WO2009107119A2

WO2009107119A2 - System and method for reduction of heat treatment in metal casts

Info

Publication number: WO2009107119A2
Application number: PCT/IL2008/000238
Authority: WO
Inventors: Ginat Muginstein; Ran Rosen; Yael Shwartz
Original assignee: Netanya Plasmatec Ltd.
Priority date: 2008-02-25
Filing date: 2008-02-25
Publication date: 2009-09-03
Also published as: WO2009107119A3

Abstract

A method and system for the reduction, and even fully elimination of the heat treatment phase of metal casting are disclosed. The method and system involve applying at least one moving electrical arc over the upper surface of the molten metal in the cast while it solidifies in order to stir it by the forces induced by the electrical arc. The stirring caused by the applied moving electrical arc improves at least one of the casting features including mechanical features, chemical homogeneity and microstructure refinement. This phenomenon enables reduction and even elimination of the heat treatment phase.

Description

SYSTEM AND METHOD FOR REDUCTION OF HEAT TREATMENT IN

METAL CASTS

Background of the invention

[001] Casting a defect free casting with uniform microstructure and chemical composition is almost a mission impossible. During casting, a liquid metal or alloy transform to crystalline solid. During this phase transformation the metal shrink by several percents, causes porosity and internal stresses in the cast. Alloying elements are pushed from the main metal matrix to the grain boundary area and the center of the casting. Due to uneven heat flow, grain size or dendrite size has a large size variation. In order to over come those problems, as cast parts, such as ingots, molded parts, gravity die casting parts, etc. are usually subject to one or more of several types of heat treatments or thermo mechanical treatments, depending on the alloy and the casting type.

Annealing

[002] In casting shaped parts the first heat treatment will be annealing. Annealing is usually performed in relatively low temperature, typically below 0.5T_m (melting temperature) and is aimed to stress relief. For example, as may be found in Casting by

John Campbell (Butterworth-Heinemann Pub., 1999, p. 237) for Al-Cu-Si alloys the

^■7 annealing temperature is around 300^°C, for Brass around 400^°C, for gray iron 600^°C and for steel 700 C. Holding the casting for at least one hour in those temperatures will result in reducing the internal stresses almost to zero. Annealing is also performed to very large steel ingots (5-70 tons) before forging, by heating them to relatively high temperature 1200-1300^°C, before forging. US patent 6,719,858 presents a method for producing large diameter Nickel based alloy ingots that will require annealing by heating the ingot to at least 1200^°C for at least 10 hours.

Homogenization and solution treatments

[003] Semi continuous casting, mainly for casting Aluminum Copper and Magnesium alloys, for producing slabs, billets and blooms, requires different approach. The as cast parts in this case are not the final product and are expected to undergo massive thermo-mechanical process like extrusion, rolling, wire drawing etc. Those processes include high level of plastic deformation, thus require good chemical homogeneity and as little as possible grain boundary phases. In most alloys the grain boundary composed from metallic compound that are strong and brittle, and can barely undergo plastic deformation. In order to solute the grain boundary phases into the main matrix and to have better chemical homogeneity, a homogenization heat treatment is done. The as cast billets, blooms or slabs, are heated up slowly in a furnace and are held in the homogenization temperature for 4-48 hours typically, depending on the chemical composition of the alloy. For example the homogenization parameters (heating temperature, holding time, cooling duration and method) for Aluminum alloys are presented in Table 1 :

References for Table 1 :

1. J.D. Robson, Mater. Sci. and Eng A388, 2002, p. 219-229

2. S. Sequeira Tavares and S. Sgobba, J. of Mater. Proc. Tech. , vol; 143-144, 2003,p.584-590

3. Zhihong Jai et al, Mater. Sci. and Eng A444, 2007, P. 284-290 4. Grazyna Mrowka-Nowotnik et al, J. of Mater. Proc. Tech. , vol;162-163, 2005, p. 367-372

5. J.D. Robson, Mater. Sci. and Eng A382, 2004, p. 112-121

6. E. Cerri, Mater. Sci. and Eng A197, 1995, p.181-198 [004] There has been a large effort in the past to optimize the homogenization heat treatment due to its high costs and the time and energy consumption. US patent 3,953,247 presents a method for preheating slabs or billets in a bath that will allow to shorten the heating time (from room temperature to the desired heat treatment temperature) that will result in over all shortening of the heat treatment time, and applying an apparatus for continuous and more economical heat treatment. US patent 4,042,227 offers a method and apparatus for continuously homogenizing and quenching Aluminium billets, to obtain uniform crystalline structure and improve metallurgical properties. US patent 6,679,956 presents homogenization process for making Copper-tin-Zinc alloys by heating them for at least one hour to 1000-1450^°C. Another patent that proposes a process for production of Cu (copper) alloys is US patent 4,067,750. When Zr (Zirconium) is the main alloying element the homogenization treatment will be at a temperature of 900-1000 C duration of 2 hours.

Heat treatment for continuous casting of steel

[005] Steel billets and slabs are casted in full continuous casting process. The metal is being continuously poured from a tundish to a cooled mold and than continuously thereafter undergo heat treatment and thermo-mechanical processes, until it is finally cut in to shots, tubes, or profile pieces in a desired length. [006] In US patent 5,461,770 optimization of the entire heat treatment and hot deformation process, mainly for steel billets, is presented in order to keep the production period in the production line as short as possible and to prevent heat losses during the heat treatment processes. [007] The above referenced solutions are aimed to solve the need to optimize, and investigate the best way to heat treat as cast alloys. None of them suggest a method to treat a cast alloys during the solidification in such way that will result in dramatically reduction or even substantially fully elimination of the heat treatment process as much as time and heat are involved.

Summary of the Invention

[008] The present invention discloses method and system for dramatically reducing the time and heat energy invested during the heat treatment processes, such as annealing and homogenization processes, compared with time and heat energy spent for achieving similar results according to prior art methods and systems. In one embodiment of the present invention a use of a method for stirring the metal during the solidification is disclosed, using plasma arc electrode such as, for example, a plasma arc as disclosed in US patent 6,169,265 for the applicant of the present invention or using the method described in US patent 7,243,701 for the applicant of the present invention, so as to create improved uniformity of microstructure and chemical homogeneity in as cast parts and as a result to reduce or even completely eliminate the after-cast homogenization and annealing treatments. [009] The arc is typically moving in the horizontal plan in close circular or rectangular path or in a line shaped path, yet the movement of the electric arc, as different from a movement of the electrode, may draw a different pattern. It can be applied during the entire solidification stage, such as with continuous casting; semi- continuous casting and some cast parts, or in intervals in other cast parts. Example for applying heat in intervals: In 30 kg of aluminum-made cylinder head of an internal combustion engine with 6 risers, 6 rotating arcs were applied over the risers. A tension of 35 Volts was applied to each of the arcs, over each riser in the following sequence: The arcs were applied 12 seconds after the end of pouring, a current of 200 Amps was applied in each arc for 40 seconds; then the arcs were switched off for 20 seconds, and then each arc was switched ON with current of 100 Amps for another 40 seconds.

[0010] The maximum energy used to treat a casting very much depends on the type of alloy, casting weight and the casting geometrical complexity. General example of estimation of maximum required energy is presented in table 2:

Table 2: Estimation of the maximum energy required to stir a metal during the solidification stage in order to reduce the need for heat treatment stage.

It will be apparent for a person skilled in the art that the energy values presented in Table 2 above are much lower than energy values required, in similar conditions, for melting the alloy. For example, for shaped parts of the first row of Table 2 the energy required for melting aluminum (Al) alloys, copper (Cu) alloys and Ferrous alloys will be in the magnitude of order of 0.5-0.6 KWH/Kg.

Brief Description of the Drawings

[0011] The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which: Fig. 1 is a schematic illustration of a cross section of a billet presenting positions at which samples of the micro structure were examined;

Figs. 2A - 2F are a set of micrographs comparing samples from casting taken from three different points of a billet, between a billet that was treated according to the present invention and a billet that was not treated this way;

Fig. 3 is a bar graph which presents the Dendrite Arm Spacing (DAS) in three different points along the radius of a billet treated according to the present invention, as compared to a billet that was not treated this way;

Figs. 4 A - 4F are a set of micrographs comparing results of homogenization in three different points along the radius of a billet, for a billet that was treated according to the present invention and for a billet that was not treated this way, after one hour of holding time; and

Figs. 5 A - 5F are a set of micrographs comparing results of homogenization in three different points along the radius of a billet, for a billet that was treated according to the present invention and for a billet that was not treated this way, after four hour of holding time

[0012] It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.

Detailed Description of the invention

[0013] Reduction of heat treatment time and energy in as cast alloys after casting may be achieved if uniformity of the chemical composition and fine microstructure of the as cast alloy is improved.

[0014] According to one embodiment of the present invention saving of at least 30% of the heat treatment time and energy as compared with casting methods and systems which do not employ electric arc stirring during solidification, and in some cases completely eliminating the need for heat treatment.

[0015] According to embodiments of the present invention for as cast parts the following method is disclosed for the reduction of annealing treatment time and / or energy: 1. Casting the part in Gravity Die Casting (GDC) or sand casting;

2. Applying moving electric arc(s) on top of the riser(s) in order to stir the metal during solidification:

The as cast part will have a better chemical homogenization and/or finer microstructure and/or better mechanical properties compared with a as cast part in which arc stirring was not applied during solidification. This effect of the present invention may allow the as cast part to undergo annealing treatment shorter by at least 30% than the time required for an as cast part with which no arc stirring treatment was applied during solidification, while achieving substantially the same or better results as may be measured by any applicable measuring parameter indicating microstructure, chemical homogeneity and/or mechanical properties.

[0016] According to yet additional embodiments of the present invention for reduction of annealing treatment time in casting ingots in a mold may be achieved by: 1. Casting ingot in a mold; 2. Applying moving electric arc(s) on top of the molten metal in the mold in order to stir the metal during solidification:

The ingot will have a better chemical homogenization and/or finer microstructure and/or better mechanical properties, than an ingot with which electric arc stirring was not applied. As a result, ingot which has undergo electric arc treatment during solidification may undergo annealing treatment shorter by at least 30% than the annealing treating time needed for cast parts that were not treated by arc stirring during the solidification, and still achieve substantially same or better results as may be measured by any applicable measuring parameter indicating microstructure, chemical homogeneity and/or mechanical properties. [0017] According to yet additional embodiments of the present invention for casting non ferrous alloys such as Aluminum and Copper in semi-continuous casting for reducing homogenization (solution) treatment time, the following method is disclosed:

1. Casting billets, slabs or blooms; 2. Applying moving electric arc(s) on top of the molten metal in the mold in order to stir the metal during solidification;

The billets, bloom or slabs which are treated as described above will have a better chemical homogenization and/or finer microstructure and/or better mechanical properties, than a billets, bloom or slabs casted without arc stirring. Furthermore, the billets, bloom or slabs may undergo homogenization treatment shorter by at least 30% than treating time needed for casts that were not treated by arc stirring during the solidification, and potentially eliminating the need for homogenization at all and still achieve substantially same or better results measured in microstructure, chemical homogeneity, casting cracks and mechanical properties. Additionally, the homogenization temperature during annealing treatment may also be reduced. [0018] According to yet additional^' embodiments of the present invention for as cast parts for reducing solution treatment time and temperature the following method is disclosed:

1. Casting the part in GDC (Gravity Die Casting) or sand casting,

2. Applying moving electric arc(s) on top of the risers in order to stir the metal during solidification.

The as cast part that were treated as described above will have a better chemical homogenization and/or finer microstructure and/or better mechanical properties, than a part casted without arc stirring. These as cast parts may undergo solution treatment shorter by at least 30%-90% than treating time needed for cast parts that were not treated by arc stirring during the solidification, and potentially eliminating the need for solution treatment at all and still achieve substantially same or better results measured in microstructure, chemical homogeneity and mechanical properties. Furthermore, the solution treatment temperature may be reduced as well. [0019] According to yet additional embodiments of the present invention for casting ferrous alloys in continues casting the following method is disclosed: 1. Casting billets, slabs or blooms, 2. Applying moving electric arc(s) on top of the molten metal in order to stir the metal during solidification.

The billets, bloom or slabs which were treated as described above will have a better chemical homogenization and/or finer microstructure and/or better mechanical properties, than a billets, bloom or slabs casted without arc stirring. The billets bloom or slabs that are continuously treated will need shorter time by at least 30% than treating time needed for casts that were not treated by arc stirring during the solidification, and potentially eliminating the need for homogenization at all and still achieve substantially same results measured in microstructure refining, chemical homogeneity and mechanical properties. Furthermore, the treatment temperature may be reduced as well.

Example 1 : Casting of AI 6063 alloy billets where electric arc stirring according to embodiments of the present invention was applied during solidification

[0020] Feasibility test was done in semi-continuous casting of Al 6063 alloy billets with a diameter of 8 inches according to casting methods known in the art. During casting and during solidification the billets were treated by rotating electric arc in a circular close path over the billets. The electric arc was applied with a current of 130A and tension of 28 Volts during the entire solidification stage of 70 minutes. Argon gas served as a protecting gas to avoid oxidation of the metal surface. The test showed very good results with respect to homogenization time, as shown herein below. The as cast microstructure was examined by optical microscopy in three points along the radius of a cross-section of the billet, at three different distances from the center of the billet, as exemplified in Fig 1, which is a schematic illustration of a cross section 10 of a billet presenting positions at which samples of the microstructure were examined. As seen in Fig, 1 point 12 corresponds to a sample taken close to the center of the cross-section, point 14 corresponds to a sample taken substantially at mid-radius and point 16 corresponds to a sample taken next to the outer surface of the billet.

[0021] Attention is made now to Figs. 2A - 2F, which are a set of micrographs comparing samples from casting taken from three different points 12 - Figs. 2E and 2F, 14 - Figs. 2C and 2D and 16 - Figs. 2A and 2B of a billet (Fig. 1), between a billet that was treated according to the present invention (represented by Figs. 2A₅ 2C and 2E) and a billet that was not treated this way (Figs. 2B, 2D and 2F). Attention is also made to Fig. 3, which is a bar graph which presents the Dendrite Arm Spacing (DAS) in three different points 12, 14 and 16 along the radius of a billet treated according to the present invention, as compared to a billet that was not treated this way. The micrographs of a comparison between billets treated according to embodiments of the present invention described above and casting done with out using electric arc stirring treatment during solidification which are presented in Fig 2 and the measurements of the DAS (Dendrite Arm Spacing) of billets treated according to the present invention and billets which were not treated according to the present invention which are presented in Fig 3 clearly show substantial changes. As may be seen from Figs. 2 and 3 the microstructure of billets which were treated according to embodiments of the present invention is 30-40% finer than the counterparts which were not treated this way and has better size uniformity distribution compared with the counterparts which were not treated this way.

[0022] Dendrite Arm Spacing (DAS) is a commonly used parameter for measuring microstructure features of non-ferrous cast alloys. The DAS results received with billets treated according to embodiments of the present invention and with billets which were not treated with electric arc during solidification are presented also in Table 3 below:

Table 3:

DAS size [μm]

Table 3: DAS of arc treated billets vs. non arc treated casting.

[0023] The as cast billets of the example, both billets treated with electric arc stirring during solidification and billets which were not treated during solidification were heated to, and homogenized at 585 ⁰C for holding time of 1, 2, 3 and 4 hours, respectively, as presented in Table 4.

Table 4: Heat treatment of the samples sample Heat time to 585 ^υC Holding time at Cooling method 585 ⁰C

1 4 hrs l hr Water quench

2 4 hrs 2 hrs Water quench

3 4 hrs 3 hrs Water quench

4 4 hrs 4 hrs Water quench

[0024] Attention is made now to Figs. 4A - 4F and 5 A - 5F, which present the microstructure of the samples described above after 1 hour (Figs. 4A, 4B and 4C representing a billet that was not treated according to the present invention and Figs. 4D, 4E and 4F representing a billet that was treated according to the present invention) and after 4 hours of heat treatment (Figs. 5A, 5B and 5C representing a billet that was not treated according to the present invention and Figs. 5D, 5E and 5F representing a billet that was treated according to the present invention). The grain boundary phase (the dark gray needle shape phase) is much finer in the samples treated by rotating arc according to the present invention after 1 hour of homogenization than in the normal casting sample after 4 hours of homogenization. The solution of the alloying elements in the arc treated billets is much faster than in the normally casted billet. This benefit of the present invention is in addition to a benefit from the stirring of the solidifying metal, which is reduction of casting cracks due to better chemical homogenization and finer microstructure. [0025] It would be apparent to person skilled in the art that scope of the invention is not limited to the examples described above and is defined by the claims that follow.

Claims

Claims:

1. A method for reducing required heat treatment of metal casting comprising: applying at least one moving electric arc by an electrode over the top of said metal casting to stir the molten metal in said casting during solidification to improve at least one of mechanical features and/or chemical homogeneity, and/or microstructure refinement of said solidifying metal casting so as to enable reduction of heat treatment time and/or temperature when applied.

2. The method of claim 1 wherein the casting method of said metal casting is one of a list comprising gravity die casting, sand casting, semi-continuous casting and continuous casting.

3. The method of claim 1 wherein the heat treatment is one of a list comprising; homogenization, annealing, and solution treatment

4. The method of claim 1 wherein said metal casting is one of a list comprising ferrous cast, ferrous alloy cast and non-ferrous cast.

5. The method of claim 1 wherein the electrical energy provided via said electric arc electrode is lower than the energy required to melt said metal casting.

6. The method of claim 1, wherein said reduction of heat treatment time is by at least 30%-50% and more preferably by at least 75% compared with the time and/or temperature required to heat-treat said metal casting without said electric plasma arc stirring.

7. The method of claim 1, wherein the casts will have less cracks than casting without said electric plasma arc stirring.