Alloys of the 7000 series (“AA7xxx”) are frequently used for aerospace and transportation applications. However, AA7xxx alloys are difficult to cast as both hot and cold cracks can occur in a cast product. A hot crack is a crack that is generated in a cast product before the solidification of the melt is complete. A cold crack is a crack that forms in the cast product when the melt is completely solidified, and the cast product has reached a lower temperature or even room temperature. A crack is also known as a tear. Both types of cracks are undesirable in a cast product as they negatively influence the properties of the cast product. To avoid the formation of cracks when casting AA7xxx alloys, in particular AA7075, which is known to be difficult to cast, it has been found effective to use a, in comparison to the casting of other AA alloys such as 6xxx alloys, lower casting speed. However, this results in a lower efficiency of a casting system, as it takes more time to produce a cast product.
The present invention provides a method for casting that allows more efficient casting of AA7xxx alloys. The inventors have found that the higher tendency of AA7xxx alloys to from hot and cold cracks during casting is due to their chemistry. That is, long solidification intervals, low-melting brittle intermetallic phases on grain boundaries and between dendrites combined with high thermal expansion coefficients of the phases constituting the microstructure of AA7xxx alloys make these alloys prone to both hot and cold cracking. The inventors found that hot cracks initiate during solidification of melt in the coherent mushy zone, when liquid feeding is restricted and deformation due to high residual thermal stresses exceeds the material strength. The inventors further found that cold cracks propagate during cooling of the solidified material when the material is in its brittle state. The inventors also found that hot cracks are potential initiation sites for cold-cracks.
Accordingly, to alleviate the afore-mentioned problems, the present invention provides a method for casting that allows efficient casting without cracks in a cast product. The method according to the invention comprises a.) determining a diameter (D) of a cross section of a product to be cast in meter (m), b.) determining an intended steady-state casting speed (V) of the product to be cast using direct chill casting in meter per second (m/s), c.) determining a Si content (cSi) in percent by weight based on the total weight of a melt (wt-%) for the melt to be used for casting the cast product, wherein the intended diameter (D), the intended steady-state casting speed (V) and the intended Si content (cSi) are determined such that the equations (I) V*D≤0.00057-0.0017*cSi and (II) V*D≥0.00047-0.0017*cSi and (III) cSi≤0.1 are fulfilled, d.) preparing a melt comprising Zn: 5.30 to 5.9 wt-%, Mg: 2.07 to 3.3 wt-%, Cu: 1.2 to 1.45 wt-%, Fe: 0 to 0.5 wt-%, Si: according to cSi, impurities up to 0.2 wt-% each and 0.5 wt-% in total, and balance aluminium, e.) casting the melt into the cast product having the intended diameter (D) using direct chill casting, wherein the casting is carried out using the intended steady-state casting speed (V).
According to embodiments of the invention two out of the three variables V, D and cSi may be determined based on product or process requirements and the third variable may be determined using equations (I) to (III).
According to embodiments of the invention, the casting of the melt into the cast product may be carried out using between 14 and 20 cubic meter per hour and meter of intended diameter (m3/(h*D)) cooling water for the direct chill casting.
According to embodiments of the invention, in the preparing the melt, between 0.025 and 0.1 wt-% grain refiners based on Al, Ti and/or B may be added to the melt.
According to embodiments of the invention, the diameter (D) of the product to be cast may be the largest circle equivalent diameter in a (for example with respect to the vertical casting direction horizontal) cross section of the product to be cast. The largest circle equivalent diameter may be the diameter of the largest circle that fits into the profile (cross section) of a cast product while only covering material.
According to embodiments of the invention, the diameter (D) of the product to be cast may be larger than 450 mm. Optionally, a wiper may be used to remove water from the cast product. The wiper may be arranged neighboring a sump or bottom, that is on the vertical height of the lower end of the solidification zone during steady-state casting. The wiper may prevent that cooling water from the direct chill mold runs down along the surface of the cast product by providing a physical barrier for the water. The wiper may be designed such that cooling water cannot pass between the wiper and the cast product, e.g. by providing no or a narrow gap between the wiper and the cast product, so that water flowing along the surface of the casted product is diverted away from the surface of the cast product. The removal of cooling water may reduce the cooling rate of the cast product and may also result in an increase of the surface temperature of the cast product by heat transmission from the center of the cast product towards the surface, which may lower cracking tendencies. Accordingly, the temperature of the casted product can be precisely controlled by using a wiper to further mitigate hot and cold cracking tendency.
Herein, SI units or derived SI units are used. Temperatures are given in degree Celsius. Compositions are generally given in percent by weight based on the total weight, wherein the balance is aluminium. When describing the numerical simulations, some phases are described using atomic percent (at %) for a more convenient description of the stoichiometry.
Numerical simulations as well as industrial trials were carried out. The computer simulations involve microstructure simulations as well as casting process simulations. The industrial trials involve casting of billets (generally cylindrical cast products) having a diameter of 405 mm with varying chemical compositions. The billets were cast using a casting system as described e.g. in European Patent Specification EP1648635B1, which is incorporated herein by reference, or in A. Hakonsen, J. E. Hafsas, R. Ledal, Light Metals, TMS, San Diego, Calif., USA, 2014, 873-878.
Numerical Simulations
The numerical simulations involved the development of models that were then, in combination with appropriate data as described below, used for simulations to confirm the effectiveness of the embodiments of the present invention.
Microstructure Model
The Scheil model coded in the software Thermo-Calc (Version S by Thermo-Calc Software AB, Solna, Sweden) together with the TTAL7 database (developed by Thermotech Ltd., available via Thermo-Calc Software AB) has been used to calculate the solidification paths. The Scheil model is not able to predict how the cooling rate influences the microstructure formation. It is built on the assumptions that no diffusion occurs in the solid and that there is complete mixing in the liquid during solidification. Therefore, only the effect of alloy chemistry on the solidification path evolution is considered, while this model ignores kinetic factors such as diffusion.
Process Model
The Alsim model (e.g. described in D. Mortensen: Metallurgical and Materials Transactions B, 1999, 30B, 119-133. H. G. Fær and A. Mo: Metallurgical Transactions B, 1990, 21B, 1049-1061 and H. J. Thevik, A. Mo and T. Rusten: Metallurgical and Materials Transactions B, 1999, 30B, 135-142) is a finite element model for transient simulations of heat, fluid flow, macrosegregation, stresses and deformation for continuous casting processes. For direct chill (DC) casting, boundary conditions are described with a very high level of details regarding contact zones, air gap sizes, and water hitting points. The effects of stresses and displacements on contact zones, i.e. air gap formation between ingot and mould or bottom block, are accounted for in the thermal boundary conditions. Transient temperature and fraction of solid fields are input to a two-phase mechanical model presented in detail in the article: H. J. Thevik, A. Mo and T. Rusten: Metallurgical and Materials Transactions B, 1999, 30B, 135-142. The mechanical analysis is carried out both in the fully solid regions of the ingot as well as in the coherent part of the mushy zone. The upper boundary of the coherent mushy zone corresponds to the solid volume fraction at coherency that is input to the model. The hot cracking susceptibility is estimated by the integrated critical strain (ICS) as further described e.g. in M. M'Hamdi, A. Mo, H. G. Fjær, Metallurgical and Materials Transactions A, 2006, 37, 3069. The criterion is taking into account both the lack of melt feeding during solidification and thermal deformation, as these two phenomena are the main driving forces for hot tearing during DC casting:
This hot cracking indicator ensures that no hot cracking occurs without insufficient feeding. This is taken care of by introducing a critical liquid pressure drop, pc. Above this value it is assumed that liquid feeding will prevent the formation of hot cracks even in the presence of a tensile stress state. When the pressure drop is lower than the critical value, volumetric and deviatoric viscoplastic (weighted by the functions wv and wd) straining of the material are assumed to contribute to the widening of existing pores and their growth into hot cracks. The parameter “gsnof” denotes the solid fraction at which coalescence and bridging between the grains in the microstructure of the cast product are fairly advanced and the alloy has obtained sufficient ductility to prevent the formation of a hot crack.
For cold cracking, the cracking susceptibility is estimated using a critical crack size (CCS) criterion as described in detail e.g. in the article: M. Lalpoor, D. G. Eskin, L. Katgerman, Metallurgical and Materials Transactions A, 2010, 41, 2425. The principle idea of the criterion is that if the defect size (i.e. a hot crack) exceeds the CCS at temperatures when the material is brittle, cold cracking will occur. The criterion accounts for the geometry of the initial defect (e.g. penny-shaped or thumbnail-shaped) as well as the temperature dependent plane strain fracture toughness (Klc). For example, for a penny-shaped (volumetric) crack the criterion is given by:
where σ11 is the first principal stress σ11.
Microstructure Simulations
A series of simulations have been carried out for the alloys listed in Table 1 to simulate how variations in alloying content influence the solidification path and the phase formation towards the end of solidification. The alloying components, Zn, Mg, and Cu are kept fixed while the alloying components Fe and Si are added with different ratios.
It is apparent that the alloys with the highest Si content has a wider solidification interval by 15° C. The reaction which terminates the solidification for the alloys with low Si is
Liquid→Mg2Si+MgZn2 (3)
where the MgZn2 phase also contains Cu, i.e. the phase composition is 33 at % Mg, 30 at % Cu, 16 at % Zn and 11 at % Al. Increasing the Si content leads to a longer solidification interval as Si reacts with Mg to form Mg2Si. Less Mg will then be available for formation of the MgZn2-phase. If the amount of MgZn2 phase is insufficient to tie up all the Cu in liquid solution, low melting Cu containing phases, e.g. Al2CuMg_S and Al7Cu2M will form resulting in a wider solidification range. The iron bearing phases, are early forming phases and the variations in Fe are found to have no influence on the end of solidification and the solidification interval length.
Process Simulations
Cracking tendencies of the model alloys A2, A3, A6 and A7 have been compared by process modelling. Fully coupled heat transfer, flow and mechanical simulations were performed for casting of billets of the model alloys with diameter 405 mm using the LPC casting technology as described e.g. in EP1648635B1. The 2D axis-symmetric start up geometry and mesh is shown in
Transient simulations were performed until a casting length of 1 meter was reached. For all experiments, the casting speed was ramped up from 30 to 36 mm/min (millimeter per minute) after a short holding period of 30 s seconds and then kept constant (steady-state casting speed). The water amount was set to 7 m3/h (cubic meter per hour).
The critical crack size criterion is shown together with the peak principal stress and the mean stress for alloy A2 in
Physical Experiments
A series of billets with varying chemical compositions as given in Table 2 were produced using direct chill casting as described in EP1648635B1, which is incorporated herein by this reference. Generally speaking and with reference to
Six billets were cast in parallel for the present experiments. The cooling conditions were kept similar for all castings. After reaching the steady state, the casting speed was slowly ramped up until cold cracking in two billets occurred. The casting speed when two billets had a cold crack is denoted “critical casting speed” (VCritical) and is given in millimeter per minute. The cold cracking was observed through the audible sound when the cold crack was forming. It was found that the alloys with higher Si content cracked at lower casting speeds, whereas the alloys with a low Si content cracked at higher casting speeds or did not crack. The correlation between the Si content and the critical casting speed is shown in
The inventors found that the critical casting speed is generally independent of the content of Mg, Cu, Fe, and Zn of the melt. The inventors also found that the critical casting speed and the Fe/Si-ratio are independent from each other. However, to improve casting efficiency and product properties, the alloy used in the method according to the present invention may optionally comprise a minimum of 0.01 wt-% Si.
Accordingly, to achieve efficient casting and to produce an efficient cast product, the contents of Mg, Cu, Fe and Zn may be chosen based on desired product properties. However, to ensure good mechanical properties and corrosion resistance of the cast product, Zn is limited to 5.30 to 5.9 wt-%, Mg is limited to 2.07 to 3.3 wt-%, Cu is limited to 1.2 to 1.45 wt-%, and Fe is limited to 0 to 0.5 wt-%. According to embodiments, the Zn content may be limited to 5.60 to 5.80 wt-%. According to embodiments, the Mg content may be limited to 2.30 to 2.50 wt-%. According to embodiments, the Cu content may be limited to 1.20 to 1.40 wt-%. Said narrower limits for Zn, Mg and/or Cu may give the cast product better mechanical properties and corrosion resistance while the tendency to form cracks remains low when the casting is carried out according to the present invention. According to the invention, the balance is aluminium. Impurities may be included in the alloy according to the invention up to 0.20 wt-% for each element and up to 0.50 wt-% in total.
When the casting conditions in direct chill casting for such an alloy do not fulfill equation V*D≤0.00057−0.0017*cSi, wherein V is the casting speed (that is the vertical speed of the bottom block) in meter per second, D is the diameter (for example the largest circle equivalent diameter in meter) of the cast product in meter and cSi the silicon content of the alloy in weight percent, cracking occurs resulting in a cast product with poor quality. On the other hand, when the casting conditions do not fulfill equation V*D≥0.00047−0.0017*cSi, then there is no efficient use of the casting equipment and the production rate of the cast product is insufficient.
When the silicon content of the melt, cSi, is higher than 0.1 wt-%, (and consequently also the silicon content of the alloy that forms the cast product after solidification of the melt), the mechanical product properties deteriorate and in addition the alloy/melt requires a casting speed that is too low.
Accordingly, as shown in
Number | Date | Country | Kind |
---|---|---|---|
20180311 | Mar 2018 | NO | national |
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/EP2019/051364 | 1/21/2019 | WO | 00 |
Publishing Document | Publishing Date | Country | Kind |
---|---|---|---|
WO2019/166156 | 9/6/2019 | WO | A |
Number | Name | Date | Kind |
---|---|---|---|
4157728 | Mitarriura et al. | Jun 1979 | A |
20110297278 | Xiong et al. | Dec 2011 | A1 |
20120087826 | Senkov | Apr 2012 | A1 |
20120186773 | Shaber | Jul 2012 | A1 |
20150336165 | Wagstaff et al. | Nov 2015 | A1 |
20190062886 | Leyvraz | Feb 2019 | A1 |
Number | Date | Country |
---|---|---|
1 648 635 | Apr 2006 | EP |
Entry |
---|
International Search Report dated Apr. 16, 2019 in International (PCT) Patent Application No. PCT/EP2019/051364. |
Dons, Anne Lise, et al., “The Alstruc Microstructure Solidification Model for Industrial Aluminum Alloys”, Metallurgical and Materials Transactions A, 1999, vol. 30A, pp. 2135-2146. |
Mortensen, Dag, “A Mathematical Model of the Heat and Fluid Flows in Direct-Chill Casting of Aluminum Sheet Ingots and Billets”, Metallurgical and Materials Transactions B, 1999, vol. 30B, pp. 119-127. |
Thevik, Havard, et al., “A Mathematical Model for Surface Segregation in Aluminum Direct Chill Casting”, Metallurgical and Materials Transactions B, 1999, vol. 30B, pp. 135-142. |
Fjaer, Hallvard, et al., “ALSPEN—A Mathematical Model for Thermal Stresses in Direct Chill Castings of Aluminum Billets”, Metallurgical Transactions B, 1990, vol. 21B, pp. 1049-1061. |
M'Hamdi, Mohammed, et al., “TearSim: A Two-Phase Model Addressing Hot Tearing Formation during Aluminum Direct Chill Casting”, Metallurgical and Materials Transactions A, 2006, vol. 37A, pp. 3069-3083. |
Håkonsen, Arild, et al., “A new DC casting technology for extrusion billets with improved surface quality”, Light Metals, 2014, pp. 873-878. |
Lalpoor, M., et al., “Cold Cracking Development in AA7050 Direct Chill-Cast Billets under Various Casting Conditions”, Metallurgical and Materials Transactions A, 2010, vol. 41A, pp. 2425-2434. |
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20210053112 A1 | Feb 2021 | US |