WO2005090628A1

WO2005090628A1 - Al/si cast alloy containing zn and mg, and method for producing a cast part from one such alloy

Info

Publication number: WO2005090628A1
Application number: PCT/EP2005/002826
Authority: WO
Inventors: Matthew Otte
Original assignee: Hydro Aluminium Deutschland Gmbh
Priority date: 2004-03-20
Filing date: 2005-03-17
Publication date: 2005-09-29
Also published as: DE102004013777B4; DE102004013777A1

Abstract

The invention relates to an Al/Si cast alloy which combines good mechanical properties with a low sensitivity to the creation of internal residual stresses and distortions of the fine structure of the alloy. To this end, the inventive Al/Si cast alloy contains (in wt. %) Zn: 4 - 5 %, Mg: 0,3 - 0,5 %, Fe: 0,2 - 0,6 %, Mn: 0,15 - 0,45 %, and optionally Cu in proportions of 0,1 - 0,5 %, the remainder consisting of aluminium, silicon and unavoidable production-related impurities. The invention also relates to a method whereby cast parts with an appropriately optimised combination of properties can be obtained in a reliable and targeted manner.

Description

AL / SI CAST ALLOY WITH ZN AND MG AND METHOD FOR PRODUCING A CAST PART FROM SUCH AN ALLOY

The invention relates to an Al / Si cast alloy and a method for producing a cast part from such an alloy.

With the known Al / Si alloys, which are used for the production of castings, and the heat treatments used in the production of the castings, it is often only possible with difficulty to achieve an optimal combination of the mechanical properties with at the same time minimized internal stresses and warpage according to the respective To ensure heat treatment safely. Castings that have been cast, for example, from the Al-Si-Cu-Mg alloy known under the name “226S” can be quenched without the risk of residual residual internal stresses in air and can be subjected to artificial aging to improve their processability. It turns out, however, that the cast parts made from the 226S alloy after heat treatment do not meet the requirements for their mechanical properties, such as tensile strength, yield strength and elongation, in many applications. Other Al / Si alloys guarantee the required mechanical properties. However, their use draws the presence of high residual stresses and the risk of Formation of structural distortions in the finished casting itself.

Attempts to overcome these disadvantages by changing the known alloys or by adding other or further alloying elements have as a rule been unsuccessful. Thus, an alloy known under the name "Autodur" (AlSilOZnδ) has, in addition to magnesium, additionally high proportions of zinc of approximately 10% by weight. These make it possible to process a casting made from the Autodur alloy without heat treatment. However, it turns out that the microstructure obtained in the casting and the mechanical properties of the known alloy are unstable. In addition, there is an increased sensitivity to corrosion and quenching of the known material.

The object of the invention was to create an Al / Si cast alloy in which good mechanical properties are combined with a low sensitivity to the development of residual internal stresses and warpage of its microstructure. In addition, a method is to be specified with which castings can be produced safely and in a targeted manner with a correspondingly optimized combination of properties.

With regard to the material, this task is solved by an Al / Si cast alloy, which in addition to aluminum, silicon and impurities (in% by weight) that are unavoidable in production (4 - 5% Zn, 0.3 - 0.5% Mg, 0 , 2 - 0.6% Fe, 0.15 - 0.45% Mn and optionally 0.1 - 0.5% Cu. The silicon contents are usually in the range from 5 to 18% by weight. The invention is based on an Al alloy which has silicon contents which are in the usual range for good castability. Magnesium and zinc contents are added to this basic alloy in order to effect precipitation hardening based on the Al-Mg-Zn precipitation system. The Zn and Mg contents provided according to the invention ensure a flexibly adaptable starting point for setting the properties of the alloy according to the invention.

The Si contents of the alloys according to the invention can be varied within wide limits, on the one hand to optimize the castability of the alloy or on the other hand to achieve improved wear properties. The Si contents are preferably varied in the range from 5 to 18% by weight.

The Mg content has been limited to a maximum of 0.5% by weight, since the formation of Mg-Fe 'pi' phases in primary or secondary aluminum cannot be ruled out at higher Mg contents. These phases are difficult to resolve in the further process and have a negative impact on the ductility of the material. The range of Mg contents in an alloy according to the invention is therefore preferably limited to 0.3-0.4% by weight of magnesium.

According to the invention, the Zn contents are limited to 4-5% by weight, on the one hand to use the strength-increasing effect of Zn and on the other hand to avoid an increase in the sensitivity to quenching and the formation of cracks as a result of stress corrosion. If necessary, the alloy according to the invention can contain Cu up to the predetermined upper limit in order to improve the heat resistance of the alloy. This improvement occurs at a content of at least 0.1% by weight of Cu. However, the positive influence of Cu can be used particularly effectively if the Cu content is at least 0.3% by weight.

Fe can be present up to the specified upper limit in Al / Si compositions according to the invention as a component of the primary or secondary aluminum from which the alloy was produced. To this extent, an alloy according to the invention behaves like any other Fe-containing Al / Si alloy. However, excessively high Fe contents lead to a decrease in the elasticity and can impair the effectiveness of the heat treatment by forming "pi" phases (FeMg ₃ Si6Al ₈ ).

With regard to the method, the above-mentioned object is achieved by a method for producing light metal castings, in which the following work steps are carried out:

a) casting an Al / Si melt composed according to the invention into a casting;

b) solution annealing followed by controlled cooling of the casting;

c) carrying out an aging treatment of the solution-annealed and cooled cast part, comprising cl) keeping the cast part at room temperature for one to five days (cold aging), c.2) subsequent controlled heating of the casting to an aging temperature of 150-240 ° C with a heating rate of at most 1200 ° C / h, c.3) holding the casting at aging temperature for at least 2 hours, c.4) cooling of the aging treated casting.

The properties of the alloy according to the invention can be used particularly well if it is used in the T6 or T7 state. For this purpose, the cast part is first subjected to solution annealing after casting. The annealing temperature is preferably in the range from 430 ° C to 540 ° C. Annealing temperatures in the lower section of this range prove to be favorable in the majority of such applications in which Mg and Zn are to be homogenized. These alloying elements are dissolved in particular at temperatures of 430 ° C - 450 ° C. If, on the other hand, rounded silicon phases are to be produced in the workpiece, it is advantageous to anneal in the higher temperature section above 450 ° C. At temperatures above 480 ° C, the hardening elements will be brought into solution so that increases in strength can be achieved when heated to these temperatures.

After solution annealing, the cast part is cooled in a manner known per se. To do this, it can be cooled in quiescent air in a controlled manner or quenched by targeted application of air or water. The workpiece obtained has a stable Micro structure and equally stable mechanical properties.

Controlled cooling is followed by natural aging. The cast part is kept at room temperature for a sufficient period of time, which is preferably 1 to 5 days. The workpiece is then heated to the aging temperature at which it is held for at least 2 hours.

The aging temperature is advantageously 160 ° C to 220 ° C. It has been found that particularly favorable properties are obtained when these temperatures are maintained. Tests have shown that, particularly when producing components for internal combustion engines, good property profiles are obtained when working with high aging temperatures.

The heating takes place at a heating rate of 400 ° C / h to 1200 ° C / h, preferably approximately 800 ° C / h.

The heating to the aging temperature can take place if the temperature does not increase continuously. In this case, it preferably comprises at least one holding section in which the workpiece is held at a holding temperature of 90 ° C.-110 ° C. for a specific holding time. In the above-mentioned temperature range around 100 ° C there is an increased excretion of GPII phases. These support the formation of η phases, which increase the strength.

With such a procedure according to the invention, castings can be produced which have an elastic limit of at least 140 MPa and at least 250 MPa tensile strength have at least 5% up to 12% and more elongation. At the same time, the cast parts produced according to the invention prove to be insensitive to the formation of residual stresses and the development of distortions in the structure as a result of the heat treatment.

According to current knowledge, this is achieved in that the composition of the alloy according to the invention and the manufacturing process according to the invention result in the formation of GP zones in the workpiece at room temperature or at the elevated holding temperature. Subsequently, during the holding at the aging temperature, n 'precipitates (MgZn ₂ ) are formed from the GP zones, which then form n precipitates.

To demonstrate the effects achieved with the invention, a 7% by weight Si, 0.3% by weight Mg, 4% by weight Zn, 0.12% by weight Fe, 0.5% by weight Cu and the remainder melted aluminum and inevitable impurities containing Al / Si melt S1 according to the invention. Likewise, an Al / Si melt S2 was melted, which matched the melt S1 in terms of its contents of Al, Si, Mg, Zn and Fe, but did not contain any Cu.

The melts S1 and S2 were cast into test pieces. These test pieces were then solution annealed at a temperature of 530 ° C for 6 hours.

A first group of castings cast from the melts S1 and S2 is then accelerated with water and a second group of castings from the melts S1 and S2 cast castings have been accelerated in air to room temperature.

The castings were then heated to a holding temperature of 90 ° C, at which they were held for 4 hours.

The castings were then heated to an aging temperature of 180 ° C., at which they were also kept for 8 hours.

The heating rate was approx. 1000 ° C / h in each case.

After finishing the aging treatment, the following properties could be determined on the various specimens:

*) Quality index Q = tensile strength + 150 x log (elongation)

Regardless of how the castings were cooled after solution annealing, the specimens examined each showed particularly high values of their mechanical properties. At the same time, the residual stresses found in them were reduced to a minimum and they had a homogeneous structure free of faults.

Claims

PATENT CLAIMS

1.Al / Si casting alloy with (in% by weight) Zn: 4 - 5%, Mg: 0.3 - 0.5%, Fe: 0.2 - 0.6%, Mn: 0.15 - 0.45%, optionally Cu in contents of 0.1 - 0.5%, the rest aluminum, silicon and inevitable impurities due to the manufacturing process.

2. Al / Si casting alloy according to claim 1, d a d u r c h g e k e n n z e i c h n e t, because it contains 0.3 - 0.4 wt .-% magnesium.

3. Al / Si casting alloy according to one of the preceding claims, that the copper content is at least 0.3% by weight.

4. A method for producing light metal castings, in which the following working steps are carried out: a) casting an Al / Si melt, which is composed according to one of the preceding claims, into a casting; b) solution annealing followed by controlled cooling of the casting; c) carrying out an aging treatment of the solution-annealed and cooled cast part, comprising cl) holding the cast part at room temperature for a holding time; c.2) subsequent controlled heating of the cast part to an aging temperature of 150-240 ° C with a heating rate of at most 1200 K / h, c.3) holding the cast part at the aging temperature for at least 2 hours, c.4) cooling of the aging-treated cast part ,

5. The method of claim 4, d a d u r c h g e k e n n z e i c h n e t, that the holding time is one to five days.

6. The method according to any one of claims 4 or 5, characterized in that the casting is heated during the solution annealing to a solution annealing temperature of 430 ° C to 540 ° C.

7. The method of claim 6, d a d u r c h g e k e n n z e i c h n e t, that the cast part is heated during solution annealing to a solution annealing temperature of 470 ° C to 540 ° C.

8. The method according to any one of claims 4 to 7, d a d u r c h g e k e n n z e i c h n e t, that the aging temperature is 160 ° C to 220 ° C.

9. The method according to any one of claims 4 to 8, d a d u r c h g e k e n n z e i c h n e t, that the heating rate is 400 ° C / h to 1200 ° C / h.

10. The method of claim 9, d a d u r c h g e k e n n z e i c h n e t, that the heating rate is about 800 ° C / h.

11. The method according to any one of claims 4 to 10, d a d u r c h g e k e n n z e i c h n e t, that the cast part is held for a holding time at a holding temperature of 90 ° C - 110 ° C during the heating to the aging temperature.