FR2787470A1 - New zinc-aluminum-copper-magnesium alloy, for hot chamber pressure die casting of high surface quality parts, has a relatively high copper content and relatively low aluminum and magnesium contents - Google Patents

Abstract

Zinc-aluminum-copper-magnesium alloy, has a relatively high copper content and relatively low aluminum and magnesium content. A novel zinc alloy has the composition (by wt.) 1.8-2.2% Al, 1.5-3.9% Cu, 0.02-0.06% Mg, balance Zn and impurities. Independent claims are also included for the following: (i) a method of producing parts by hot chamber pressure die casting of the above alloy; and (ii) parts produced by hot chamber pressure die casting of the above alloy.

Description

The present invention relates to zinc-based alloys

  allowing high quality foundry parts.

  Numerous zinc-based alloys have already been proposed for this purpose.

  We thus know a family of alloys called "zamak", 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 standards NF EN 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 AI 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 the standard NF EN 1774 is ZL5. The purity of 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 of which is that part of its injection system is immersed 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 with high productivity parts of varying degrees of complexity, dedicated to multiple applications (luxury industry, hardware, automotive, building etc., this list

not being exhaustive).

  In such a process, the presence of aluminum is essential to limit the aggressiveness of the zinc with respect to the steel or cast iron parts constituting the casting machine. The addition of aluminum thus allows the parts to be injected by a process of the hot room type where the

  injection system is immersed in the molten alloy.

  Likewise, it is known that the presence of magnesium at contents between 0.02% and 0.06% is recommended to avoid problems of intercrystalline corrosion of the parts and to improve the mechanical properties such as the tensile strength, the hardness and embrittlement at low temperatures. Above 0.06%, it is known that the

  magnesium produces a decrease in impact flexural strength.

  Finally, it is known that for the zamak family, the aluminum content of which varies between 3.9 and 4.3%, copper makes it possible to avoid intercrystalline corrosion. Many efforts have been made to improve the mechanical properties of the ZL 5 alloy, in particular its resistance to creep, which is a very important parameter in applications such as the automobile.

or the building.

  In general, it is desired that the appearance of the parts made of such zinc alloys is of high quality. However, coated or uncoated parts very often have surface defects. Two frequent and particularly critical faults 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 step when the subsequent coating step is perfectly controlled and it does not generate

  its specific defects (stings or scrapes for example).

  Cold drops are junctions of several streams of poorly re-welded injected alloys, often favored by an excessively cold mold temperature. However, those skilled in the art know that certain part designs make this surface defect almost inevitable whatever

be the injection conditions.

  Many zinc-based foundry parts have a decorative role, for example in the so-called luxury industries (high-end or costume jewelry, 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 pieces of appearance injected in zamak and then coated is judged by a visual examination which rejects any piece having surface defects called "appearance defects", these defects being able to be linked to one

  foundry and coating steps.

  The company Stolberger Zink proposed an alloy with a lower aluminum content 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 underline the beneficial effect of beryllium on the resistance to impact bending and mention a qualitative improvement in the appearance of the parts. The tensile strength of this alloy is however

  reduced by 10-15% compared to that of ZL 5.

  In the document BE-846.899, there is described a family of alloys with AI: 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 drawback of providing parts having only an unsatisfactory surface quality, the slight aesthetic improvement being accompanied by a lowering of the qualities.

mechanical parts.

  The object of the present invention is to provide a family of zinc alloys whose surface quality of the parts injected as they are, raw foundry, or else ready to be coated, for example after polishing, in such alloys is significantly improved. while maintaining

  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:

AI: 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 makes it possible in particular to improve the surface quality of the injected parts by reducing the number of porosities close to the surface, porosities inherent in the conventional injection process (without the use of a vacuum mold). , and by reducing the number of cold drops for parts sensitive to this defect. This quality of the alloy family has been verified by manufacturing tests of many parts. To provide a quantified evaluation of the improvement made, the Applicant has chosen an appearance part having large flat surfaces, which is particularly indicative of appearance problems. Other aims, advantages and characteristics of the invention

  will appear on reading the detailed description which follows, made in

  reference to the appended examples and figures in which: FIG. 1 represents an injection device of a pressurized injection machine with 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; - Figure 3 shows the evolution of enthalpy of transformation as a function of temperature for ZL5 and three alloys according to the invention; - Figure 4 shows solid fraction profiles of a part in

  ZL5 alloy at different solidification times.

  - Figure 5 shows solid fraction profiles of an alloy part according to the invention; - Figures 6 to 9 show sections of injected parts

  respectively in ZL5 and in three alloys according to the invention.

  In the first example of the invention, a part with dimensions 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 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 placed under vacuum or filled with a gas such as oxygen before injection, but the most commonly used method uses a mold whose cavity 20 contains air. The liquid phase alloy 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 process, the direction of injection being parallel to the long sides of the part. The large surface that must have a nice appearance is that on the 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 glm of material to simulate the polishing practiced

  industrially in the usual way before a final coating.

  The polished surface of each of the parts is then observed using an optical microscope (Olympus, reference PMG3) with which

  detected surface defects with a span greater than 50 p.m.

  Such defects are indeed likely to lead to porosities. These faults were quantified 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 AI% Cu% Mg% Zn by mass by mass by mass by mass ZL 5 4 1 0.04 Rest N 1 2.2 1.75 0.04 Rest N 2 2.1 2.90 0.04 Rest N 3 2.1 3.90 0.04 Rest N 4 2.1 0.95 0.04 Rest Alloys 1 to 3 have an aluminum content close to 2% by 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 for Stolberger Zink, to the

  difference that that of Stolberger Zinc contained Beryllium.

  For the alloys of the invention, a mass content of magnesium is adopted of between 0.02 and 0.06%, and preferably between 0.03

and 0.06%.

  The experiment parameters are reported in the following table Temp'Ure Speed Velocity to Temp Time Time of the piston post-mold attack

  casting in 16th in 2nd C pressure cooled

   C phase phase s ssement m / sm / s S ZL 5 430 0.03-0.06 38 - 47 150 -180 1.5 - 2.5 2 - 3.5 Alloy 450 0.03-0.06 38 - 47 150 -180 1.5 - 2.5 2 - 3.5 n01 Alloy 450 0.03-0.06 38 - 47 150 -180 1.5 - 2.5 2 - 3.5 no 2 Alloy 450 0, 03- 0.06 38-47 150 -180 1.5- 2.5 2 - 3.5 n0 3 Conventionally, the piston is driven in two phases, the first being a slow phase and the second a rapid phase. The post-pressure time 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 piece below.

  above described and mechanical tests were carried out with an Instron traction machine, at a speed of 2 mm / min at 20 C, (the test pieces

having a length Io of 40mm).

  The results obtained from defect counting and mechanical tests are presented in the table below: Alloy Average number of defects> 50 tLm on Hardness Resistance an area of 50x50 mm2 after Vickers latraction polishing MPa (force 10 kg) ZL 5 Base: 100 290 102

N 1 56 290 107

No 2 47 290 112

N 3 80 297 126

N 4 - 260 102

  The results clearly show the increase in surface quality, 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, it can be seen 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%.

  FIG. 2 shows 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 condition for the Cu contents of between 1.7% and 3.5% by weight with a number of defects substantially less than 55, even more advantageous for the Cu contents ranging from 2.0 to 3.1% by weight with a particularly reduced number of defects, substantially less than 50 as in the case of alloy 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 have been able to observe 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 lower surface quality is much more

  sensitive to changes in parameters than the alloys according to the invention.

  The quality of parts manufactured with the new family of alloys is therefore in practice much less sensitive to changes in injection parameters that may occur during the manufacture of parts (starting up of

  machine, machine stop etc.).

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

  traction is improved, to 297 MPa.

  The inventors have found that the tensile strength becomes very low again for a Cu content of less than 1% to an aluminum content of approximately 2% as illustrated by the case of alloy 4 which has a resistance only of 260 MPa, the hardness, measured by the well-known Vickers method, 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% of Cu.

  In the second example of the invention, conventional measurements by differential technical analysis (ATD) were carried out on the ZL 5 and on the family of alloys with AI: 1.8-2.2%, Cu: 1.5-3.9%, Mg: 0.02-0.06%, zinc residue. 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 at C is plotted on the abscissa and the enthalpy H of transformation into MJ / m3 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 grow progressively over a wide temperature range from approximately

380 C to around 400 C.

  A 2D finite element model representing a section of the previous part perpendicular to the direction of injection and composed of quadratic elements with 4 knots was produced 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 / m2, thus that 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

  parts as the alloy solidifies in the part.

  The results of these calculations are represented in FIG. 4 and in FIG. 5. FIGS. 4 and 5 thus represent the solid fraction within the alloy according to the point where one is between the core of the part and its

area.

  Figures 4 and 5 thus show on the abscissa the distance d to the heart 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.

  The solid fractions F are plotted on the ordinate. In FIG. 4, the distribution of the solid fraction is shown in an injected part, made of ZL 5 alloy, during solidification, with three solidification times, ie at 0.5s, ls and 2s after the injection, these three solidification times corresponding respectively to the plots 61, 62

and 63.

  In FIG. 5, the solid fraction profile has been represented in two parts in accordance with Example 1, of alloy n 1 and n 3 respectively according to

  the invention, both at 2.8 seconds of solidification time.

  The layout for alloy n 1 is referenced 71 and the layout for alloy

n 3 is referenced 73.

  Those skilled in the art find, by this model, solidification times 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 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 large area going from the heart of the part to about a quarter of its half-width, then suddenly passes around 1. On the contrary , in the case of alloys 1 to 3 of the invention, the solid fraction at 2.8 seconds has a slowly growing profile between the core and the surface of the part, ranging from around 0.5 - 0.6 to core at about 0.6 - 0.7 on the surface. ZL 5 therefore has a skin on the surface of approximately 200 rt completely solidified (100% solid fraction) and a rather fluid core (F substantially equal to 0.3), and the family of alloys of the invention presents a set homogeneous o solid phase and liquid phase coexist on the surface and at the core, this set being qualified as "pasty zone" at F substantially

equal to 0.5.

  Those skilled in the art can see the benefit of such behavior with regard to cold drops. In the case of the ZL5, two streams of alloys on the surface, traveling differently due to the design of the part, risk poorly rewinding 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 makes it possible to reweld together streams of alloys from different sources, thus avoiding the defect

called "cold drops".

  This difference is confirmed by the observation of microstructures of injected parts represented in FIGS. 6, 7, 8 and 9: the well-known microstructure in section of the injected ZL 5 represented in FIG. 6 has a skin near the surface of approximately 200 .m with dendrites 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 homogeneous structure at the heart (bottom of the photograph) and near the surface (top of

photography).

Claims (4)

Claims   1. Zinc alloy comprising in mass percentages: AI: 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.   2. Zinc alloy according to claim 1, comprising in percentages by mass: AI: 1.8-2.2%   Cu: 1.7-3.5% Mg: 0.02-0.06% the rest being made up of zinc, with the usual impurities inevitably   present in the aforementioned metals.   3 - Zinc alloy according to claim 2 comprising in mass percentages: AI: 1.8-2.2%   Cu: 2.0-3.1% Mg: 0.02-0.06% the rest being made up of zinc, with the usual impurities inevitably   present in the aforementioned metals.   4 - Zinc alloy according to claim 3 comprising in mass percentages: AI: 1.8-2.2%   Cu: 2.5-3.0% Mg: 0.02-0.06% the rest being made up of zinc, with the usual impurities inevitably   present in the aforementioned metals.   - A process for manufacturing pressure foundry parts   hot chamber with an alloy according to one of claims 1 to 4.   6. Part made in hot chamber pressure foundry with a   alloy according to one of claims 1 to 4.   7. Appearance piece made in hot chamber pressure foundry   with an alloy according to one of claims 1 to 4.