GB2162540A - Aluminum grain refiner containing "duplex" crystals - Google Patents

Abstract

In an aluminium-titanium-boron master alloy "duplex" crystals are made by (1) producing aluminides that contain boron in solution, and by (2) aging said aluminide in a manner to precipitate at least part of the boron to form the duplex crystals. The duplex crystals have been discovered to be extremely potent grain refining agents.

Description

SPECIFICATION Aluminum grain refiner containing duplex crystals This invention relates to a novel aluminum grain refiner; and, more specifically, to an Al-Ti-B grain refiner containing an improved structure. A method to produce the refiner is also disclosed. Typically, aluminum refiner alloys ofthis invention consist of in weight percent, 0.05 to 5 boron, 2 to 12 titanium and the balance aluminum plus normal impurities.

It is possible to classifythe prior art of commercial grain refiners in two main categories based on chemical composition, and these two categories can be broken down further into two subcategories based on structure. This classification is depicted below:

Aluminum-Titanium Aluminum-Titaniun-Boron Graln ilners GLULledlel I I I Blocky Needle Blocky Needle Tidal3 Tidal3 TiAl3 TiAl3 crystals crystals crystals crystals This classification runs counter to that usuallyfound in the prior art. In the past, the primary means used to describe a grain refiner was the bulk chemistry ofthe alloy. Even the use ofthe word "alloy" is somewhat questionable.Since the solubilities oftitanium and boron in liquid aluminum metal are small, nearly all titanium and boron are present as TiAl3 and boride crystals. Therefore, changing the bulk composition of the alloy only changes the relative proportion of these three phases: aluminum metal, aluminides, and borides.

The morphology ofthe aluminide crystals in aluminum4itanium alloys is determined by the process used to produce this material. For a needle like structure, the titanium must first be put into liquid solution at high temperature. Then Till3 will precipitate in needle form upon cooling. The size of needles is dependent on the cooling rate. The blocky structure results from a growth of TiA13 directly from the source oftitanium in the presence of a liquid solution saturated in titanium. This occurs attemperatures where the solubility of titanium in the liquid is fairly small; i.e., less than about 900"C. The blocky crystals can be very small initially and growthrough a process of agglomeration and recrystallization.

The structure of TIDAL3 present is dependent solely on the process used. It does not depend on composition. It is possible to get 100% blocky or 100% needle structures, or any mixture in between.

The structure of aluminum-titanium grain refiners containing boron have historically been an extension of what has been said above for boron-free (Al-Ti) grain refiners. For any given composition,the resulting structure has been a mixture ofTiAl3 and TiB2 crystals in a matrix of aluminum saturated with titanium and boron. The prior art has considered the borides to exist only as discreet particles (usually having a hexagonal plate morphology), and the morphology ofthe TiAI3 crystals has been either blocky or needle-like. In other words, theTiAl3 morphology in the ternary (Al+Ti+B) follows the same rules as in the binary (Al-Ti) grain refiners. The only apparent differences is the presence of "free" (AI-Ti)B2 orTiB2 crystals.

Examples of attempts to control the blocky and needle structures may be found typically in U.S.

Patents No.3,785,807; No. 3,857,705; and No.

3,961,995. These patents disclose several concepts to obtain improved grain refining alloys. These disclosures are often contradictory and do not clearly solve the problems.

One ofthe present important objects of this invention is to provide an improved, more efficient grain refiner. Another important object of this invention is to provide a novel means to control the effectiveness ofthe grain refiner. Afurtherobject of this invention is to provide controlled processing steps that will produce improved grain refiners.

SUMMARYOFTHEINVENTION Said objects are realized by the provision of novel grain refiners that contain an effective amount of a complex boron-containing aluminide crystal labeled herein as a "duplex" crystal. This so-called "duplex" crystal is obtained by (1) producing aluminide crystals which contain boron in solution (these crystals are labeled (Ti-B)AI3 and (2) aging said crystalsfora sufficienttimeto precipitate all, orat least some, ofthe boron in solution. This results in the desired "duplex" structure of (Ti-B)AI3 and (Al.Ti)B2.

Other structures may also be formed, such as TiA13 andTiB2,asarewell known in the art.

This "duplex" crystal is an extremely potent grain refining agent.

This duplex structure is a third aluminide structure as depicted below:

Aluminum-Tituniu3rBoron Crai n RFf iner Blocks I Nsedle 3uple: Tin3 Tidal3 (Ti.s)A13 + Tis2 + TiB2 +Ti52 It should be noted that this structure can be present in grain refinersofvarying bulk composition.

The "duplex" structure has been observed to occur by chance in minute quantities (lessthan about 2%) in grain refiners produced by methods available in the priorart. But, there was no discovery that it is effective in promoting the highest degree of grain refinement. Thus, this discovery, in combination with the discovery of methods to promote the formation of larger amounts of duplex crystals, is the gist of this invention.

Experimentation has been directed towards a thorough understanding of present processes and products. During the investigation of existing grain refiners, it became apparent that two batches of the same product, apparently produced in nearly the same manner, behave differently when used as a grain refiner. When the bulk chemistry was checked, no significant difference could be found, so the reason forthe difference between the two products was obscure.

As a result, a procedure was devised to reveal the three dimensional morphologyofthealuminidesin situ. By using an iodine-methanol solution, the aluminum matrix was etched away, leaving the aluminides in relief. The deep etched samples were examined with a Scanning Electron Microscope (SEM). Itwasthrough this method that an understanding ofthestructureofthealuminideswas obtained.

Using the dissolution procedure, it was noticed that better grain refiners exhibited some aluminides whose morphology deviated from the typical blocky type of TiAl3, which is illustrated in Figure 1. All the photographs of aluminides are secondary electron images obtained on SEM. Figure 1 a showsthe blocky crystal at 2000x magnification. Figure 1 b shows the crystal at4000x magnification. The blocky crystal had 10 sides, and one major dimension was largerthan the other two dimensions. The surfaces were very smooth and the overall appearance was almost like a cut gemstone. Also visible in the SEM photos was some occasional marbling or streaking of a siliconcontaining phase, which did not change the shape of the crystal.

Thesecondtypeofaluminide, present in the better products, displayed a wide range of morphologies.

Some aluminides showed only some surface rough: ness; others bear little resemblance to TiAl3. The surfaces of the latter are very rough, and have pock marks and bumps. The numberofsides has dropped from 10 to 6, and edges sometimes have an appear ance of being layered. In addition, the surface ofthe "duplex" aluminides are covered with smalll boride particles. Figure 2 shows various types ofthe irregular "duplex" structure. Figure 2a shows the duplex crystals at 1500x magnification. Figure 2b shows the duplex crystals at 2000x magnification.

Figure 2c shows the duplexcrystals at3000x magnification. As used herein, the term "duplex" aluminide structure defines this type ofaluminide. The "duplex" aluminide structure is the most critical aspect of this invention.

From this structural characterization, it was disco vered that grain refining performance could be predicted from the structure of the aluminides, but the process to produce the desired structure remained unknown. In otherwords, normal process variations in prior art would sometimes accidentally produce a small amount ofthis superiorstructure. It was then decided to examine the processing variables more carefully to see what the "accident" was.

Then it would be possible to produce the desired structure more efficiently. In fact, by careful control of the structure in a scientific way, it should be possible to produce a grain refinersuperiorto anything which the priorarthas made, once the importantfactors were established. A series of scoping experiments were then instituted; and itwasfound thatthe important process parameters were reaction temperature, flux ratio, stirring speed, order of addition of the reactants, and the amount of holding time. A more detailed explanation of each of these items and a brief description of the process follow.

The process consists of placing aluminum metal intoafurnaceand bringing ittothe reaction temperature. Atthistimea mechanicalstirreris placed in the molten metal and broughtto the correct stirring speed. (Electromagentic stirring may also be employed.) Titanium-bearing salts, and/or possibly titanium sponge (ortitanium alloy chips), are added.

Then a titanium- and boron-bearing flux is fed to the surface ofthe melt. When the chemical reaction is complete, the spent (reacted) salt is decanted and the Al-Ti-B grain refiner is placed in a holding furnace, where it is stirred for a predetermined period oftime.

The more important process parameters are defined below.

1. Reaction Temperature This is the average temperature ofthe molten aluminum during the reaction as measured bya thermocouple immersed in the aluminum bath. Since the reaction between the salts and the molten metal can be rapid and violent, it is not feasible to measure the actual reaction temperature at the salt/melt interface. However, the average metal temperature defined here has been found to correlatewell withthe structure produced.

2. Flux Ra tio Aflux is defined as a mechanical mixture oftwo or more salts. For this investigation, the two salts used were K2TiF6 and KBF4. The flux ratio is the weight ratio of contained titanium divided by the contained boron in the salt nixture.

3. Stirring Speed All experiments were conducted with a mechanical stirrer having a flat two-bladed propeller. For con veniencethe energy input is expressed in RPM since the size ofthe propeller and crucible was a constant.

4. OrderofAddition Fora given grain refiner composition, a number of combinations of salt and/or flux additions can be used. Forexample, theflux can be a blend of all necessary components. Forth is case, the procedure would only be to include the flux addition. A second example would be this: If the flux contains half ofthe required titanium, the other half could be added as a salt (K2TiF6 or as titanium sponge. This remaining, or "excess" titanium, over what is contained in the flux, can be added either before or afterthe flux addition.

5. Holding Time This is the amount oftime thatthe melt is held after the chemical reaction between salts and metal has gone to completion. The holding temperature may or may not be the same as the reaction temperature.

Also, mechanical or electromagnetic stirring is maintained during the holding time where the alloy is liquid. The stirring speed during the holding period may or may not be the same as in the reaction period.

Figures 3 and 4 show the effects of holding time.

Figure3showstheboron-containing aluminides- (Ti.B)Al3-at2500xmagnification in an alloy priorto the holding step. Figure 4 shows the aluminides at 1500x magnification in the same alloy after a holding time of 60 minutes. The aluminides after holding are no longer a single phase; borides have precipitated on the surface, forming the desired "duplex" structure. It is clearfrom this resu It that the holding time is critical for the formation of the "duplex" structure.

Practical Limits ofthe Process Variables There are a large number of combinations ofthe above conditions that will result in the production of a good grain refiner. At the present time the ranges that can be suggested are: 1. Reaction Temperature-700-900"C The lower limit of 700"C is a practical lower limit to maintain the metal as a liquid. The upper limit of 900 Cwill produce a structure that is 90% or more "blocky" with some needles.

2. Flux Ratio-2.2 to 22.5 Depending on the target composition, (for example, 5%Ti-1 % B or 5%Ti-0.2% B), the flux ratio to be used should allowforsometitaniumto be added separately. Thus, if a 5%Ti-1 %B grain refiner is to be made, and all Ti and B are added as a flux, then the flux ratio would be 5.0.Butthe 5.0 flux would notyield the best grain refiner because it does not have a separate titanium addition. (Our experiments show that best results are obtained when 10% or more of theTi addition is made separately.) Thus, the maximum flux ratio for some commercial alloys would be: Composition Flux ratio (Ti:B) 5%Ti-1%B 4.5 5%Ti-0.6%B 7.5 5%Ti-0.2%B 22.5 The maximum limit (22.5) isforcertain existing commercial alloys only. If composition is allowed to change to lower boron levels, as noted in the discussion below, this flux ratio may also increase.

The lower limit (2.2) is imposed because belowthis ratio there is an excess of boron so that separate crystals of TiB2 are formed, which is not desirable.

3. StirringSpeed-Gentleto Vigorous The amount of stirring is dependent on the product being produced, the temperature and the flux ratio.

Stirring speed during reaction is not of first order of importance, but can help to improve the comparison made in the other variables.

4. Holding Time The holding time required depends on the holding temperature, as shown in Figure 5. It seems probable thatthe precipitation of borides occurs during holding. From theoretical considerations, thetime requiredfora precipitation process to occur is logarith micwith the reciprocal ofabsolutetemperature.

Hence, a semilogarithmic plot has been employed in Figure 5. The solid bands indicate the optimum holding times found experimentally for a series of high purity laboratory grain refiners having the composition of 5%Ti and 0.2%B. Shorter times (i.e., in the lower portion ofthefigure) are underaged, so aluminidesaresimilartothoseshown in Figures 1 and 4. The "duplex" aluminide (examples are shown in Figures 2 and 4) occurs attimeswithin the band given bythetwosolid lines. As shown below,there can be a very substantial improvement in the grain refining performance of materials held forthe proper time. The lower and upper solid lines in Figure5 represent respectively the beginning and the end of this improvement. The optimum performance is found roughly in the center ofthetwo lines.

At excessively long holding times the "duplex" structure and an "overaged" condition is found.

Examples are given at 5000x magnification in Figure 6. Figure 6a shows an aluminide produced by holding 144 hours at600 C. Figure 6b shows an aluminide formed by holding 504 hours at the same tempera ture. There areveryfew borides on the surface of these particles; and they are larger in size. Also, the aluminides now have an irregularscalloped or cellular shape on the surface.

It should be noted thatthe desired "duplex" structure has been produced by aging both in solid and liquid states. (The melting pointofaluminum is 660"C.) The lowest practical holding temperature has not been established experimentally, but may be estimated from the lines in Figure 5. For example, if one is not prepared to hold for more than 1000 hours, the minimum temperature will be about 420"C.

(788"F).

Since the data on holding time in Figure arefor laboratory alloys, and since commercial alloys will have varying amounts of im pu rities -- Fe, Si, V and Cu are most common - it is possible that the correct holding time for commercial alioys mayshiftsome- whatfrom the results indicated by the two solid lines.

The extent of the shift is not possible to predict a priori, but it most probably would change by no more than a factor of 1.5, as indicated by the dashed lines in Figure 5.

5. OrderofAddition The excess Ti should be added first. If it is added last, it has a harmful effect on the metallurgical quality and also on the recovery.

6. Effective Contents of Duplex Crystals As stated earlier, duplex crystals have been observed to occur adventitiously in the prior art. It has been observed that such crystals may occur up to about 5% ofthe aluminides present in the grain refiner. Furthermore, it appears that some beneficial effects ofthe duplex crystals are noted in contents as low as 2% ofthe grain refiner. The benefits ofthis invention are provided when the grain refiner contains more than the range of 2 to 5% duplex crystals as a result of deliberate processing.

The percent of duplex crystals can be determined by measuring the number of duplex and conventional aluminides. One merely needs to divide the total number of duplex aluminides by the total number of all the aluminides and then to multiply by 100to convertto percent.The numberof duplexand conventional aluminides is obtained by examining the deep-etched grain refiner and by using a scanning electron microscope (SEM) as a point-counting machine. In this method, a network of scan areas in the SEM is disposed uniformly over a typical random sample area. The number of duplex and conventional aluminides are tabulated in each scan area, repeating the process until a sufficient number of measurements have been obtained.

Figure 7 is the preferred flow of processing steps to obtain the optimum benefits of this invention. Critical operating parameters are also indicated in Figure 7.

(Figure 7 is drawn forthe case of holding in the liquid state. Forthe case of holding in the solid, step No. 5 is omitted, and holding at elevated temperature occurs after casting.) Following are the preferred (optimum) parameters for each ofthe operation steps, as shown in Figure 7.

Operation 1) The stirring speed may be gentleto vigorous with the temperature above the melting point.

Operation 2) The reaction temperature may be 725-825"C with vigorous stirring speed and 10 to 80% excess titanium.

Operation 3) The reaction temperature may be about 760"C (+50"C) at vigorous stirring speed and a flux ratio about 2.2 to 2.8.

Operation 4) The time to decant salt may be as small as reasonably possible, so that sedimentation of solid particles does not occur.

Operation 5) The holding time and temperature may be in the range of values indicated bythe dashed lines in Figure 5.

When making alloys of high Ti:B ratios (15 to 50 or more),the following parameters are suggested: 1) when excess titanium is about 1 0%,theflux ratio maybe 13.5to45; 2) when excess titanium is over 80%, the flux ratio may be 2.5 to 3.0.

The duplex crystal structure has been seen to be produced by a well defined sequence of processing steps. Firstly, there is the simultaneous reduction of boron and titanium-containing salts by stirred liquid aluminum. This produces an aluminide crystal which appears to contain boron in solution: the (Ti,B) Al3 phase shown in Figure 3. Then after a specified holding period at elevated temperature; as shown in Figure 5; boride particles precipitate and the duplex structureforms.

Thatthiswell-defined processing sequence producesthe duplex structure has been shown earlier by the SEM photographs in Figures 2 and 4. The effect on grain refining response is shown in Figure8fora commercial heat of grain refiner having 5% Ti and 0.2% B. Verysmall quantities of this grain refiner (0.001 % Ti addition level) were added to a melt of 99.7% Al held at 800"C, and small castings were made attimes of 1,2,5,10,25,50, and 100 minutes after the addition. The castings were then etched with acid to reveal the grain structure, and the average grain size was measured under a stereo microscope by using the line intercept method.The time afterthe grain refiner addition is called the "contact time",- that is,the time the grain refiner has been in contact with the 99.7% Al melt.

These tests represent an addition level of one part of grain refinerforeach 5000 parts of liquid metal.

This is a very severe test, and clearly establishes the difference in performance between the prior art and the novel duplex structure. Curve (a) in Figure 8 (the uppercurve) represents a sampleofan alloy cast at the end of processing step number4 in Figure 7. (That is, holding period was omitted.) Curve (b) in Figure 8 isforthe same alloy as in curve (a), only it has been heldfor5 hours at about 700"C. In otherwords, two portions of the same heat are shown here. Curve (a) is for a structure not held for times sufficientto produce the duplex structure. Less than about 2% ofthe aluminides were duplex.The grain sizefound in the castings containing this refiner isfairly large, and the grain refining effectfades after 25 minutes. Curve (a) is typical of product produced according to the prior art. The product in curve (b), however, is much better, since aboutone-fifth ofthe aluminides in this grain refinerwereduplex. Notonlywerefinergrains obtained, but no fading was observed at contact times of 100 minutes.

A single, well-defined, sequence of processing steps has been found to produce a superior grain refiner. However, now that the discovery of the duplex structure has been made, it is easy to envision other processesthatwould produce a similar product.

One simple example of a different process would be the use of a differentflux. K2TiF6 and KBF4were used here,butothertitanium-and boron containing halogens are available (e.g., NaBF4and Na2TiF6). One could also envision the simultaneous reduction of TiO2 and B203, which have a small, butfinite, solubility in potassium- and sodium cryolte melts. For this reason, the partial or complete substitution of KBF4orK2TiF6with other compounds-as long as the same structure is produced -- must be considered as part of this discovery.

It is also possibleto imagine another possibility.

The active role of boron in the duplex structure is apparently to act as a catalyst to change the structure ofthe aluminide-TiAl3. It is well known that neighboring elements ofthe periodic table have similar chemical properties, so the partial substitution of boron with these elements (such as C, Si, N, P, Be, and Mg) must also be considered to be part of this discovery.

In a similar fashion, one could partially replace Titanium with its neighbors (V, Zr, Cb, Hf, and Ta).

Claims (9)

1. A grain refiner; consisting of aluminum, tita nium and boron; thatcontainsmorethan 2% duplex crystals obtained by producing aluminide crystals containing boron in solution, and aging said aluminide crystals for a sufficient time and temperature to precipitate at least part ofthe boron to form said duplex crystals.

2. The grain refinerofclaim 1 containing more than 5% duplex crystals.

3. The grain refiner of claim 1 wherein the duplex crystals have a structure of (Ti-B)AI3.

4. The grain refinerofclaim 1 wherein the combination of aging time and temperature employed fall between the dashed lines of Figure 5.

5. The grain refinerofclaim 1 wherein the grain refiner consists essentially of, in weight percent 0.05 to 5 boron. 2 to 12 percenttitanium and the balance aluminum plus normal impurities.

6. A grain refiner according to claim 1 and substantially as herein described.

7. A method of producing a grain refinerconsisting ofaluminum,titanium and boron that contains more than 2% byweight duplex crystals wherein aluminide crystals containing boron in solution are aged for a sufficient time and temperature to recipitate at least part ofthe boron to form said duplex crystals.

8. A method according to claim 7 wherein the combination of aging time and temperatures employedfall between the dashed lines of Figure 5.

9. Amethod of producing a grain refinersubstantially as herein described.