EP3215647A2 - Grain refiner for magnesium alloys - Google Patents

Abstract

The grain size of pure magnesium or Mg alloys is refined by employing Mg_1-X A1_XB₂ as a grain refiner wherein x is above 0 and below 1. A1B₂ may be used as a precursor, for example by being added in powder form (pre-synthesised).

Description

Grain refiner for magnesium alloys

The present application relates to the use Mg_1-X Al_xB₂ to refine the grain size of pure magnesium (Mg) or of Mg-Al alloys and to an associated method.

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 grain refiners that are also called inoculants. Grain refinement can improve the mechanical properties of an alloy which involves the formation of fine equiaxed grains at the expense of dendrites. 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 machinability, good surface finish and resistance to hot tearing (along with various other desirable properties). Magnesium alloys have shown great potential in structural applications due to their many appreciated properties. The efforts by automotive sector to reduce fuel consumption and vehicle emissions have increased the demand for lightweight magnesium alloys as a structural material. Apart from automotive sector, magnesium alloys have also attracted application in electric and aerospace industries.

Inoculation of melt is the most commonly used method for grain refinement of Mg alloys as Mg has a hexagonal close packed (HCP) structure and it is difficult to decrease the grain size through thermo-mechanical processing due to limited slip systems. Zirconium has been found to be an effective grain refiner for aluminium- free magnesium alloys (such as ZE43, ZK60 and WE43). However, it has not been possible to employ zirconium as a grain refiner for aluminium-containing magnesium alloys (AZ series alloys and AM series alloys) due to the undesirable reaction between zirconium and aluminium forming stable intermetallic phases which adversely effects grain refinement. CN101457312 (Shengfa Liu) discloses a Mg-Ti-B grain refiner where an elemental magnesium, titanium and boron powder compact is used to grain refine magnesium alloys. US8016957 (B2) and US2007181226 (Al) disclose the addition of titanium to grain refine magnesium.

Addition of boron in the form of an A1-4B master alloy has been shown to grain refine AZ91D alloy [1,2], where A1B₂ is identified as the heterogeneous nucleating site.

Prior research on bismuth addition to AZ91 alloy is shown to refine the Mgi₇Ali₂ precipitates at 2 wt% Bi additions [3]. Such high additions of Bi to Mg-Al alloys results in compositional changes in the Mg-Al alloy and the result is that Bi becomes a major alloying element.

WO 2012/110788 in the name of the present applicant employs niobium diboride in various forms as a grain refiner for magnesium alloys (amongst other things). WO 2014/027184 in the name of the present applicant discloses the preparation of an Al-Nb-B master alloy for refining the grain of magnesium alloys (amongst other things).

US 6,395,224 A (Nishino) discloses the grain refinement of magnesium alloys through the combined addition of boron and manganese to magnesium.

CN 101457312 A (Univ Wuhan Sci & Eng) discloses the use of compounds such as TiB₂, AI4C3, A1N, SiC, B₄C, or TiC to grain refine Mg-Al alloys.

JP 2002115020 A (Toyota) discloses a method of manufacturing thin sections of a wrought Mg alloy suitable for hot working. Particles selected from the group of titanium chloride, titanium boride, aluminium nitride, titanium nitride, magnesium boride, aluminium boride, molybdenum boride, vanadium boride and aluminium chloride with particle sizes 10 μπι or less are dispersed in magnesium in the amount 0.1 to 10 vol.% during the solidification of the base material.

Material Science Forum, Vol. 710, 2012, p. 161-166 (Suresh) discloses the grain refinement effect of an 96wt%Al-4wt%B master alloy in Mg. It attributes the grain refinement in Mg to the growth restriction factor of Al and the A1B₂ nucleant particles. 'Effects of Α13ΤΪ4Β master alloy on microstructure and properties of Mg-7Al-0.4Zn-0.2Mn alloys' in Chinese Journal of Nonferrous Metals, Vol. 15, No. 3, March 2005, pp. 478-484 reports the grain refinement in Mg-7Al-0.4Zn-0.2Mn alloy by addition of 0.3% Α13ΤΪ4Β master alloy. The grain refinement in this article is assumed to be through TiB₂ and A1B₂ heterogeneous nucleation.

CN 102383013 A (Byd) discloses the development of a deformed magnesium alloy containing 2.0-4.0 percent of Al, 0.40-1.70 percent of Zn, 0.20-0.50 percent of Mn, 0.05-1.0 percent of B and the balance of Mg or Mg and impurities. The addition of boron could be either in the form of ingot or as an Al-B master alloy.

'Influence of boron addition on the grain refinement and mechanical properties of AZ91 Mg alloy', Materials Science and Engineering, Vol. 525, No. 1-2, Nov 2009, pp. 207-210 shows that the addition of A1-4B master alloy could grain refine the AZ91 alloy. This grain refinement is attributed to the presence of A1B₂ phases.

In accordance with a first aspect of the invention, there is provided the use of Mg_1-X Al_xB₂ to refine the grain size of pure magnesium or Mg-Al alloys, wherein x is above 0 and below 1. In accordance with a second aspect of the invention, there is provided a method of refining the grain size of pure magnesium or of a Mg-Al alloy, including the step of employing Mg_1-X A1_XB₂ as a grain refiner wherein x is above 0 and below 1.

Without wishing to be constrained by theory, it is believed that A1B₂ addition produces Mg rich boride (Mg_1-X A1_XB₂). The presence of Mg-rich boride can be confirmed when A1B₂ is added by measuring the superconducting signature of the Mgi_-XA1_XB₂ phase.

In a preferred embodiment therefore, A1B₂ is added to the Mg-Al alloy, wherein the A1B₂ converts to Mg_1-X A1_XB₂ in the alloy. Preferably, A1B₂ is added in powder form (pre- synthesised). We have identified that Mg_1-X A1_XB₂ is the effective grain refining compositions in Mg alloys Lattice constants a and c of Mgi_-XA1_XB₂ decreases from 3.084 to 3.064 and 3.528 to 3.4466 as x increases from 0 to 0.4. At x=0.5, MgAlB₄ super structure forms. In 2001,

superconductivity was discovered in this crystalline phase. When Al is partially doped in Mg site, the crystal structure remained unchanged, but the lattice parameters decreased. The doped phase Mgi_-XA1_XB₂ is also a superconducting phase, but the transition temperature decreases with increasing Al content 'x'. In this application we have utilised the

superconducting property of Mgi_-XA1_XB₂ to detect boride phases present in the grain refined Mg alloys. Master alloys containing Mgi_-XA1_XB₂ have shown to refine the grain size significantly in Mg-Al alloys.

Al based master alloys when used as a grain refiner for Mg alloys, leads to increase in formation of interdendritic grain boundary phase Mg₁₇Al₁₂, which will limit the ductility of the matrix. A weak Mg/Mgi₇Ali₂ interface will form as the BCC structure of Mgi₇A ₂ is not coherent with the HCP structure Mg structure. Also, grain boundary sliding can take place at elevated temperatures due to poor thermal stability and discontinuous precipitation of this phase [4]. We have discovered that the use of Mg based master alloys is an effective approach to improve mechanical properties in magnesium alloys.

Also, we have identified that Mg_1-X A1_XB₂ acts as a heterogeneous nucleating site.

In an alternative embodiment, B₄C is added to the Mg-Al alloy, wherein the B₄C converts to Mgi_-xAl_xB_2-yC_y, in the alloy, wherein y>0.

Our master alloy Mg-B C has been produced through melt infiltration technique and has been show to grain refine Mg-Al alloy castings at 1 wt% additions. A separate research by Cafri et al, [5] has studied infiltration of B C pre-forms with liquid Mg and AZ91D alloy but fails to study the effect of grain refinement on Mg or Mg-Al alloys.

In an alternative embodiment, there is provided the use of Mg_1-X Al_xB₂ to refine the grain size of pure magnesium or Mg-Al alloys, wherein x is from 0 to 1. References:

1. M. Suresh et al, 'Influence of boron addition on the grain refinement and mechanical properties of AZ91D', Materials Science and Engineering A, 525 (2009) 207-210

2. Nishino et al, 'Grain refinement of magnesium casting alloys by boron addition', In: Proceedings of the International Conference on Magnesium Alloys and their Applications, September 2000. Germany: Wiley, 59-64

3. Guangyin, Y.Yangshan, S. and Wenginag, D., 'Effects of bismuth and antimony additions on the microstructure and mechanical properties of AZ91 magnesium alloy', Material Science and Engineering A, 308 (2001) 38-44

4. Cao, W. et al., 'In Situ Synthesis and Compressive Deformation Behaviors of TiC Reinforced Magnesium Matrix Composites', Materials Transactions, 49 , 11 (2008) 2686- 2691

5. Cafri, M. et al., 'Boron carbide/magnesium composites: Processing, microstructure and properties', Journal of European Ceramic Society, 32 (2012) 3277-3483

A number of preferred embodiments of the present invention will now be described, with reference to the drawings, in which: Figure 1 depicts micrographs of an AM50 alloy showing the grain size when refined by Mg- 5%(Mg,Al)B₂;

Figure 2 depicts what is believed to be the phase transformation of A1B₂ into Mgi_-XA1_XB₂ in liquid Mg;

Figure 3 is a graph showing how the grain size of an AZ91D alloy changes with A1B₂ concentration;

Figure 4 is a graph showing how the grain size of an AZ3 IB alloy changes with A1B₂ concentration;

Figure 5 shows photographs of cooled AZ31B samples with and without grain refining; Figure 6 is a graph showing how the grain size of an AM50 alloy changes with A1B₂ concentration;

Figure 7 shows photographs of cooled AM50 samples with and without grain refining; Figure 8 depicts micrographs of an AZ31 alloy and an AM60 alloy showing the grain size when refined by B₄C;

Figure 9 shows the superconducting signal for the transformation of B₄C into Mgi_-xAl_xB2-_yC_y Figure 10 shows photographs of a billet formed from an AZ3 IB alloy and an AZ91D alloy showing grain refinement from A1B₂ grain refiner precursor; and

Figure 11 shows mechanical test data plots depicting yield strength and elongation for different alloys.

The alloys discussed below and their compositions under consideration are as given in Table 1 below:

Table 1

EXAMPLES

All the alloys employed in the below examples were melted in an electric furnace in the temperature range 690-720°C and were held for at least 1 hour before any grain-refiner was added. The melt was exposed to the grain refiner compound for 20 minutes in each case. Picral acetic solution was used as an etchant for these alloys and a Zeiss polarized optical microscope with an Axio 4.3 image analysis system was used to observe the grain size of all the alloys under consideration. Example 1 : Mg-5%(Mg,Al)B? master alloy addition to AM50

Pre-synthesised powder of Mg- 5%(Mg,Al)B₂ (that is, 5wt%AlB2(powder) and

95wt%Mg(powder)) is compacted in pellet form of 32 mm diameter and varying heights. The pellet is then infiltrated with liquid Mg in an electric furnace at 700°C. The master alloy obtained through this process is then introduced in AM50 melt held in the temperature range 690-720°C for 1 hour in an electric furnace. The mould used for casting is a cone-shaped steel mould preheated to 250°C in an oven. As seen in the microstructures in Figure 1, AM50 reference sample with 950 μπι is reduced to grain size of 250 μπι after 0.35wt% addition of the master alloy Mg-5%(Mg,Al)B₂.

Without wishing to be constrained by theory, it is believed that A1B₂ undergoes a

phase transformation into Mgi_-XA1_XB₂ in liquid Mg as shown in Figure 2. Addition of AlBi powder to AZ and AM series alloys

Example 2: A1B? powder addition to AZ91D alloy

We have introduced pre-synthesized aluminium boride A1B₂ phase (99.9% purity) powder into AZ91D alloy which is melted in an electric furnace at the temperature range 690-720°C and held for 1 hour after melting. A1B₂ powder wrapped in Al foil is introduced in the melt and a steel rod is used to push the powder packed foil into the melt. Experiments were conducted in a wide range (0.1% to 0.4%) of A1B₂ addition levels. The mould used for casting is a cone-shaped steel mould preheated to 250°C in oven. As shown in Table 2 below and in Figure 3, grain size decreases as A1B₂ concentration increases until 0.2wt% and further addition of A1B₂ results in an increasing trend of average grain size of AZ91D alloy. Table 2

Example 3 : A1B? powder addition to AZ31B alloy

AZ3 IB alloy is melted in an electric furnace at the temperature range 690-720°C and the melt is held for 1 hour before adding the refiner. A1B₂ powder wrapped in Al foil is introduced in the melt and a steel rod is used to push the powder packed foil inside the melt. SF₆+N₂ gas mixture was used to protect the melt from oxidation. Experiments included addition of 0.2% and 0.4% of A1B₂ and the melt is cast after 20 minutes of refiner addition. The mould is a cone-shaped steel mould preheated to 250°C. Figure 4 shows that addition of pre-synthesized aluminium boride A1B₂ powder into AZ31 alloy will give a reduction in average grain size with increase in A1B₂ amount. Effect of cooling rate on AZ31B alloy

Under similar casting conditions as the previous examples, a wedge shape Cu-mould is used to observe the cooling rate effect on the Mg alloys. Mould is preheated to 250°C for at least an hour. As seen in Figure 5, the AZ31B grain-refined sample shows fine grains along the length of the sample which suggests that the addition of (Mg, A1)B₂ is less sensitive to the cooling rate.

Example 4: A1B? powder addition to AM50 alloy AM50 alloy is melted in an electric furnace at the temperature range 690-720°C and held for 1 hour after melting. SF₆+N₂ gas mixture was used to protect the melt from oxidation. A1B₂ powder wrapped in Al foil is introduced in the melt and a steel rod is used to push the powder packed foil inside the melt. AM50 melt containing refiner is held for 20 minutes before pouring in the mould, which is cone-shaped and preheated to 250°C in oven. It is observed from Figure 6 that the average grain size of AM50 alloy reduces by more than 70% at 0.1% A1B₂ addition levels.

Effect of cooling rate on AM50 alloy:

Under similar casting conditions as the previous examples, a wedge mould is used to observe the cooling rate effect on the Mg alloys. Mould is preheated to 250°C for at least an hour. As shown in Figure 7, the grain size is finer due to (Mg,Al)B₂ inoculations and is finer at both the thick and thin sections. This suggests that our chosen grain refiner is less sensitive to cooling rates.

Example 5: Additions of B₄C powder to AZ31 and AM60 alloys

We have introduced pre-synthesized boron carbide (B₄C) phase powder into AZ31 and AM60 alloy which are melted in an electric furnace at the temperature range 690-720°C. B₄C powder wrapped in Al foil is introduced in the melt and a steel rod is used to push the powder-packed foil into the melt and the melt is exposed to the refiner for 20 minutes before casting. The mould used for casting is a cone-shaped steel mould preheated to 250°C.

The microstructures shown in Figure 8 suggest that at 0.05wt% B₄C addition, AZ3 IB samples have an average grain size reduction from around 420 μπι to 250 μπι and that AM60 samples have an average grain size reduction from 420 μπι to around 300 μπι

Example 6: DC casting

AZ3 IB alloy is melted in an electric furnace in the temperature range 690-720°C and held at that temperature for 1 hour before adding the refiner. The A1B₂ grain refiner precursor is added to the melt 20 min before casting in a specially designed static direct-chill simulator mould. The cylindrical mould is pre-heated to 800°C in a furnace while a Cu-base used for this experiment is pre-heated to 250°C in an oven. Macroetching reveals that the bottom of the billet at the Cu-mould edge has columnar grains while the top of the billet has coarse equiaxed grains in the reference sample. The grain refined billet gives a uniform distribution of equiaxed fine grains (see Figure 10). The experiment is repeated for the AZ91D alloy (also shown in Figure 10).

MECHANICAL TEST DATA As seen from the mechanical test data plots in Figure 11, an improvement in yield strength and elongation is observed on addition of the selected grain refiners.

All optional and preferred features and modifications of the described embodiments and dependent claims are usable in all aspects of the invention taught herein. Furthermore, the individual features of the dependent claims, as well as all optional and preferred features and modifications of the described embodiments are combinable and interchangeable with one another.

The disclosures in UK patent application number 1419715.6, from which this application claims priority, and in the abstract accompanying this application are incorporated herein by reference.

Claims

Claims

1. The use of Mg_1-X Al_xB₂ to refine the grain size of pure magnesium or Mg-Al alloys, wherein x is above 0 and below 1.

2. A method of refining the grain size of pure magnesium or of a Mg-Al alloy, including the step of employing Mg_1-X A1_XB₂ as a grain refiner wherein x is above 0 and below 1.

3. The use or method as claimed in claim 1 or 2, wherein A1B₂ is added to the Mg-Al alloy, and wherein the A1B₂ converts to Mg_1-X A1_XB₂ in the alloy.

4. The use or method as claimed in any preceding claim, wherein B₄C is added to the Mg-Al alloy, and wherein the B₄C converts to Mgi_-xAl_xB_2-yC_y (wherein y>0) in the alloy.

5. The use or method as claimed in any preceding claim, wherein the total amount of borides is at least 0.02% by weight of the alloy.

6. The use or method as claimed in any preceding claim, wherein borides are added to the alloy in a pre-synthesised form.

7. The use or method as claimed in any preceding claim, wherein the boride compound is added in an elemental form or boron carbide form and borides are formed in situ.

8. An alloy obtainable by the method or use as claimed in any preceding claim.