1. Introduction
In zinc alloy foundries for centrifugal casting into silicone molds (Tekcast method), only the primary alloy is used for the production of castings. The casting design and number of castings placed in the mold determine the ratio between the castings volumes and the gating system volume. The gating system volume has a constant value when using the centrifugal casting method. That is why a large amount of scrap is created during the production of small castings. The role of recycling in the metallurgical sector has always been crucial and predominant and has had a major impact not only on the global economy and energy consumption, but also on the ecological environment and human life [
1].
In terms of energy consumption, secondary or recycled zinc consumes less than 10% of the energy required to produce primary zinc. By using more recycled metal, a huge amount of air pollution generated by power plants can be eliminated. Research by Hsien Hui Khoo [
2] displays the Final Weighted Scores (indirect emissions) for the primary-to-recycled metal mixes of 100% primary zinc, 50–50% primary-to-recycled zinc, 40–60% primary-to-recycled zinc, and finally 100% recycled zinc. From the first (100% primary metal) to the second graph and third graph (metal mixes), up to about a 50% reduction in power-plant emissions can be reduced. Moreover, when 100% recycled zinc is used, the overall environmental load drops by 90%. However, due to the adverse effects on material characteristics and product quality, it is impossible to use 100% recycled zinc for casting. However, efforts to increase the content of recycled zinc, from its present value in castings, should continue [
2].
One of the main objectives in order to increase economic efficiency in foundries is to determine the ratio of the scrap material which can be used in the further manufacture (after the recycling process). This factor is important for determining the final prices of products and their competitiveness, given that the prices of zinc on world markets have increased significantly in the recent years [
3].
At present, zinc is the fourth most widely used metal in the world, after iron, aluminum, and copper. The recycling and production of secondary zinc alloys accounts for 30% of the global zinc consumption. The level of recycling is increasing in line with advances in zinc production and zinc recycling technologies [
4,
5,
6]. Zinc is most often used as a coating element or as an alloying element. The production of zinc alloys accounts for 15% of its consumption [
7,
8]. Zn-Al alloys are classified on the basis of chemical composition: hypereutectic zinc alloys (over 5.1 wt.% Al), eutectic zinc alloys (5.1 wt.% Al), and hypoeutectic zinc alloys (less than 5.1 wt.% Al) [
9]. Hypoeutectic alloys are known as Zamak alloys and make up the majority of the zinc alloys produced. The ZAMAK 2 alloy has the best mechanical properties of Zamak alloys in terms of tensile strength, creep resistance, and hardness. It is a commercial material that is widely used for the production of mechanically stressed components [
10].
The presence of Cu (3% wt.%) plays an important role in the alloy itself [
11,
12]. Cu improves mechanical properties by forming CuZn4 precipitates; however, a Cu content greater than 1.25 wt.% causes undesirable dimensional instability. By forming the intermetallic phase CuZn4, the copper content in the matrix precipitates, and this intermetallic phase crystallizes at the eutectic temperature (377 °C) as a result of the following reaction: L → α + η + E. Moreover, at the temperature 268 °C, the following reactions occur: α + E → η + T’, forming a stable phase Al4Cu3Zn [
13,
14]. These phase formations reduce the solid solution strengthening effect. The hardness of the alloy increases with the formation of CuZn4 particles. These particles are harder than the matrix, but there is an increasing tendency to crack [
15,
16,
17]. The formation of these Cu-rich intermetallic phases leads to a reduction in Cu content in primary η (Zn-rich phase in hexagonal morphology), which causes a decrease in strength due to reduced solid solution solidification [
16,
18].
Cadmium in zinc alloys is classified as an unwanted contaminant in terms of its negative effects on the human organism. Under European conditions, ZAMAK alloys with Cd are not allowed. On other continents, its usage is allowed, claiming that Zn with Cd creates eutectics that do not cause harmful effects on the human body. On the other hand, it can improve the fluidity, especially in castings with complicated shapes [
19]. Cadmium as an alloying element in zinc alloys is currently not described at all, and there are almost no scientific articles dealing with this issue. Pola states that the presence of Cd in a zinc alloy improves the wear resistance, especially at high loads (30–45 N) [
8]. On the other hand, Cd causes inter-granular corrosion in Zn-Al alloys and reduces the physical properties of the alloy [
20].
The aim of this paper is to analyze the actual effect of multiple remelting for the “pure” ZAMAK 2 alloy and the ZAMAK 2 alloy with cadmium addition. Another objective is to evaluate how multiple remeltings or the addition of Cd affects the casting and mechanical properties with connection to the microstructure of experimental alloys. Scientific studies focusing on a given alloy and Cd as the alloying element are very limited, so deeper research could help to increase its applications in practice.
Author Contributions
Conceptualization, D.B. and M.B.; methodology, M.M., D.B. and M.B.; software, M.M.; validation, D.B. and M.B.; formal analysis, M.M. and D.B.; investigation, M.M. and M.B.; resources, D.B. and M.B.; data curation, M.B. and M.M.; writing—original draft preparation, D.B. and M.M.; writing—review and editing, D.B., M.B. and M.M.; visualization, M.B. and M.M.; supervision, D.B.; project administration, D.B.; funding acquisition, D.B. and M.B. All authors have read and agreed to the published version of the manuscript.
Funding
The article was created as part of the VEGA grant agency project: VEGA 1/0160/22. The authors thank the agency for its support.
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
Data available upon request. The data presented in this study are available upon request from the corresponding author.
Conflicts of Interest
The authors declare no conflict of interest.
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Figure 2.
Scheme of spiral fluidity test (dimensions in mm): (a) front view and (b) top view.
Figure 3.
Dependence of spiral fluidity test length on the number of remelts: (a) ZnAl4Cu3 and (b) ZnAl4Cu3 with 3 wt.% Cd addition.
Figure 4.
Dependence of ultimate tensile strength on the number of remelts: (a) ZnAl4Cu3-P1 to P9 alloys and (b) ZnAl4Cu3 with 3 wt.% Cd addition C1 to C9 alloys.
Figure 5.
Dependence of ductility on the number of remelts: (a) ZnAl4Cu3-P1 to P9 alloys and (b) ZnAl4Cu3 with 3 wt.% Cd addition C1 to C9 alloys.
Figure 6.
Dependence of ductility on the number of remelts: (a) ZnAl4Cu3-P1 to P9 alloys and (b) ZnAl4Cu3 with 3 wt.% Cd addition C1 to C9 alloys.
Figure 7.
Microstructure of the ZnAl4Cu3 sample after first remelting (P1 alloy): (a) etched 0.5% HF and (b) etched Dix–Keller.
Figure 8.
Microstructure of the ZnAl4Cu3 sample after fifth melting (P5 alloy): (a) etched 0.5% HF and (b) etched Dix–Keller.
Figure 9.
Microstructure of the ZnAl4Cu3 sample after ninth melting (P9 alloy): (a) etched 0.5% HF and (b) etched Dix–Keller.
Figure 10.
Quantitative analysis of P2: (a) dendrite-phase CuZn4 and (b) eutectics-Al4Cu3Zn phase.
Figure 11.
Comparison of the presence of pores in the microstructure of ZnAl4Cu3 alloy: (a) after first remelting (P1 alloy) and (b) after ninth remelting (P9 alloy).
Figure 12.
Comparison of ZnAl4Cu3 alloy microstructure with Cd addition, etched 0.5% HF; (a) after first remelting (C1 alloy); (b) after ninth remelting (C9 alloy).
Figure 13.
Comparison of the presence of pores in the microstructure of ZnAl4Cu3 alloy: (a) after first remelting (C1 alloy) and (b) after ninth remelting (C9 alloy).
Figure 14.
Microstructure of ZnAl4Cu3 alloy with addition 3 wt.% Cd (C9) with quantitative analysis of eutectic E3 and dendrite D.
Figure 15.
Fractography evaluation of ZnAl4Cu3 alloy: (a) after first remelting, P1; (b) after third remelting, P3; and (c) after ninth remelting, P9.
Figure 16.
Fractography evaluation of ZnAl4Cu3 alloy with addition 3 wt.% Cd: (a) after first remelting, C1; (b) after third remelting, C3; and (c) after ninth remelting, C9.
Table 1.
ZnAl4Cu3 alloy chemical composition.
Element Content (wt.%) |
---|
Al | Cu | Mg | Zn |
3.93 | 3.06 | 0.047 | rest |
Permissible Elements Content (wt.%) |
Pb | Cd | Sn | Fe |
0.0037 | 0.002 | 0.0078 | 0.010 |
Table 2.
Mechanical properties of the ZnAl4Cu3 alloy.
Rm (MPa) | A50 (%) | Hardness HBS |
---|
HPDC | Sand Mold | HPDC | Sand Mold | HPDC | Sand Mold |
355 | 215 | 5 | 2 | 102 | 90 |
Table 3.
Casting conditions.
Temperature of the cast metal | Tk = 420 ± 5 °C |
Initial temperature of the mold | Tf = 60 ± 3 °C |
Rounds per minute of the mold | n = 700 rpm |
Solidification time | t = 30 s |
Table 4.
Chemical composition of ZnAl4Cu3 alloy depending on the number of remelts.
Alloy | Al (wt.%) | Cu (wt.%) | Mg (wt.%) | Pb (wt.%) | Cd (wt.%) | Sn (wt.%) | Fe (wt.%) | Zn (wt.%) |
---|
P3 | 3.92 | 2.92 | 0.040 | 0.0036 | 0.002 | 0.0078 | 0.011 | rest |
P5 | 3.97 | 2.92 | 0.042 | 0.0042 | 0.002 | 0.0079 | 0.012 | rest |
P7 | 3.91 | 2.98 | 0.039 | 0.0040 | 0.002 | 0.0091 | 0.011 | rest |
P9 | 4.04 | 2.85 | 0.045 | 0.0047 | 0.003 | 0.0077 | 0.0072 | rest |
Table 5.
Chemical composition of ZnAl4Cu3 alloy with Cd addition depending on the number of remelts.
Alloy | Al (wt.%) | Cu (wt.%) | Mg (wt.%) | Pb (wt.%) | Cd (wt.%) | Sn (wt.%) | Fe (wt.%) | Zn (wt.%) |
---|
C1 | 3.92 | 2.42 | 0.036 | 0.0047 | 3.39 | 0.0096 | 0.0150 | rest |
C3 | 3.84 | 2.41 | 0.035 | 0.0048 | 3.38 | 0.0110 | 0.0078 | rest |
C5 | 3.82 | 2.37 | 0.035 | 0.0051 | 3.50 | 0.0110 | 0.0065 | rest |
C7 | 3.55 | 2.54 | 0.031 | 0.0046 | 2.99 | 0.0120 | 0.0140 | rest |
C9 | 3.76 | 2.35 | 0.035 | 0.0050 | 3.38 | 0.0110 | 0.0078 | rest |
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