EP1029936B1 - Zinc alloy for production of high quality pieces - Google Patents

Description

The present invention relates to zinc-based alloys allowing to make high quality foundry parts.

Numerous zinc-based alloys have already been proposed for this purpose. We thus know a family of alloys called "zamak", at 4% aluminum, with varying copper and magnesium contents (no copper for zamak 3 or ZL3, 3% copper for zamak 2 or ZL2).

The compositions of these alloys are defined by the NF EN standards 1774 "Zinc and zinc alloys - Foundry alloys - ingots and liquid" and by standards EN 12844 "Zinc and zinc alloys - Castings - Specifications ”as well as the international standard ISO 301.

The most widely used alloy is the Al alloy: 3.9-4.3%, Cu: 0.75-1.25%, Mg: 0.03-0.06%, remains of high purity zinc commonly called zamak 5. Its abbreviated designation according to standard NF EN 1774 is ZL5. The purity zinc to be used is defined in these standards, it corresponds to a zinc type Zl, purity 99.995% defined in standard NF EN 1179.

This family of alloys is particularly suitable for applying a conventional pressure injection process in a hot chamber, the particularity is that part of its injection system is submerged in the molten alloy to be injected. It should be understood, however, that all other known methods can also be used.

The hot chamber pressure injection process is commonly used to produce parts with high productivity of varying degrees of complexity, dedicated to multiple applications (luxury industry, hardware, automobile, building etc., this list not being exhaustive).

In such a process, the presence of aluminum is essential for limit the aggressiveness of zinc with respect to steel or cast iron parts components of the casting machine. The addition of aluminum thus allows injection of the parts by a hot chamber type process where the injection system is immersed in the molten alloy.

Likewise, it is known that the presence of magnesium at levels between 0.02% and 0.06% is recommended to avoid problems of intercrystalline corrosion of parts and to improve properties mechanical such as tensile strength, hardness and embrittlement at low temperature. Above 0.06%, it is known that the magnesium produces a decrease in impact flexural strength.

It is finally known that for the zamak family whose content in aluminum varies between 3.9 and 4.3%, copper prevents corrosion intergranular.

A welding alloy is known from FR-A-2 093 821 with from 0.5 to 4.5% by weight of aluminum, from 0.1 to 4% by weight of copper, from 0.005 to 0.08% by weight weight of magnesium and the rest zinc.

Many efforts have been made to improve the properties mechanical properties of the ZL 5 alloy, in particular its creep resistance which is a very important parameter in applications such as automotive or the building.

Generally, it is desired that the appearance of the parts in such zinc alloys are of high quality. However, the coated parts or very frequently have surface defects. Two faults frequent and particularly critical are called respectively porosity and cold drops.

The porosities appear as small holes in the surface of the coated part and are attributable to the foundry stage when the stage subsequent coating is perfectly controlled and does not generate its specific defects (stings or scrapes for example).

Cold drops are junctions of several streams of alloys injected poorly re-welded together, often favored by a temperature mold too cold. However, those skilled in the art know that some part drawings make this surface defect almost inevitable whatever be the injection conditions.

Many zinc-based castings have a role decorative, for example in so-called luxury industries (high-end jewelry range or fantasy, perfumery, cosmetics) but also in particular in the automotive and building industries.

These so-called "appearance parts" are, after their injection, subjected to a surface treatment giving them the final aesthetic appearance desired (chrome, gold, silver, paint, or other coating).

The quality of the appearance parts injected in zamak and then coated is judged by a visual examination which rejects any part showing defects surface defects called "appearance defects", these defects can be linked to one foundry and coating steps.

Stolberger Zink has offered a lower grade alloy aluminum than ZL5, having the composition: AI: 2%, Cu: 1%, Mg: 0.03-0.06%, Be: 0.0005-0.0050%, zinc residue. The results obtained with this alloy underlines the beneficial effect of beryllium on resistance to bending by impact and mention a qualitative improvement in appearance pieces. The tensile strength of this alloy is however reduced by 10-15% compared to that of ZL 5.

Document BE-846.899 describes a family of Al alloys : 0.2-3%, Cu: 0.2-5%, and at least one of the following elements Mn: 0.3-3%, or Cr: 0.01-0.5%, or V: 0.01-0.5%, or Ni: 0.2-0.5%, zinc residue, presenting improved creep characteristics compared to ZL 5.

The various alloys mentioned above have the disadvantage of providing parts having only an unsatisfactory surface quality, the low aesthetic improvement accompanied by a lowering of qualities mechanical parts.

The object of the present invention is to provide a family of alloys zinc, the surface quality of the parts injected as is, raw foundry, or ready to be coated, for example after polishing, in such alloys is significantly improved while retaining mechanical characteristics at least similar to those of ZL 5.

According to the invention, these aims are achieved with a zinc alloy comprising in mass percentages: al 1.8-2.2% Cu 1.5-3.9% mg 0.02-0.06% the rest being made up of zinc, with the usual impurities inevitably present in the aforementioned metals.

It has been found that this family of alloys allows in particular improve the surface quality of the injected parts by reducing the number of porosities near the surface, porosities inherent in the conventional injection process (without using a vacuum mold), and by reducing the number of cold drops for parts sensitive to this default.

This quality of the alloy family has been verified by manufacture of many parts. To provide an assessment quantified of the improvement made, the plaintiff chose a appearance part with large flat surfaces, particularly revealing appearance issues.

• la figure 1 représente un dispositif d'injection d'une machine d'injection sous pression à chambre chaude connue en elle-même;
• la figure 2 est un tracé représentant pour une pièce de l'invention un nombre de défauts en fonction de sa teneur en cuivre ;
• la figure 3 représente l'évolution d'enthalpie de transformation en fonction de la température pour le ZL5 et trois alliages selon l'invention ;
• la figure 4 représente des profils de fraction solide d'une pièce en alliage ZL5 à des temps de solidification différents.
• la figure 5 représente des profils de fraction solide d'une pièce en alliages selon l'invention ;
• les figures 6 à 9 représentent des coupes de pièces injectées respectivement en ZL5 et en trois alliages selon l'invention.

Other objects, advantages and characteristics of the invention will appear on reading the detailed description which follows, made with reference to the examples and appended figures in which:

• FIG. 1 represents an injection device of a pressurized injection machine with a hot chamber known in itself;
• Figure 2 is a plot showing for a part of the invention a number of defects according to its copper content;
• FIG. 3 represents the evolution of enthalpy of transformation as a function of temperature for ZL5 and three alloys according to the invention;
• FIG. 4 represents solid fraction profiles of a part made of ZL5 alloy at different solidification times.
• FIG. 5 represents solid fraction profiles of a part made of alloys according to the invention;
• Figures 6 to 9 show sections of parts injected respectively in ZL5 and in three alloys according to the invention.

In the first example of the invention, a piece of dimensions 110 x 60 x 2.5 mm was injected with a foundry machine under usual hot chamber pressure which is shown in Figure 1.

With such a machine, the molten alloy is brought in due to the thrust of an injection piston 10 from a crucible 15 to a footprint 20 of a mold passing through an injection system. The mold cavity 20 can be evacuated or filled with a gas such only oxygen before injection, but the most common method employee uses a mold whose imprint 20 contains air. The alloy in liquid phase arrives at this imprint 20 via a nozzle intermediate 25 and a machine nozzle 30.

The alloy is injected along a 60mm edge using a double tangential attack, the direction of injection being parallel to large sides of the room. The large surface that must have a nice appearance is that attack side. Its large size usually makes it difficult to obtain perfect surface quality.

The parts produced then undergo a polishing removing a thickness of 20 to 40 µm of material to simulate polishing industrially in the usual way before a final coating.

The polished surface of each piece is then observed using an optical microscope (Olympus, reference PMG3) with which detected surface defects with a span greater than 50 µm.

Such defects are indeed likely to lead to porosities. The quantification of these defects was made by image analysis using a Micromechanical motorized stage and special software ETC-3000 and AMC-2000. This gives a quantitative and objective criterion of comparison of the surface quality of the parts injected after polishing.

The ZL5 and the family of alloys according to the invention were injected under various conditions defined by an experimental design. The composition of the alloys used is given in the following table: Alloy al%
en masse Cu%
en masse mg%
en masse Zn
en masse ZL 5 4 1 0.04 Rest # 1 2.2 1.75 0.04 Rest # 2 2.1 2.90 0.04 Rest # 3 2.1 3.90 0.04 Rest # 4 2.1 0.95 0.04 Rest

Alloys 1 to 3 have an aluminum content close to 2% in mass, a magnesium content of 0.04%. The copper content of these alloys 1 to 3 are respectively 1.75%, 2.90% and 3.90% by mass.

Alloy No. 4 which has a low copper content of 0.95%, corresponds to an alloy of the type proposed by Stolberger Zink, to the difference that that of Stolberger Zinc contained Beryllium.

For the alloys of the invention, a mass content of Magnesium between 0.02 and 0.06%, and preferably between 0.03 and 0.06%.

The experience parameters are shown in the following table: Temp ° C casting ^ture Piston speed in ^1st phase m / s Speed to the attack by 2 ^nd stage m / s Temp ^ture of mold ° C Post-pressure time Cooling time s ZL5 430 0.03-0.06 38 - 47 150 - 180 1.5 - 2.5 2 - 3.5 Alloy # 1 450 0.03-0.06 38 - 47 150 - 180 1.5 - 2.5 2 - 3.5 Alloy # 2 450 0.03-0.06 38 - 47 150 - 180 1.5 - 2.5 2 - 3.5 Alloy # 3 450 0.03-0.06 38 - 47 150 - 180 1.5 - 2.5 2 - 3.5

Conventionally, the piston is driven in two phases, the the first being a slow phase and the second a fast phase. Time post-pressure is, in known manner, the total time during which the piston 10 exerts pressure on the alloy. The cooling time is the time between the injection and the opening of the mold.

Tensile test pieces were cut from the part described above and mechanical tests were carried out with an Instron tensile machine, at a speed of 2 mm / min at 20 ° C. (the test pieces having a length I ₀ of 40mm).

The results obtained from defect counting and mechanical tests are presented in the table below: Alloy Average number of defects> 50 µm on a surface of 50x50 mm ² after polishing Tensile strength MPa Vickers hardness
(force 10 kg) ZL 5 Base: 100 290 102 # 1 56 290 107 # 2 47 290 112 # 3 80 297 126 # 4 - 260 102

The results clearly show the increase in the quality of surface, i.e. the decrease in the number of surface porosities, due to the use of alloys 1, 2 and 3 of the invention.

More specifically, we note that, for an average number of defects of 100 for ZL5, the alloys according to the invention have a number of defects reduced by half for the contents of Cu by mass of 1.75% and 2.90% and a number of defects reduced by a third for a grade of Cu by mass of 3.9%.

There is shown in Figure 2 a plot showing on the abscissa the mass percentage of Cu in the alloy according to the invention and in ordered the number N of faults recorded.

The inventors have found that the alloys according to the invention provide a particularly advantageous surface finish for the contents in Cu between 1.7% and 3.5% by weight with a number of defects significantly lower than 55, even more advantageous for the contents of Cu ranging from 2.0 to 3.1% by weight with a number of defects particularly reduced, significantly lower than 50 as in the case of alloy n ° 2 presented here. The Cu concentration range is optimal for Cu between 2.5 and 3% by weight, for which the number of defects is minimal, close to 47.

These results are average values and the inventors were able to find that the new family of alloys makes it possible to produce parts whose surface quality is superior and, surprisingly, constant for various and variable injection conditions. The ZL5 which has a poorer surface quality is much more sensitive to changes in parameters than the alloys according to the invention. The quality of the parts manufactured with the new family of alloys is therefore in practice much less sensitive to changes in parameters injections that may occur during the manufacturing of parts (starting of machine, machine stop etc.).

The tensile strength and hardness of ZL 5 are retained and even improved with the alloys according to the invention. Indeed, the measured tensile strengths are equal to the resistance of 290 MPa ZL5 except for the 3.9% copper alloy for which the resistance in traction is improved, to 297 MPa.

The inventors have found that the tensile strength becomes again very low for a Cu content of less than 1% at a content of Aluminum of around 2% as illustrated by the case of alloy 4 which has no resistance than 260 MPa. Hardness, measured by the Vickers method well known, is better for alloys 1 to 3 according to the invention than for alloys ZL5 and n ° 4, the hardness being particularly high for the alloy with 3.9% Cu.

In the second example of the invention, conventional measurements by differential thermal analysis (ATD) were carried out on the ZL 5 and on the AI alloy family: 1.8-2.2%, Cu: 1.5-3.9%, Mg: 0.02-0.06%, remains zinc. This method allows to know an enthalpy of transformation as a function of temperature for each of the alloys.

FIG. 3 shows for each of the alloys 1, 2 and 3, and for ZL5 this enthalpy as a function of temperature.

In FIG. 3, the temperature in ° C is plotted on the abscissa and the enthalpy H of transformation into MJ / m ³ on the ordinate. The lower dotted line 50 corresponds to ZL5, and the lines 51, 52 and 53 are respectively the enthalpy lines of alloys 1, 2 and 3. The line of ZL5 shows a sharp increase in the value of the enthalpy at 380 °, while the enthalpies of the alloys of the invention gradually grow over a wide temperature range from about 380 ° C to about 400 ° C.

A 2D finite element model representing a section of the part previous perpendicular to the direction of injection and compound quadratic elements with 4 knots was carried out in order to simulate the filling of the part using a known software called Ansys.

By taking values of numerical thermal parameters usual for the basic thermal models, ie a mold temperature of 180 ° C, and an enthalpy of exchange between the part and the mold of h = 2300 W / m ² , as well as the zinc conductivity known to those skilled in the art and the enthalpies determined by ATD for each alloy, the model made it possible to observe the evolution of solid fractions in the thickness of the pieces as and when solidification of the alloy in the part.

The results of these calculations are shown in Figure 4 and in the Figure 5. Figures 4 and 5 thus represent the solid fraction within the alloy according to the point where one is between the heart of the part and its area.

Figures 4 and 5 thus show on the abscissa the distance d to the core of the part in mm, the value 0 corresponding to the heart of the part and the value 1.25 on the surface of the room.

On the ordinates are reported the solid fractions F.

FIG. 4 shows the distribution of the solid fraction in an injected part, made of ZL 5 alloy, during solidification, three times solidification time, i.e. 0.5s, 1s and 2s after injection, these three solidification times corresponding respectively to traces 61, 62 and 63.

In FIG. 5, the solid fraction profile has been represented in two parts conforming to example 1, respectively of alloy n ° 1 and n ° 3 according to the invention, both at 2.8 seconds of solidification time.

The layout for alloy # 1 is referenced 71 and the layout for alloy n ° 3 is referenced 73.

Those skilled in the art find, by this model, times of solidification of the order of 2 to 3 seconds, this value depending on the massiveness of the injected part.

There is a clear difference in behavior on solidification between the ZL 5 and the family of alloys of the invention at times of similar solidification.

In the case of ZL5 at 2 seconds after injection, the solid fraction is close to 0.3 over a wide area from the heart of the room to about one quarter of its half-width, then suddenly passes around 1. At on the contrary, in the case of alloys 1 to 3 of the invention, the solid fraction to 2.8 seconds has a slow growing profile between the heart and workpiece surface, ranging from approximately 0.5-0.6 in the core to approximately 0.6-0.7 in area. ZL 5 therefore has a surface skin of around 200 µm completely solidified (100% solid fraction) and a rather fluid core (F substantially equal to 0.3), and the family of alloys of the invention has a homogeneous whole where solid phase and liquid phase coexist on the surface and at heart, this set being qualified as a “pasty zone” at F substantially equal to 0.5.

A person skilled in the art sees the benefit of such behavior with regard to cold drops. In the case of ZL5, two streams of alloys on the surface, moving differently due to the design of the part, risk of becoming reweld because they will solidify at least in skin before joining completely, whereas in the case of the family of alloys of the invention, the existence of this pasty area on the surface allows to weld together streams of alloys from different sources, thus avoiding defect called "cold drops".

This difference is confirmed by the observation of microstructures of injected parts shown in Figures 6; 7, 8 and 9: the microstructure well known in section of the injected ZL 5 represented in FIG. 6 presents a skin near the surface of about 200 μm with dendrites and then a homogeneous equiaxed zone, whereas the parts injected in alloys n ° 1, 2 and 3 shown respectively in Figures 7, 8 and 9 have a structure homogeneous at the heart (bottom of the photograph) and near the surface (top of photography).

Claims (7)

1. Zinc alloy comprising, in percentages by weight: Al 1.8 - 2.2% Cu 1.5 - 3.9% Mg 0.02 - 0.06% the balance consisting of zinc, with the usual impurities inevitably present in the aforementioned metals.
2. Zinc alloy according to Claim 1, comprising in percentages by weight: Al 1.8 - 2.2% Cu 1.7 - 3.5% Mg 0.02 - 0.06% the balance consisting of zinc, with the usual impurities inevitably present in the aforementioned metals.
3. Zinc alloy according to Claim 2, comprising in percentages by weight: Al 1.8 - 2.2% Cu 2.0 - 3.1% Mg 0.02 - 0.06% the balance consisting of zinc, with the usual impurities inevitably present in the aforementioned metals.
4. Zinc alloy according to Claim 3, comprising in percentages by weight: Al 1.8 - 2.2% Cu 2.5 - 3.0% Mg 0.02 - 0.06% the balance consisting of zinc, with the usual impurities inevitably present in the aforementioned metals.
5. A process for manufacturing parts by hot-chamber pressure die casting with an alloy according to one of Claims 1 to 4.
6. A part produced by hot-chamber pressure die casting with an alloy according to one of Claims 1 to 4.
7. An appearing part produced by hot-chamber pressure die casting with an alloy according to one of Claims 1 to 4.

Images