In 2018, China’s automobile production and sales were 22,116,800 and 21,984,100 respectively, and the number of vehicles in the country is increasing. Correspondingly, the requirements for energy-saving and environmental protection performance of automobiles have been continuously improved, and automobile materials should also meet this requirement. As a corrosion-resistant material, galvanized sheet can increase the life of parts, and has been used as the main steel plate of the main engine factory. European and American cars mostly use Galvanized (GI) pure zinc coating material, which has regular and fine crystal flowers on the surface, and the zinc content in the coating is more than 99%. Japanese cars mostly use Galvaneal (GA) zinc-iron alloy coating material, which converts all the pure zinc layer on the surface into a Zn-Fe alloy coating plate through a heat treatment method.
Nippon Steel developed a zinc-aluminum-magnesium coating called ZAM in the 1990s, with a composition of Zn-6%Al-3%Mg, and its corrosion resistance is 1.5-18 times that of ordinary zinc coatings. This is because after magnesium is added, the zinc-aluminum eutectic structure becomes a granular structure, and a ternary eutectic structure of zinc, aluminum and magnesium is precipitated. After more and more studies have shown that zinc, aluminum and magnesium coatings have excellent corrosion resistance, wear resistance, welding performance, paint performance, and are considered to be the next generation of automotive protective coatings. However, domestic OEMs have not used the actual results, and there are few articles describing the influence of zinc, aluminum and magnesium coating on stamping.
This paper takes the rear floor of a mass-produced model as a representative, based on the performance of mass-produced materials, selects the zinc-aluminum-magnesium materials provided by three steel mills that currently have the ability to produce zinc-aluminum-magnesium coatings, and conducts grid tests and stamping comparisons. The comparison of appearance, inflow, thinning rate, margin, etc. shows that the zinc-aluminum-magnesium material has no obvious impact on stamping, and can realize fast and seamless switching on the molds of mass-produced models.
Part shape of rear floor
Figure 1 shows the shape of the rear floor of a certain auto parts, which is symmetrical, with a maximum dimension difference of 224.30mm in the Z-axis direction. Typical deep-drawn parts, mass-produced materials use GA material (zinc-iron alloy coating) DX53D-ZF suitable for deep drawing. The basic performance parameters are: Young’s modulus 205GPa, Poisson’s ratio 0.24, yield strength 165MPa, tensile strength 300MPa, elongation 47%, anisotropy index 2.1.
Figure 1 Rear floor of a certain model
Material performance description
Based on the performance of mass-produced GA materials, three steel mills with the ability to produce zinc-aluminum-magnesium coatings each provide a set of materials for comparison. Three sets of ZAM, one set of GA have four sets of materials, and the three sets of ZAM material coatings with aluminum content no more than 3 %, magnesium does not exceed 2%. The mechanical performance parameters of each group of materials are shown in Table 1. The values all meet the requirements of the national standard, and the yield strength varies. Except for the third group, the elongation differs greatly, and the rest such as tensile strength, n value, and r value are not much different.
Table 1 Four sets of material performance parameters
After selecting a certain model of the test subject, the floor mold is in good condition, and has been stably supplied for mass-produced models. After the four groups of materials are cut into a uniform blank shape, the results are compared without changing the production conditions.
Because the test object is the inner panel, the appearance surface is not strictly required, and the final parts meet the quality requirements, and there are no defects such as cracks and wrinkles. Figure 2 shows the comparison of ZAM and GA drawing process parts. The ZAM coating (upper part of the figure) is brighter than GA (lower part). GA parts have a dim appearance because the zinc-iron alloy coating contains about 10% iron.
Figure 2 Comparison of ZAM (upper) and GA (lower) drawn parts
The inflow refers to the distance that the outer end size of the blank flows into the mold after the drawing process, which can reflect the fluidity of the material. Due to the ease of measurement, this article measures the distance from the outermost end of the drawn part to the drawbead at the same position to compare the inflow. The larger the value, the less surface flow and the poorer fluidity. As shown in Figure 3, select four areas around to compare the inflow.
Figure 3 Schematic diagram of the inflow measurement area
The results are shown in Fig. 4, it can be seen that the inflows have little difference. The three groups of ZAM materials are compared with mass-produced GA materials. The values are all within ±5mm, which can be judged to be feasible. However, the value is positive and negative, and it fails to show that the friction performance of the zinc-aluminum-magnesium coating is better than that of the zinc-iron alloy coating.
Comparison of FLD margin and thinning rate
As shown in Figure 5, select 7 areas with larger deformations to compare FLD margin and thinning rate.
Figure 4 Comparison results of inflow
Figure 5 Schematic diagram of margin and thinning rate measurement area
The margin refers to the minimum value of the vertical distance between the strain of a certain area and the FLC curve, which can reflect the safety margin of forming. The larger the value, the safer it is. Generally, the value should be at least 10%. It can be seen from Figure 6 that the margins are all above 10%, which meets the formability requirements.
The thinning rate is also one of the indicators reflecting the forming safety. It can be seen from Figure 7 that the values are all below 20%, which can meet the formability requirements. At the same time, it can be seen from Figure 6 and Figure 7 that the three groups of ZAM materials and GA materials have positive and negative values, and there is no good regularity. This also proves that only the difference in plating layer has no obvious influence on the forming of mass production molds. This is because the mass production mold has a certain forming safety field, and the material can be processed and formed within this certain performance range, and qualified parts can be obtained.
Figure 6 Comparison results of margin
Figure 7 Comparison results of thinning rate
In this paper, the rear floor of a mass-produced car model is selected as the test object. Based on the performance of mass-produced materials, three steel plants with the ability to produce zinc-aluminum-magnesium coatings each provide a set of zinc-aluminum-magnesium materials. Three groups of ZAM, one group of GA, a total of four groups of materials are compared for stamping formability. Through comparison of appearance, inflow, thinning rate, margin, etc., the results show that the stamping and forming properties of the two coatings are equivalent, only the zinc-iron alloy coating is changed It is a zinc-aluminum-magnesium coating material, which has little effect on stamping formability. Taking into account that the zinc-aluminum-magnesium coating has excellent corrosion resistance, wear resistance, welding performance, paint performance and other advantages, it is considered to be the next generation of automotive protective coatings. The zinc-aluminum-magnesium coating material can be quickly applied to the molds of mass-produced models Switch seamlessly.