Deep-drawing and backmolding process for plastic-magnesium hybrids
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Plastic-steel hybrids have been used as economical lightweight components since the late 1990s. The properties of a thin-walled, deep-drawn metal profile are specifically combined with thoses of a thin-walled plastic ribbing. Even greater lightweight construction potential is offered by the use of wrought magnesium alloys used at the IKV and the IUL as metal components.
Magnesium has only 22 % of the density of steel, or 64 % of the density of aluminium. Due to the lower strength, a higher sheet thickness is required compared to steel. However, the weight saving with identical mechanical properties is more than 50 % compared to steel, depending on the load. Since the mechanical properties are comparable to aluminum, the weight saving is even about 30 % compared to aluminum [2, 3]. However, compared to fiber-reinforced plastics with similar densities, magnesium has the typical metallic properties such as high electrical and thermal conductivity and higher ductility.
Combined Deep-drawing and Backmolding Process
In the first forming step, the sheet metal component is formed by means of a deep- drawing die. The required deep-drawing force is applied via the clamping unit of the injection molding machine. In order to improve the deep-drawing result, the flow of the metal into the die is controlled via the blank holder, and pleat in the flange area is avoided. The blank holder force is applied decoupled from the clamping unit via core pulls. In the first deep-drawing step, approximately 90 % of the metal component is formed. In the second forming step, the remainder is carried out by injecting the plastic melt into the closed cavity according to the principle of high-pressure sheet metal forming. Due to the larger-dimensioned die gap, the plastic melt opens the cavity (between die and sheet metal) by the residual forming of the metal component . The degree of forming can be adjusted by the injection molding parameters. A latent reactive adhesion promoter, applied to the sheet metal, is used to join the two components when activated by the melt heat. For the processing of wrought magnesium alloys, the combined mold has to be adapted to the forming conditions of magnesium. In addition, an adhesion promoter for joining plastics and magnesium has to be developed.
Challenges in the Processing
Due to their low cold formability, magnesium alloys are mostly used as casting materials. Compared to cast alloys, however, wrought magnesium alloys have better mechanical properties (higher yield strength, strength and elongation at breakage) . Wrought alloys can be processed into flat semi-finished products by rolling or extrusion . Thin magnesium sheets made of the wrought magnesium alloy ME20 (composition in % by weight: 1.8-2.0 Mn, 0.6-0.9 Ce/La, rest Mg) of the project partner TWI are used in the project.
Two process routes are investigated for heating the magnesium sheet blanks.
The limited cold formability is due to the crystalline structure of magnesium. Only when the forming temperature of 225 °C is exceeded, sufficient sliding planes are activated in hexagonal dense spherical packing and a deformation is possible [7, 8]. In the combined process, the magnesium sheet must be heated for forming and cooled again after injection of the plastic melt in order to dissipate the melt heat (minimum ΔT = 125 °C).
Variants of Magnesium Heating
Both process routes have advantages and disadvantages. The external heating of the sheets is easy to realize. A continuous furnace upstream of the injection molding process is used to heat the sheets in a defined manner, which means that there is no need for adapting the injection mold. An extension of the cycle time is also avoided due to processes running parallel. However, sheet metal cools down during handling between the furnace and injection mold. This is influenced by interfering factors such as ambient temperature or humidity and makes it difficult to carry out a precise process investigation. Since the sheets have to be heated well above the required forming temperature, adhesion promoter layers are exposed to high temperatures over a long period of time. The layers often contain organic, thermally sensitive components, so an influence of the temperature load on the bond strength cannot be ruled out. The integrated heating in the injection mold, on the other hand, requires a sufficiently dynamic temperature control of the mold components to prevent an uneconomical extension of the cycle time. The integrated heating allows for precise control over the forming temperature.
Forming by External Heating
For cooling, the heated blanks are laid over the entire surface and radially on tool steel or on a 5 mm thick insulation plate over the entire surface. Both the tool steel and the insulating plate are tempered to 80 °C by means of water heating (mold temperature for processing polyamide 6). The temperature of the blanks over time is recorded in the center and at the edge of the blanks (110 mm pitch circle) with a Thermo Vision A40M thermal imaging camera from Flir Systems.
Between the removal of the blanks from the furnace, the placement of the blanks and the start of the measurement, 2 s elapse. During this time, the magnesium sheets cool down to 175 °C in the edge area in the case of tool steel, or to 190 °C if they are placed on an insulating plate, Figure 3 (left). This means that the blanks in the edge area cool down too much during handling for subsequent forming. A much slower cooling takes place in the middle of the blanks. The full-surface contact shows the fastest cooling behavior. After approximately 7 s, the required forming temperature of 225 °C in the middle of the blank is reached. Due to the radial contact, the cooling time could be increased to 9 s or 18 s for the insulating plate until the minimum forming temperature is reached. However, cooling times are still too short due to the low forming speed of 2 mm/s for magnesium wrought alloys.
Based on the results, the combined deep-drawing and backmolding mold must be adapted. Both draw punch and draw ring radius must be increased for forming wrought magnesium alloys. Only an integrated tempering concept allows for the experimental investigation of process influences. For this purpose, the blank holder, draw ring and draw punch temperature should be adjustable independently in order to set a uniform temperature profile over the blank. Thus, a possible disturbance variable could be eliminated.
Joining by Backmolding
For adhesive bonds, force is applied over the entire joining surface. Due to the more homogeneous stress distribution, thinner sheet metals can be used and additional fasteners can be dispensed . In the combined process, adhesive joining offers the additional advantage of eliminating the need for precise positioning devices.
As described above, no adhesion promoter is available on the market for adhesive bonding of plastic magnesium hybrid components. For this reason, a system for polyamide molding compounds is being developed in cooperation with project partner Jubo Technologies, Wuppertal. Even though maximum bond strength is the primary goal of the work, a number of constraints must be considered. These include sufficient elasticity and plasticity during forming, a short activation time, the use of a wide range of polyamides and resistance to media and other environmental influences. In addition, the forming temperature of the wrought magnesium alloys is considered as a further aspect.
Draw punch and draw ring radius must be increased for forming wrought magnesium alloys.
As a starting system, a conversion layer for wrought magnesium alloys, a so-called pretreatment, is investigated. This is applied in a multi-step dipping process and forms a necessary corrosion protection layer on the surface and serves as a primer for subsequent coatings, for example cathodic dip coating. The influence of the injection molding parameters and the blank temperature is investigated with full-factor test plans. The results for polyamide 610 are shown as an example.
As the pretreatment is not only intended to function as an adhesion promoter for a molding compound, polyamide 6 and polyamide 66 are also examined. In order to compare the pretreatment with other adhesion promoters, a reference system, consisting of pretreatment and primer topcoat 310025-70 from Hühoco-Metall Oberflächenveredlung, is also backmolded.
In the further course of the project, the approach is to modify the pretreatment in order to avoid the additional costs associated with another system and the necessary process step of a knife coating application.
Thanks to the good mechanical properties of hybrid components, coupled with the lightest construction metal in the world, plastic- magnesium hybrid components can represent a key technology for lightweight design. By manufacturing hybrid components in a combined process, unit costs can be reduced by saving process steps. The combination of the different materials and the integrated process are challenges that have to be solved. Since it is not possible to examine the combined process in detail by means of external heating, a combined deep-drawing and backmolding mold with an integrated heating concept must be developed. The geometry of the test specimen must be adapted to the special challenges of magnesium forming. The combination of pretreatment and bonding agent in one system is a promising approach for joining the materials. Although the tensile shear strengths achieved are still below those of commercially available adhesion promoters for steels, a suitable modification is promising. This would mean that both flat and three-dimensional magnesium semi-finished products would be protected against corrosion by a single system and could also be functionalized by polyamides.
The tensile shear strength can be increased in the range of 8 to 10 MPa.
The project “Development of hybrid plastic/magnesium composites for ultra-lightweight applications — KuMag” with the funding designation ERDF-0800113 is funded by the European Regional Development Fund (ERDF). We would like to take this opportunity to thank all the institutions for their support and encouragement.
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