy systems solidify into different phases . These form at different temperatures and have different compositions. Resulting in a heterogeneous structure. But not necessarily detrimental.
    The aim should be to minimise the detrimental effects and maximize the benefits.
    Eutectics are a classic example, where the alloy properties are better than the individual elements.
    Pearlite is an intimate mixture of ferrite and iron carbide. But has better properties than either pure iron, ferrite, iron carbide or carbon. It's like having an insitu composite.

    ------------------------------
    Rahul Gupta
    Managing Director
    N D Gupta Enterprises
    Pune
    +919423009829
    ------------------------------



  • 7.  RE: Segregation during solidification

    West Michigan Admin
    Posted 12-02-2020 08:55
    Rahul,

    Thank you for your feedback - you give an interesting insight!

    David

    ------------------------------
    David Betz
    Sr. Laboratory Engineer
    Hydro Aluminum Metals, USA
    dbetzasm@gmail.com
    ------------------------------



  • 8.  RE: Segregation during solidification

    Posted 12-03-2020 08:52
    If you're interested in the physical metallurgy of aluminum specifically, this book is pretty good. It's old and there may well be newer versions out there but it has a lot of information on the various phases present in different aluminum alloy systems and their formation.

    https://www.asminternational.org/home/-/journal_content/56/10192/06236G/PUBLICATION/#:~:text=A%20collective%20effort%20of%2053%20recognized%20experts%20on,Aluminum%20Association%20Inc.%20and%20ASM%20International%20Published%3A%201984​

    ------------------------------
    Sean Piper
    Product / Process Metallurgist
    Ellwood Texas Forge Houston
    Houston TX
    773-524-8985
    ------------------------------



  • 9.  RE: Segregation during solidification

    Kansas Admin
    Posted 12-03-2020 08:58

    One of my best classes that I took as an undergrad, was a graduate level Solidification course. It helped be explain many things I saw later in my work experiences on welds, cast irons, and aluminum castings.

    I recall that the text book was written by Merton Flemings?

     

    Patrick Mizik, P.E.

    Principal Metallurgical Engineer
    Haldex Brake Systems

     

    Direct:  816-801-2568 (Lab Office)

    Fax:     816-801-2101

    E-mail: pat.mizik@haldex.com

    www.haldex.com

     

    Haldex AB - The information contained in this communication may be confidential, is intended only for the use of the recipient named above or if improperly named, the intended recipient, and may be legally privileged. If the reader of this message is not the intended recipient, you are hereby notified that any dissemination, distribution, or copying of this communication, or any of its contents, is strictly prohibited. If you have received this communication in error, please re-send this communication to the sender and delete the original message and any copy of it from your computer system






  • 10.  RE: Segregation during solidification

    Detroit Admin
    Posted 12-04-2020 08:11
    There were two very useful texts used in my graduate solidification class that I would suggest.  The first is Solidification Processing by Merton C. Flemings, 1974 (ISBN: 007021283x).  This book is from the McGraw-Hill Series in Materials Science and Engineering and will be somewhat hard to find as it is out of print.  The other is Fundamentals of Solidification by W. Kurz and D.J. Fisher, 1998 (ISBN: 0878498044).  

    I think one of the things that is being overlooked in the discussion so far is that typical OES sample buttons need to be cast in such a way that the face used for the chemical analysis has been chill cast to an extent as to eliminate the normal liquid phase diffusion ahead of the solidification front.  Further this high cooling rate leads to a fast solidification rate and is meant to lock in the an even distribution of solute.  Due to the high solidification rates, you really can't rely on the equilibrium phase diagram to tell you what is going to happen to the phases.  I'd be interested in know if you have done any microstructural analysis showing your OES samples contain a segregated microstructure that aligns with the equilibrium phase diagram.  If so, then I would look at a way to better cool the OES sample to minimize the effect.  

    For example, the steel foundry where I worked, we cast the OES samples on a copper plate leading to a high solidification rate and minimal to no segregation at the plate chilled surface.  As I recall there was an ASTM requirement for these samples.  We did have to grind into the sample in order to get a clean face for analysis as the as cast surface would not be acceptable for analysis.  I'm sure you have already reviewed the relevant ASTM E716 standard but it might be helpful for others to review as well.  

    Hope this helps a little.  
    Thanks
    Dan




    ------------------------------
    Daniel Baker
    Lead Metallurgical Engineer
    GENERAL MOTORS
    West Bloomfield MI
    ------------------------------



  • 11.  RE: Segregation during solidification

    Posted 12-04-2020 08:16
    Hi Mr. Betz,

    When I was in graduate school I did a fair amount of modeling of the semi-continuous direct chill (DC) casting process for aluminum alloys which also commonly has a compositionally enriched area near the surface. There are several works that discuss this phenomena in detail, below are a few of them:

    Vreeman, C. J., Schloz, J. D., and Krane, M. J. M. (September 11, 2002). "Direct Chill Casting of Aluminum Alloys: Modeling and Experiments on Industrial Scale Ingots ." ASME. J. Heat Transfer. October 2002; 124(5): 947–953. https://doi.org/10.1115/1.1482089

    Založnik, M., Kumar, A., Combeau, H., Bedel, M., Jarry, P., Waz, E., 2011. Influence of Transport Mechanisms on Macrosegregation Formation in Direct Chill Cast Industrial Scale Aluminum Alloy Ingots. Adv. Eng. Mater. 13, 570–580. https://doi.org/10.1002/adem.201000341

    Eskin, Dmitry G., and Laurens Katgerman. "Macrosegregation Mechanisms in Direct-Chill Casting of Aluminium Alloys." Materials Science Forum, vol. 630, Trans Tech Publications, Ltd., Oct. 2009, pp. 193–199. Crossref, doi:10.4028/www.scientific.net/msf.630.193.

    Kyle Fezi, Alex Plotkowski & Matthew John M. Krane (2016) Macrosegregation modeling during direct-chill casting of aluminum alloy 7050, Numerical Heat Transfer, Part A: Applications, 70:9, 939-963, DOI: 10.1080/10407782.2016.1214508


    The mechanism in DC casting is due to shrinkage driven flow deep in the mushy zone which redistributes the interdendritic liquid perpendicular to the solidification front. This flow causes the surface to become locally enriched.

    Regards,
    Kyle Fezi


    ------------------------------
    Kyle Fezi
    Fort Wayne Metals
    Columbia City IN
    1.260.438.9149
    ------------------------------



  • 12.  RE: Segregation during solidification

    West Michigan Admin
    Posted 2 days ago
    Kyle,

    Thank you for the help and recommendations! My interest got sidelined by life and I am still working through all the responses. A couple of these articles look like exactly I am interested in knowing, so I will be downloading them and reading them.

    Thanks!

    David

    ------------------------------
    David Betz
    Sr. Laboratory Engineer
    Hydro Aluminum Metals, USA
    dbetzasm@gmail.com
    ------------------------------



  • 13.  RE: Segregation during solidification

    Posted 12-04-2020 11:05
    David

    Segregation during solidification can be either macro-segregation, or micro-segregation. Macro-segregation is mainly due to density differences between the alloy components and elements. This results in regions of the cast metal rich in certain elements than other. regions, e.g., heavy elements segregate to the bottom of the mold. good stirring before casting can eliminate this defect. The micro-segregation occurs at the micro-scale for short distances and typically observed within grains and between grains. The extent of micro-segregation depends on the partition coefficient or sometimes called distribution coefficient "f"of the alloy system. This coefficient equals f = Cs/Cl, where Cs = the solute concentration in the solid phase, while Cl = solute concentration in the liquid phase during solidification. These values are obtained from the phase diagrams. As long as f is not equal to 1, micro-segregation occurs during solidification.
    For f < 1, the solute or alloying element segregates toward the liquid phase during solidification, and the last liquid to solidify will be enriched with solute element. This is very common and the formation of intermetallic phases and second phases at grain boundaries and at inter-dendritic regions is a result of this phenomenon. The formation of Fe-intermetallic phases in aluminum alloys at grain boundaries and inter-dendritic regions is an example.  Particles of about 40 wt % Fe forms at grain boundaries from liquid alloys containing only 0.3 wt% Fe. For f >1, the first solid to solidify is richer in solute than the subsequent solid layers.

    There are some equations that describe this phenomena such as Scheil Equation (complete solubility in the liquid phase, and no back diffusion), lever rule equation (complete solubility in solid and liquid phases) , and other modified equations that take into account other factors such as the back diffusion (diffusion in the solid phase).  

    One of the best books dealing with this subject is "Solidification Processing" by Flemings, and a good discussion can also be found in the book " Microstructure Development During Metalcasting" by John Gruzleski.

    The following link is for a good paper dealing with this in Al-Si-Fe alloys:

    https://link.springer.com/article/10.1007/s11661-003-0116-y

    Hope this helps.

    ------------------------------
    Waleed Khalifa
    Principal and CEO
    Arabic Consultancy Center for Engineering Materials, Inspection
    Maadi, Cairo
    01098163293
    ------------------------------



  • 14.  RE: Segregation during solidification

    Posted 12-04-2020 13:03
    David 
    I forgot to explain something in my previous reply. The engineering alloys are chemically inhomogeneous at the microscale. So the alloy solidifies into different phases with different compositions and physical nature. Even in single phase alloys, segregation occurs within the solid phase (coring phenomenon) and by formation of intermetallic phase particles at the end of solidification. This means the composition of the solid phase changes with time during solidification. This kind of phases is called solid solution and it has a range of comositions and is most common in type of phases in engineering alloys. If the segregation level is beyond the composition range of the solid solution, another phase will form in the alloy.

    ------------------------------
    Waleed Khalifa
    Principal and CEO
    Arabic Consultancy Center for Engineering Materials, Inspection
    Maadi, Cairo
    01098163293
    ------------------------------



  • 15.  RE: Segregation during solidification

    West Michigan Admin
    Posted 2 days ago
    I know it has been a while, but I wanted to take a moment and thank everyone for their help! I have been slowly working my way through the various suggested articles and books. I even tracked down my old Reed-Hill Physical Metallurgy text book to review! I am hopeful from some of the abstracts that you all have provided the answers I was looking for. Thank you!

    David

    ------------------------------
    David Betz
    Sr. Laboratory Engineer
    Hydro Aluminum Metals, USA
    dbetzasm@gmail.com
    ------------------------------