Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
  • C22C1/02—Making non-ferrous alloys by melting
  • C22C1/03—Making non-ferrous alloys by melting using master alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
  • C22C1/02—Making non-ferrous alloys by melting
  • C22C1/026—Alloys based on aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C21/00—Alloys based on aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C21/00—Alloys based on aluminium
  • C22C21/02—Alloys based on aluminium with silicon as the next major constituent
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C21/00—Alloys based on aluminium
  • C22C21/02—Alloys based on aluminium with silicon as the next major constituent
  • C22C21/04—Modified aluminium-silicon alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C23/00—Alloys based on magnesium

Definitions

• the present application relates to a method of making a master alloy (also known as a masterbatch alloy) for refining the grain size of a metal alloy, and to the subsequent use as a grain refiner of the metal alloy.
• a master alloy also known as a masterbatch alloy
• it relates to the preparation of a master alloy for refining the grain size of aluminium-silicon alloys and magnesium alloys (both including and excluding aluminium).
• grain refinement An important objective in the production of metal alloys is the reduction in grain size of the final product. This is known as “grain refinement” and is commonly addressed by adding so-called “grain refiners” which are substances thought to promote inoculation of metal alloy crystals. Grain refinement by inoculation brings many benefits in the casting process and has significant influence on improving mechanical properties.
• the fine equiaxed grain structure imparts high yield strength, high toughness, good extrudability, uniform distribution of the second phase and micro-porosity on a fine scale. This in turn results in improved
• WO 2012/110788 it is disclosed that, instead of salt addition, one can add the niobium diboride grain refiner in the form of a small metal piece of Al-Nb-B master alloy to an Al-Si based liquid alloy to obtain a fine grain size. Addition of concentrated Al-Nb-B alloy ensures the uniform dispersion of NbB 2 into the aluminium melt.
• WO 2012/110788 discloses that a commercial pure Al ingot was melted in an electric furnace at the temperature range 800-850 °C and held for 2 hours. 5wt % NbB 2 (mixture of Nb and KBF 4 ) was added to the melt in order to form an NbB 2 phase. After stirring and removing dross, the liquid metal was cast into a mould in order to result in an Al- Nb-B grain refiner master alloy.
• WO 2012/110788 discloses that a commercial Al-lONb master alloy was melted at 900 °C and added to pure Al to dilute the alloy to form Al-2Nb master alloy. Then the lwt% Boron is added to the melt to result in a master alloy composition of Al-2Nb-B.
• US 3,933,476 discloses a method for the grain refining of aluminium using an addition of titanium, aluminium and KBF 4 .
• GB 1 244 082 discloses a method for adding an alloying or grain refining constituent in the form of a wire or strip to a main metal, wherein the constituent consists of aluminium and one or more of boron, titanium or zirconium.
• a method of producing a master alloy for refining the grain size of a bulk alloy comprising the step of providing an Al-B alloy and adding Nb in elemental form to form an Al-Nb-B master alloy.
• a master alloy in accordance with the invention is first that it can be produced without using a corrosive salt such as KBF 4 ; secondly that the addition of a concentrated Al-Nb-B alloy to the bulk alloy ensures the uniform dispersion of Nb-based phases into the melt; and thirdly that the it results in a finer grain size in the final alloy (in other words, it is a more effective grain refiner).
• the Al-B alloy is prepared by providing an Al-B alloy with a higher boron content than is required and diluting it with elemental aluminium.
• the commercially available 95wt%Al-5wt%B alloy is diluted by adding pure aluminium to produce a 99wt%Al-lwt%B alloy.
• Sufficient elemental niobium is then added until the desired 97wt%Al-2wt%Nb-lwt%B alloy is obtained.
• the metal alloy to which the master alloy is added is (i) an Al-Si alloy comprising at least 3% w/w silicon or (ii) a magnesium alloy .
• the masterbatch (also called a master alloy) may comprise niobium and boron in amounts sufficient to form sufficient niobium diboride in the final alloy product so that when this master alloy is added to the Al-Si or Mg alloy melt, it can refine the grain size of solidified structures. It is conventional when representing the formula of an alloy to omit the weight percent of the highest alloy component.
• a masterbatch alloy for adding to an aluminium alloy may have the general formula Al-Xwt%Nb-Ywt%B where X can be from 0.01 to 99 and Y can be from 0.002 to 25 and the weight percent of the aluminium component is the balance to take the total to 100.
• Fig.1 Typical microstructural features in A1-5B master alloy.
• the boride phase particles (AIB 12 ) are dark in contrast.
• Fig.2 Al-4.05Nb-0.9B master alloy microstructure, showing NbB 2 phase particles Fig. 3 Al-2Nb-2B master alloy microstructure, showing NbB 2 phase particles Fig.4 Al-2Nb-B master alloy microstructure, showing NbB 2 phase particles
• Fig. 5 Photograph of macro-etched surfaces of cross-sections of Al-lOSi cast samples. Photo on left is Al-lOSi alloy and the photo on right is for the sample produced after the addition of Al-2Nb-2B to the melt.
• Fig.6 Optical micrographs of samples shown in Fig.5. (a) & (b) are for Al-lOSi and (c) & (d) are for Al-lOSi with Al-2Nb-2B addition. Finer primary Aluminium and finer eutectic particles can be seen in the sample with Al-2Nb-2B addition.
• Fig.7 Photographs of top surfaces of A380 ingots cast from liquid metal (a) without and (b) with the addition of Al-2Nb-B .
• Fig. 8 Schematic illustration of spatial variation of grain structure in Al-lOSi alloy billets processed without and with Al-2Nb-B master alloy addition.
• FIG.10 Schematic illustration of cross-section of wedge mould.
• Tl, T2, T3 represents positions of thermocouples in the mould. Microstructures of etched surfaces at these three different positions are also shown.
• Fig.11 Microstructures of cross-sections of tensile bars produced using high pressure die casting process for AM50 alloys (a) without and (b) with addition of Al-2Nb-B master alloy.
• Fig. 12. Sketch of the moulds used in the experiment of Example 7. Typical cooling rates obtained in these moulds are depicted in these sketches. Cooling rate decreases as the thickness of cast structure increases.
• Fig. 13 Macroetched cross-sections of Alloy A wedge shape and cylindrical specimens: a) without and b) with addition of Al-2Nb-2B master alloy.
• Fig. 14 Macroetched cross-sections of Alloy B wedge shape and cylindrical specimens: a) without and b) with addition of Al-2Nb-2B master alloy.
• Fig. 15 Macroetched cross-sections of Alloy C wedge shape and cylindrical specimens: a) without and b) with addition of Al-2Nb-2B master alloy.
• Fig. 16 Microstructures of Alloy D. (a) without and (b) with O. lwt of Nb equivalent master alloy addition with Al-2Nb-2B composition. Al-2Nb-2B addition refined grain structures significantly both Al grains and eutectic Si needle size.
• Fig. 17. Microstructures of Alloy E. (a) without and (b) with addition of Al-2Nb-2B master alloy. Gain structure is significantly refined for both Al grains and eutectic Si needle size.
• Fig. 18 Anodised microstructures of Alloy F. Microstructure on left is for cylindrical mould specimen without grain refiner addition. Microstructure on right is with Al-2Nb-2B addition.
• Fig. 19 Macroetched cross-sections of Alloy G wedge shape and cylindrical specimens: a) without and b) with addition of Al-2Nb-2B master alloy.
• Fig. 20 Comparison of microstructures in cylindrical cast samples (a) Alloy G without addition, (b) Alloy G with Al-2Nb-2B master alloy addition. Grain refiner in the form of Al- 2Nb-2B master alloy addition resulted in very fine (-150 microns) grain structure in comparison to large dendrite structures seen in reference samples. Finer primary Si size particles (dark contrast particles) are after Al-2Nb-2B addition.
• Example 1 Processing of Al-Nb-B master alloys
• Nb is in the form of elemental powder, procured from Alfa Aesar, A Johnson Matthey Company.
• Figure 1 shows the microstmcture of Al-B master alloy. The particles which are dark in contrast and spherically shaped are aluminium borides.
• This master alloy together with commercial pure Al ingot with required contents were melted in an electric furnace at the temperature range 800-850 °C and held for 2 hours with appropriate concentrations listed in Table 1. The melt was stirred with a non-reactive ceramic rod.
• Al-lOSi alloy was melted in an electric furnace at 800 °C and held for 2 hours.
• a reference sample is cast in a conical shaped mould.
• the mould was pre-heated to 250 °C and temperature of the melt was maintained at 740 °C, prior to pouring into a conical mould.
• a small piece of Al-2Nb-2B master alloy (equivalent to 0.05wt%NbB 2 w.r.t weight of Al in Al- 10 Si alloy) was added to the remaining melt. 15 minutes later, the melt was stirred for about 1 minute and cast into a conical mould.
• Figure 5 reveals the grain size of Al-lOSi alloy without addition and with addition of Al-2Nb-lB master alloy.
• Example 3 Application of Al-2Nb-lB master alloy to A380 alloy 3 Kg of A380 alloy was melted in an electric furnace at 750 C and held for 1 hours and cast into a steel mould. Another batch of 3 Kg was melted and a small piece of Al-2Nb-B mas ter alloy (equivalent to 0.05wt%NbB 2 w.r.t weight of A380 alloy) was added to the melt. 15 minutes later, the melt was stirred for about 1 minute and cast into a mould.
• Figure 7(a) shows the grain size of this alloy and Fig 7(b) shows the alloy added with Al-Nb-B master alloy.
• the detailed analysis of the ingots revealed that addition of Al-Nb-B mater alloy reduced the grain size from 1 cm to 0.4 mm. The macro-porosity is also significantly reduced.
• Al-lOSi alloy melt is prepared in a graphite crucible with electric resistance furnace.
• the melt temperature is maintained at 800 °C.
• Both ends opened cylindrical steel mould is placed in a vertical tube furnace.
• the hot zone for this furnace is controlled by three zone heating system to maintain uniform temperature along longitudinal direction of the tube.
• the temperature along the axis of the steel mould is maintained at 720 °C.
• the bottom part of the steel tube is closed with a Cu block.
• the melt temperature is reduced to 740 C and then the melt is poured into steel mould.
• the Cu block Prior to pouring the melt, the Cu block is cooled by water jet with flow rate of 4 1/min.
• the time taken to fill the steel tube with melt is ⁇ 5 seconds. Due to cooling provided be the water jet, the melt starts to solidify from bottom.
• FIG. 8 (a) shows schematic illustration of billet produced by this method. Huge columnar grain structure forms in the melt.
• Figure 8(b) shows a schematic illustration of billet produced from the melt added with 0.05wt% of NbB 2 in the form of Al-2Nb-B addition. Columnar grain structure is absent after Al-Nb-B addition and only fine equi-axed grain structure can be seen. Macro-etched surfaces of cylindrical billets cast in unidirectional solidification are shown in Fig. 9. Much needed elimination of columnar growth is achieved through the addition of Al-Nb-B master alloy.
• AM50 alloy was melted in an electric furnace at 690C °C and held for 2 hours. SF 6 +N 2 gas mixture was used to protect the melt from oxidation. Approximately 0.1 wt% of Al-2Nb-B master alloy w.r.t to weight of AM50 was added to the melt and stirred for 1 minute with steel rod. The melt containing NbB 2 was poured into the wedge shaped mould. For comparative purpose an experiment without any NbB 2 addition was also carried out. This wedge shaped mould provides wide range of cooling rate depending on the thickness of casting. The cooling rate between position Tl and tip, the cooling rate could range between 80 °C/s to 1000 °C/s. Both cast samples were polished and chemically etched.
• Microstructures at various positions are compared in Figure 10. Grain refinement is observed, as shown in Figure 10, when Al-2Nb-B is added to the melt.
• Example 6 High pressure die casting of Magnesium (AM50) alloy with Al-2Nb-B master alloy addition
• High pressure die casting is a commonly used process to produce variety of large structures/components for automotive, electronics and construction sector applications. It is a mass production technology. It provides higher cooling rates to the melt and finer grain structure is obtained during solidification process.
• AM50 alloy melt is prepared as described in Example 5. Melt with and without addition of 0. lwt% of Al-2Nb-B is fed to the shot- sleeve of high pressure die casting machine, followed by injecting the melt in to a permanent mould with a plunger and then solidifying it under pressure. At least 15 castings are produced. Each cast structure consists of three cylindrical and three flat bars. Microstructure of cross-sections of a typical cylindrical sample is shown in Fig. 11.
• alloys compositions used to perform the study of the influence of the Al-2Nb-2B master alloy are given in Table 2. These alloys are near eutectic (Alloys A); hypo-eutectic alloy (Alloys B-F) and Hyper-Eutectic (Alloy G) commercially used alloys. Table 2. List of alloys investigated
• the alloys were placed in a clay graphite crucible, melted and, prior to casting, kept at a processing temperature of 790°C at least for 1 hour. At this point, the reference alloy was left to cool down to approximately 740 ( ⁇ 3) °C and cast into a cylindrical mould and wedge shaped copper moulds pre-heated at 250 °C. These moulds are a 30 mm diameter steel mould and a copper wedge shaped mould. In the wedge mould, the cooling rate studied by means of this configuration ranges between 2°C/s to 150°C/s as it can be seen in the sketch presented in Figure 12.
• Fig. 13 The macroetched cross-section of the Alloy A wedge-shaped samples without and with the addition of Al-2Nb-2B are shown in Fig. 13 where it can be seen that the alloy A without grain refiner addition is characterised by an important spatial variation of the grain size of the primary a-Al grains since it ranges from approximately 200 ⁇ (tip) up to almost 1mm (top of the sample). From Fig. 13b, it can be noticed that the addition of the master alloy significantly decreases the mean primary a-Al grain size as well as the spatial variation reducing this latter to between 100 ⁇ and 200 ⁇ . Similarly, the final primary a-Al grain size is less sensitive to the cooling rate and, therefore, industrial components based on Alloy A with fine and uniform grain size could be obtained with a great range of casting processes.
• Macroetched cross-section of the Alloy A cylindrical samples without and with the addition of the Al-2Nb-2B master alloy are also shown in the figure (right side).
• the microstructure of the reference material is composed of coarse primary a-Al grains and there is an spatial variation in size.
• the grain size is fine in the outer diameter, which corresponds to the material solidified in contact with the mould, and then increases noticeably and, finally, slightly decreases in the centre of the cylindrical samples.
• the addition of the master alloy led to much finer primary a-Al grains and levels.
• the alloy microstructure is also less sensitive to the local variation of the cooling rate which is of paramount importance when casting products with different wall thicknesses are manufactured.
• microstructures are presented in Fig 14 to 20.

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• Refinement Of Pig-Iron, Manufacture Of Cast Iron, And Steel Manufacture Other Than In Revolving Furnaces (AREA)

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• 2012
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