EP2113576B1 - Method for producing a structural material made of magnesium-containing aluminium-based alloy - Google Patents

Description

• The invention relates to foundry and rolling engineering. Existing methods for producing structural materials for the automobile industry are generally based on the use of the traditional method for producing ingots from aluminum-based alloys containing magnesium, lithium, zinc, and so on, and for rolling the same.
DESCRIPTION OF THE PRIOR ART
• The principal requirements made to structural materials for the automobile industry are the need to maintain the ultimate stress thereof within 300 to 400 MPa, relative elongation of 30% to 40%, and density not exceeding 2.65 g/cu.cm., good weldability, and corrosion resistance.
• No alloys meeting these requirements exist today. Existing lithium alloys of required density are unsuitable for the above purposes because of inadequate strength and plasticity, and because they are poorly weldable, and all other alloys are unsuitable for similar reasons. Magnesium-containing alloys can be rolled into sufficiently strong products, but density requirements can only be met if the alloy contains more than 9% of magnesium. Up to this day, no processes have been developed for making rolled products from alloys containing over 5% or 6% of magnesium.
• An aluminum-based alloy AMG5 (GOST Standard 478961) and a method for producing it are known in the prior art. The prior art alloy has the following chemical composition (in percent by weight):

  Magnesium	- 4.8 to 5.5
  Manganese	- 0.3 to 0.6
  Titanium	- 0.1 to 0.2
  Iron	- 0.5 to 0.6
  Silicon	- 0.1 to 0.2
  Aluminum	- the balance

• Rolled products manufactured from the AMG5 alloy are used widely in the aircraft and shipbuilding industries and are made by traditional processes described in detail in "Foil Manufacturing" by S.N Cherniak, V.I. Karasevich, and P.A. Kovalenko, Metallurgia Publishers, Moscow, 1968. Ingots are produced from the AMG5 alloy by a semi-continuous method. For this reason, there is a natural limit to magnesium solubility in the ingot matrix. Excess magnesium forms a fragile eutectic substance around a grain that blocks it and determines the final plastic properties of the ingot and alloy. Moreover, parabolic solidification fronts of a polycrystalline structure are the cause of the absence of a uniform crystallographic orientation, that is, plasticity is different across the ingot at the macro- and micro-level. In general, plasticity decreases by at least a half for this reason. Plasticity of an alloy is characterized by relative elongation of up to 4%-6%, a figure that obviously does not meet the requirements of the automobile industry.
• Processes for producing the AL8 and AL27 alloys (GOST Standard 2695-75) related closely to the chemical composition of this invention are suitable for producing corrosion-resistant castings that can be subjected to heat treatment to improve their strength. These processes, however, can be used to cast ingots from the above alloys only. Besides, they cannot, in principle, be used to make plastic ingots from alloys containing about 10% of magnesium, for which reason they are classified as foundry processes.
• A further method of forming aluminium alloys is proposed in RU 2111826 C1 . The proposed aluminium alloy additionally contains Magnesium, Zirconium, Beryllium and Titanium with the following rations in percentage of weight: Magnesium 9.5 % to 11.5 %, Zirconium 0.05 % to 0.2 %, Beryllium 0.03 % to 0.15 %, Titanium 0.02 % to 0.1 % and Aluminum the rest of this alloy.
DESCRIPTION OF THE INVENTION
• The invention is defined in the claim. This invention is intended to develop a method for producing a structural material from an aluminum-based alloy containing 9 % to 11 % of magnesium, comprising casting an ingot, subjecting the ingot to heat treatment, and rolling the ingot, in order to improve the strength and plasticity of rolled products and the quality of the process for producing sheet material.
• This objective is achieved by developing a method for producing a structural material from an aluminum-based alloy containing 9 % to 11 % of magnesium, said method comprising producing an ingot, subjecting it to heat treatment, and rolling the ingot containing magnesium, wherein the mechanical properties of the alloy are improved by further adding zirconium, cobalt, beryllium, and boron thereto at the following proportions of the ingredients (in percent by weight):

  Magnesium	- 9.0 to 11.0
  Zirconium	- 0.15 to 0.2
  Cobalt	- 0.001 to 0.01
  Beryllium	- 0.001 to 0.02
  Boron	- 0.005 to 0.007
  Aluminum	- the balance.

• Solidification is carried out in a revolving mold at a gravitation factor of 220 to 250 during a lifetime of the melt equal to between 12 and 15 sec/kg. Heat treatment and rolling are conducted according to the following algorithm:
1. (a) depending on its size, the ingot is heated for hot rolling for 2 to 4 hours at a temperature of 340°C to 380°C;
2. (b) the ingot is hot-rolled at its initial temperature of 340°C to 380°C to a thickness of 4 to 8 mm at a deformation of up to 30% in each cycle. The final rolling temperature of the semifinished rolled product must be within 310°C and 330°C;
3. (c) at the next stage, the semifinished rolled product is cold-rolled at a deformation of up to 50%, rolling alternating with intermediate annealing for 0.5 to 2.0 hours at a temperature of 310°C to 390°C until the required thickness of 0.5 to 2.0 mm is achieved; and
4. (d) the rolled product is annealed finally for 5 to 40 minutes at a temperature of 400°C to 450°C.
• The claimed method is based on the use of new physical phenomena attending solidification of melts in strong gravitation fields of centrifuges. Generally, the effect of such fields is as follows:
1. (a) diffusion processes are intensified in any multi-component melts, resulting in intrusion-substitution type solid solutions from which eutectics are recovered in minimum quantities. Moreover, the eutectics already formed are minimized in volume and coagulated into isolated combinations that do not block the matrix grain; and
2. (b) a casting or ingot, even if it has a poly crystalline structure, has a dominant crystallographic orientation in a preset direction accounting for 80% to 85% of all possible orientations.
• The claimed method, therefore, helps develop an aluminum-based structural material containing 9% to 11% of magnesium, and make rolled products therefrom.
PRINCIPLE OF PERFORMING THE INVENTION
• The method of this invention for producing a structural material from an aluminum-based alloy containing 9% to 11% of magnesium is performed at the ingot producing stage in a revolving mold, the design of which depends on the required ingot shape and weight. In this case, the thermal conditions of solidification depend on the specific design of the mold lining.
• The invention is based on an analytically computed and experimentally confirmed effect of gravity fields on the solidifying melt from the viewpoint of orientation of the crystallographic lattice axes.
• This physical phenomenon is examined in more detail below. For an emerging crystallographic orientation of a solidifying melt in any penetration type fields of force (gravity, ultrasonic, and so on) to be analyzed, it is essential to examine in dynamics the forces acting upon the nucleus, and then on the crystallite (grain) in the melt under the effect of the external field of a nonuniformly distributed force F(x^κ, II).
• The text below examines a crystallite near a support (solidification front or substrate) that is deformed under the effect of this force F(x^κ, n).
• This crystallite is assumed to have a rigid support at one end. The equilibrium equation is in this case as follows: $$\frac{\mathrm{\delta }{\mathrm{\sigma }}_{\mathrm{xx}}}{{\mathrm{\delta }}_{\mathrm{x}}}+\frac{\mathrm{\delta }{\mathrm{\sigma }}_{\mathrm{xy}}}{{\mathrm{\delta }}_{\mathrm{y}}}+\frac{\mathrm{\delta }{\mathrm{\sigma }}_{\mathrm{xz}}}{{\mathrm{\delta }}_{\mathrm{z}}}=\frac{{\mathrm{\rho }}_3\mathrm{F}\left({\mathrm{X}}_{,}^{\mathrm{K}}\mathrm{\Pi }\right)}{{\mathrm{M}}_{3\mathrm{ap}}}$$

  

  $$\frac{\mathrm{\delta }{\mathrm{\sigma }}_{\mathrm{yx}}}{{\mathrm{\delta }}_{\mathrm{x}}}+\frac{\mathrm{\delta }{\mathrm{\sigma }}_{\mathrm{yX}}}{{\mathrm{\delta }}_{\mathrm{y}}}+\frac{\mathrm{\delta }{\mathrm{\sigma }}_{\mathrm{yz}}}{{\mathrm{\delta }}_{\mathrm{z}}}=0$$

  

  $$\frac{\mathrm{\delta }{\mathrm{\sigma }}_{\mathrm{zx}}}{{\mathrm{\delta }}_{\mathrm{x}}}+\frac{\mathrm{\delta }{\mathrm{\sigma }}_{\mathrm{zy}}}{{\mathrm{\delta }}_{\mathrm{y}}}+\frac{\mathrm{\delta }{\mathrm{\sigma }}_{\mathit{zz}}}{{\mathrm{\delta }}_{\mathrm{z}}}=0$$

  
• wherein: σ_ik is the stress tensor;
• x, y, and z are coordinates;
• ρ is density;
• M_sap is the mass of the nucleus.
• For a conclusion to be made, the following boundary conditions have been accepted:
• stress tensors (σ_ik on the side surfaces are equal to zero, except for σ_xx. and
• the free end of the crystallite of a length x=L has: $${\mathrm{\sigma }}_{\mathrm{xz}}={\mathrm{\sigma }}_{\mathrm{yz}}={\mathrm{\sigma }}_{\mathrm{zz}}=0$$
  
• Free energy may be described as follows: $$\mathbf{F}=\frac{1}{2}{\mathrm{\sigma }}_{\mathrm{xx}}{\mathrm{\epsilon }}_{\mathrm{xx}}=\frac{1}{2}{\mathrm{s}}_{\mathrm{xxxx}}{\mathrm{\sigma }}_{\mathrm{xx}}^2=\frac{1}{2}\frac{{\mathrm{s}}_{\mathrm{xxxx}}{\mathrm{\rho }}^2{\mathrm{F}}^2\left({\mathrm{X}}_{\mathrm{,}}^{\mathrm{K}}\mathrm{\Pi }\right)\left({\mathrm{L}}_{\mathrm{o}}-\mathrm{X}\right)}{{\mathrm{M}}_{3\mathrm{ap}}}$$

  

  wherein: ε_ik is the deformation tensor equal to S_ikLmxσ_Lm S_ikLm are elastic flexibility constants; $${\mathrm{s}}_{\mathrm{iiLL}}=\frac{1}{2}{\mathrm{s}}_{\mathrm{ikLm}}{\sigma }_{\mathrm{ik}}{\sigma }_{\mathrm{Lm}}$$

  

  σ_ik is the Kronecker symbol.
• Hence, the surface energy of the entire crystallite is equal to: $$\mathrm{E}={\displaystyle \int \mathrm{FdV}}={\displaystyle {\int }_{\mathrm{o}}^L\mathrm{F}\left(\mathrm{x}\right){\mathrm{S}}_3\mathrm{Dx}}$$

  
• To proceed with the conclusion, it is assumed that: $$\mathrm{E}\approx {\mathrm{S}}_{\mathrm{xxxx}}$$

  
• Since Young's modulus corresponding to the direction X (along the longitudinal axis of the crystallite) is equal to: $${\mathrm{E}}_{\mathrm{x}}=\frac{1}{{\mathrm{s}}_{\mathrm{xxxx}}}$$

  

  $$\mathrm{then\; E}\sim \frac{1}{{\mathrm{E}}_{\mathrm{x}}}$$

  
• If the crystallographic plane hkL is normal to the axis OX, then: $$\frac{1}{{\mathrm{E}}_{\mathrm{x}}}={\mathrm{s}}_{11}-2\left({\mathrm{s}}_{11}-{\mathrm{s}}_{12}-{\mathrm{s}}_{44/2}\right)\mathrm{A}$$

  

  wherein: $$\mathrm{A}=\frac{{\mathrm{h}}^2{\mathrm{k}}^2+{\mathrm{k}}^2{\mathrm{L}}^2+{\mathrm{L}}^2{\mathrm{h}}^2}{{\left({\mathrm{h}}^2+{\mathrm{k}}^2+{\mathrm{L}}^2\right)}^2}$$

  
• Since it is assumed for all metals, with the exception of molybdenum, for which S₁₁-S₁₂-S_44/2<0
• then the ratio 1/E_x has a minimum value for plane (III).
• The direction (III), provided it coincides with the vector of the force F(x^κ,Π), is to be preferred over all other directions. This conclusion unambiguously suggests that the crystallite retarded in the melt is oriented in the direction of the force F(x^κ,Π) regardless of the melt type and initial growth conditions of the axes (III).
• A retarded crystallite is a fully grown nucleus distorted from the spherical shape to nearly an ellipsoid attached to the cellular solidification front.
• In principle, orientation of crystallographic axes in a growing nucleus (except for the force F(x^κ,Π)) occurs long before it is attached to the solidification front as it is separated in the melt at a variable viscosity in the direction of the force F(x^κ,Π).
• Direction of the absolute elastic energy minimum of the lattice is a maximum suitable situation for successive deformation of the solid at minimum external forces applied, for example, by the rolling mill. This is particularly important in this case, with a supersaturated solution of magnesium in aluminum, because this alloy has an improved strength.
• This method has been tested many times, and the recommendations made on the basis of the tests were used to make several hundred kilograms of rolled products 2 mm, 1 mm, 0.5 mm, and 0.1 mm thick. The rolled products were tested in laboratory conditions. The test results are shown in Table 1 for examples having alloy compositions according to the invention. It is clear from Table 1 that the best combination of strength and plasticity is achieved by following the claimed method parameters and using an alloy having magnesium content of around 10%. Table 1

  Test parameters	Rolled product thickness, mm				Magnesium concentration, %
  	2.0	1.0	0.5	0.1
  Ultimate strength, MPa	320-380	320-380	325-380	330-390	9
  	330-400	330-400	330-410	330-420	10
  	330-390	330-390	320-390	325-380	11
  Relative elongation, %	30-40	32-40	32-40	32-40	9
  	32-40	32-40	32-40	33-40	10
  	31-38	30-37	30-36	30-36	11

• This invention is suitable for application on an industrial scale because it may be performed in a revolving mold known in the art, and the result is achieved by varying alloy production conditions. This invention may be used for producing structural materials of practically any thickness from the AMG10 alloy. The invention may be used most efficiently for producing rolled materials for motor vehicle bodies and power components of motor vehicles.

Claims (1)

1. A method for producing a structural material made of magnesium-containing aluminum-based alloy, comprising solidifying a melt to obtain an ingot, subjecting the ingot to heat treatment, and rolling the ingot,
   wherein the ingot is heated for 2 to 4 hours at a temperature of 340 °C to 380 °C in advance of heat treatment and rolling,
   whereupon the ingot is hot-rolled at the above temperature to a thickness of 4 to 8 mm at a deformation degree of up to 30% in each cycle, and
   at a final rolling temperature of the semifinished rolled material within the range of 310 °C to 330 °C, followed by cold rolling of the semifinished rolled material at a deformation degree of up to 50 % in each cycle, alternating with intermediate annealing for 0.5 to 2.0 hours at a temperature of 310 °C to 390 °C to a required thickness of 0.5 to 2.0 mm, and
   the rolled material is annealed finally for 5 to 40 minutes at a temperature of 400 °C to 450 °C,
   and,
   wherein, with the purpose of producing a structural material from an alloy containing ingredients at the following proportions in percent by weight: Magnesium - 9-11 Zirconium - 0.15-0.2 Cobalt - 0.001-0.01 Beryllium - 0.001-0.02 Boron - 0.005-0.007 Aluminum - the balance, the melt is solidified in a revolving mold at a gravitation factor of 220 to 250 and a melt lifetime of 12 to 15 sec/kg.