Material and Process Modification to Improve Manufacturability of Low-Lead Copper Alloys by Low-Pressure Die Casting Method

Abstract

Copper alloys are widely used in faucet production due to their formability, enabling the casting of complex shapes, as well as to their antibacterial properties and good corrosion resistance. This study examined a faucet produced by the low-pressure die casting method, focusing on alternatives to lead (Pb) in copper alloys. Fluidity, casting rejection rates, and mechanical and microstructural properties were assessed. Additionally, lead-free and environmentally friendly brass alloy developments in the literature were reviewed. The experimental work involved producing a faucet from aluminum, antimony, and a bismuth-modified low-lead alloy using low-pressure casting. As faucet material, the antimony-supplemented alloy exhibited superior strength and optimal hardness. It also demonstrated better microstructural distribution and the highest production efficiency (at 81%). These findings highlight the significant advantages of the addition of antimony over aluminum and bismuth in faucet casting. The results are promising for both the casting process and subsequent mechanical operations, suggesting that antimony could enhance production quality and efficiency in low-pressure die-cast copper alloys.

1. Introduction

Copper alloys, known for their high electrical conductivity, corrosion resistance, and formability, often lead to the enhancement of machinability and casting properties, making them ideal for complex shapes like faucets. However, environmental and health concerns have led to regulations limiting lead content, increasing the need for lead-free alternatives. Evaluating the physical properties of these lead-free alloys is essential for understanding their performance in casting processes [1].

Copper- and lead-based bronze materials offer high-strength and low-friction coefficients, while aluminum bronzes, with various alloy compositions, are used in the marine and petroleum industries. The addition of lead has enabled the development of free-machining brass materials. Additionally, the addition of lead ensures the tightness of brass castings as lead has a low crystallization temperature and crystallizes last in the alloy, filling any voids created by shrinkage. These materials generally contain less than 4% Pb. Despite its excellent performance in machining, lead poses significant health risks, and its use is increasingly being reduced. In line with this, relevant regulations are being issued to limit its application. The European Union’s Restriction of Hazardous Substances (RoHS) directive and California’s Proposition 65 standards are among the regulations aimed at limiting the amount of lead in drinking water systems.

The Istanbul Chamber of Industry’s October 2022 report on the “Copper and Copper Alloys Manufacturing Industry” highlights the impact of developments in the drinking water sector on faucet production. Markets like Japan, the US, and the EU have already imposed strict lead restrictions, requiring nearly zero lead content in materials contacting water. The US mandates a maximum lead content of 0.25% for faucet materials, with related standards ensuring compliance [1].

Regulations on reducing lead contents, from past to present, are as follows:

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Declarations on limiting lead contents in alloys, as outlined by the hygienic compliance requirements for products in contact with drinking water published by the 4 MS (Four Member State) alliance established through a partnership between Germany, Netherlands, the United Kingdom, and France [2].

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The law enacted in the United States mandating the classification of lead content in faucet materials, specifically for fittings, under the topic of “lead-free materials” [3].

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National Sanitation Foundation; NSF/ANSI 61-2016—Drinking Water System Components—Health Effects—With Addendum Standard [4].

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National Sanitation Foundation; NSF/ANSI 372-2016—Drinking Water System Components—Lead Content Standard [5].

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The EU Parliament’s Directive 2020/2184 on the quality of water intended for consumption [6].

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AB the EU Parliament’s RoHS (Restriction of Hazardous Substances) and REACH (Registration, Evaluation, Authorization, and Restriction of Chemicals) regulations, published in 2006 as part of international sustainability regulations, focus on restricting the use of hazardous substances [7].

The use of alternative elements to replace lead in brass alloys has become a focal point of both academic and industrial research. A study shows that elements such as bismuth and selenium can provide machinability properties like those offered by lead while being safer for health. For instance, the C89550 brass alloy, which contains bismuth and selenium instead of lead, has been found to offer comparable machinability to lead brass while being more environmentally friendly. Additionally, the addition of antimony to lead-free brass alloys has been identified as an effective alternative for faucet casting, improving casting properties. The effects of approximately 1–3% lead contents, commonly used in commercial copper alloys, on the casting ability of the alloy and the mechanical properties of the products have been investigated. When the lead content was reduced by 1%, the alloy’s hardness increased by 15–18%, the tensile strength decreased by 22–33%, the elongation decreased by about 1%, and the machinability also decreased. These studies have demonstrated that the lead element has an improving effect on the alloy’s casting ability and mechanical properties. In some studies, research was initiated on copper–graphite alloys containing selenium–bismuth as alternatives to lead in the alloy, and it has been shown that these alloys provided the same beneficial effects on machinability as lead [8].

A. La Fontaine et al. investigated the sustainable development of lead-free copper alloys (lead contents of 0.5%, 1%, and 2%) for marine and other industrial applications. The samples were cast, and their leak-tightness testing was conducted according to the related standard, with lead-tightness levels measured at 95%, 98%, and 99% for the respective alloys. The results showed that the lead-free alloys did not exhibit any significant decrease in leak-tightness properties, with the 2% lead alloy showing the best performance. Furthermore, this sample demonstrated high tensile strength and low wear rate in mechanical testing [9].

There have been some papers studying the effects of Bi and Sb additions (from 0.5% to 4%) on the microstructure and mechanical properties of copper alloys. Mechanical tests have shown that the addition of Bi and Sb increased the tensile strength by 10–15%, and the hardness value also increased. In the casting analysis, it was observed that the inclusion of Bi and Sb reduced the shrinkage rate by approximately 25–30%, with casting temperatures ranging between 950 and 970 °C. Microstructural observations revealed that Bi addition refined the α-dendritic structure, while Sb addition increased the amount of δ-phase by 5–8%. These findings indicate that Bi and Sb additions enhance the casting quality and improve the mechanical properties of the copper alloys [10,11].

Post-casting microstructure examinations and the effects of lead material in casting were explored in a study by M. Ayyapan and colleagues. They referenced the powder metallurgy method, emphasizing that this method produced outputs with superior homogeneity and wear resistance in copper–lead alloys [12].

In Merola’s investigation of a lead-containing copper alloy, it was observed that the addition of lead increased the density of the alloy while contributing to improved fluidity during casting. In this study, Merola noted an increase in wear resistance but also observed a decrease in tensile and yield strength beyond a certain threshold [13].

In the optimization of the casting process, which is another important parameter, M. Koru and colleagues demonstrated the significance of different gating system designs and vacuum applications through their simulation studies, offering a novel perspective for the production process in this field [14].

Copper alloys are widely used in faucet production due to their excellent formability, which facilitates the casting of complex-shaped parts, as well as to their antibacterial properties and corrosion resistance. Recent advancements in the production process of lead-free copper and brass alloys have further improved their environmental and mechanical performance. Different gating system designs and gas evacuation systems employed in high-pressure casting have significantly enhanced process efficiency by reducing porosity. Additionally, the impact of European Union regulations on the castability of alloys has been extensively explored [15,16,17]. In line with these developments, removing impurities such as lead and bismuth from Cu-Zn alloys through pulsed electrical current has enabled the production of purer melts, meeting critical environmental standards [18]. Moreover, the addition of lanthanum to brass alloys via squeeze casting has demonstrated notable improvements in microstructure and hardness, further enhancing product quality [19,20,21].

In our preliminary experimental studies investigating the high-quality low-pressure casting capabilities of low-lead copper alloys, we conducted computer-aided filling and solidification simulations using Altair Inspire Cast Software 2024.1 for the following alloys: CW509L (CuZn40), containing 0.2% lead; CW502L (CuZn15), containing 0.05% lead; and negligible-lead-content CuAl10Ni5Fe4. The analyses and evaluations revealed that as the lead content decreased, the mold filling time increased. Furthermore, as the lead content decreased, the narrowing of the solidification range imposed a time constraint for the alloy in properly conforming to the mold shape, thereby reducing its castability. In all three alloys, microporosity and cold shut defects were observed. The highest occurrence of cold shut defects was found in the CuAl10Ni5Fe4 alloy with negligible lead content. Our preliminary trials confirmed that the lead content in copper alloys is a significant factor in optimizing the filling and solidification processes of low-pressure cast parts. Premature solidification and cooling of the casting reduce castability and lead to the formation of critical casting defects. The success criteria for this study include conducting efforts to prevent or minimize the occurrence of such defects. To achieve these goals, innovative mold designs will be developed, and process parameters will be optimized to align with the requirements of the method, enabling the high-quality production of low-lead copper alloys via low-pressure casting.

This study focuses on faucet production using low-pressure casting with lead-free copper alloys. Key parameters, such as alloy fluidity during the casting process, post-production quality, part refection rates, material microstructure, and hardness, were analyzed. Additionally, the microstructural and mechanical effects of alloying elements, such as aluminum, antimony, and bismuth, were examined in this study. These analyses aim to contribute to the development of environmentally friendly and efficient brass alloys.

2. Materials and Methods

2.1. Alloy Preparation

In this study, the alloys of a faucet produced using the low-pressure casting method were prepared by adding antimony (Sb), aluminum (Al), and bismuth (Bi) elements to the traditional/base commercial low-lead alloys used by the casting facility. Furthermore, both the production costs have been very high, and the efficiency of the production process has remained quite low so far. At this point, studies have become increasingly necessary in areas such as material selection and the modification of alloys, the characterization of alloys, novel mold and runner designs, the determination and optimization of the process parameters, and the modification of process components.

Within this scope, modifications with different alloying elements were attempted to improve the casting process of low-lead copper alloys and to address the machining issues caused by the reduction of lead. The aim is to achieve flawless mold filling through improved fluidity. To this end, six different alloys were created by adding three different alloying elements at two different levels. In each sample alloy, the following elements were added to the base alloy as modifiers at given acceptance tolerances. Each alloy was produced separately.

• Sample: Base alloy with Al added (between 0.3 and 0.5%);
• Sample: Base alloy with Al added (between 0.7 and 0.9%);
• Sample: Base alloy with Sb added (between 0.01 and 0.03%);
• Sample: Base alloy with Sb added (between 0.03 and 0.05%);
• Sample: Base alloy with Bi added (between 0.4 and 0.6%);
• Sample: Base alloy with Bi added (between 0.9 and 1.1%).

The compositions for each charge were prepared in the main melting furnace. High-purity copper (99.99%Cu) was used as the base alloy element. The chemical analysis of the commercial low-lead base alloy is provided in

Table 1. Chemical analysis of commercial low-lead base alloy [22].

%Al	%Cu	%Zn	%Sn	%Fe	%Ni	%Mn	%Si	%Pb
0.3–0.9	58–63	Remaining	0–0.5	0–0.3	0–0.2	0–0.05	0–0.05	0–0.19

.

2.2. Casting Process Conditions

The casting processes were carried out using an KWC, Unterkulm, Switzerland, Die Casting Machine via a low-pressure casting method. In this process, different combinations of pressure, temperature, and filling time were tested to observe the effect of casting parameters. No protective atmosphere/gas (e.g., Ar or N) was used in trials. The casting parameter ranges used at trials are as follows:

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Furnace temperature: 1005–1015 °C;

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Graphite pool temperature: 30–34 °C;

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Casting tressure: 550–600 mbar;

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Filling time: 4.5–7 s;

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Mold temperature: 135–136 °C.

The solid model image and dimensions of the casting mold, half made of the CuBe alloy, are presented. The solid model image of the faucet produced with these molds is also provided, with the relevant dimensions and visual representation given in Figure 1. This female part of the mold has two gravities, with a specially designed runner system.

2.3. Post-Casting Analyses

After the casting was completed, quality control tests were conducted on the samples. In high-quality parts, defects such as shrinkage, gas porosity, hot tearing, cold shut, and incomplete filling, which lead to scrap, should not be present. It is crucial that the cast parts are produced according to the nominal dimensions and within tolerances. Manufacturing a product with a highly complex geometry and narrow dimensional tolerances using conventional pressure casting is a challenging process that requires precise control methods. Therefore, the goal of this study is to achieve this success criterion in the parts produced using the low-lead alloy and low-pressure casting technique. In this context, the following tests and analyses were performed on the production parts.

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Scrap rate determination: Each part obtained from the casting was examined, and defects such as incomplete filling, cracks, or surface imperfections were subjected to visual inspection under lighted inspection benches.

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Dimensional and geometric control: Each part obtained from the casting was subjected to dimensional and geometric controls using a coordinate measuring machine (CMM). The faucet produced from the third sample was deemed qualitatively suitable after dimensional and geometric inspection. The samples’ 3D data are given in Figure 2. This faucet is a leading member of the sink group, which is very popular in the market.

In addition, surface roughness was measured 5 repeated times by Time TR 221 model surface roughness measuring device and it is measured as 1.50 µm which is below the design value of 1.60 µm. The gauge length used in the roughness test was 4.8 mm, and the measurement accuracy was 0.01 µm.

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Microstructural analyses: The cross-sections of the samples were taken, and the microstructure was examined with the help of an optical microscope. The phase distributions and grain sizes within the alloy were determined. During this process, a Nikon SMZ800 optical microscope was used.

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Mechanical tests: Hardness tests were carried out using a Brinell hardness testing machine. The effect of antimony, bismuth, and aluminum elements on the mechanical strength was evaluated. During this process, a Matsuzawa, Akita, Japan, DXT hardness measuring device was used. A load of 30 kgf was applied to the samples for a duration of 20 s. Additionally, five randomly selected castings from all charges were subjected to tensile testing on a Matest brand 10 kN universal testing machine with a strain rate of 15 mm/min for determining the tensile and yield strengths.

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Chemical analyses: Spectral analysis was conducted to check the accuracy of the alloy composition. For this analysis, the Perkin Elmer, CT, USA, Lambda 35 UV/VIS spectrophotometer was used as the spectral analysis device. The chemical analysis contents for all the samples are as shown in

Table 2. Chemical compositions of the samples.

Sample Number	%Cu	%Pb	%Zn	%Ni	%Mn	%Fe	%Si	%Al	%Bi	%Sb
1	61.96	0.190	37.18	0.002	<0.001	0.033	0.009	0.486	<0.003	0.002
2	62.83	0.194	36.17	0.003	<0.001	0.036	0.010	0.727	<0.003	0.002
3	61.96	0.190	37.18	0.002	<0.001	0.033	0.009	0.586	<0.003	0.020
4	61.89	0.194	37.26	0.002	<0.001	0.035	0.009	0.557	<0.003	0.040
5	62.76	0.197	35.95	0.001	<0.001	0.033	0.008	0.533	0.483	0.002
6	61.95	0.283	36.22	0.001	<0.001	0.035	0.009	0.516	0.968	0.002

. An average of five repeated chemical analysis has been conducted.

The effects of the casting parameters and alloy elements on the part have been observed in the results obtained. In particular, the effect of antimony in reducing shrinkage formation, along with its positive environmental impact for lead-free alloy production, have been highly evaluated. The optimal casting parameters applied to all charges and the experimental results are provided in detail in the following section.

3. Results and Discussions

3.1. Optimum Casting Parameters

In the faucet production experiments conducted using the low-pressure die casting method, the melting temperature gradually increased from aluminum to bismuth, and the filling time after casting decreased. Chemical analysis was conducted on each charge, and the results are given in

Table 3. Casting parameters and lead rates.

Samples	Furnace Temp. (°C)	Graphite Pool Temp. (°C)	Casting Pressure (mBar)	Filling Time (s)	Mold Temp. (°C)	Lead Rate (%Pb)
1. Sample	1005	30	550	7.0	135	0.190
2. Sample	1010	34	550	7.0	136	0.194
3. Sample	1010	30	525	7.0	135	0.190
4. Sample	1010	30	550	7.0	135	0.194
5. Sample	1015	30	600	4.5	135	0.197
6. Sample	1015	30	600	4.5	135	0.247

with the optimum casting parameters for each charge that causes minimum defects. They are obtained by many adjustments at multiple trials for each charge.

3.2. Material Scrap Rate

After casting, the faucet samples underwent various mechanical processes. Each charge produced was subject to a trimming operation. Mechanical processing, sanding, and polishing were carried out, followed by a coating process. After all the processes, the scrap percentage (

Table 4. Production efficiency percentages.

Samples	Quantity	Scrap Pieces	Production Efficiency (%)
1. Sample	100	68	32
2. Sample	100	53	47
3. Sample	100	19	81
4. Sample	100	44	56
5. Sample	100	56	44
6. Sample	100	50	50

) is as follows. The charge with the best scrap-to-production ratio was the third faucet production charge, where the effect of antimony was observed. The rejection rate does not principally depend on the mechanical properties. The parts are used for the sanitary conditions, and quality requirements are more focused on the visual defects. In addition, internal tests showed that the low-lead alloys did not exhibit any significant decrease in the leak-tightness properties of the subjected faucets.

3.3. Mechanical Test Results

The hardness measurements of the samples varied according to the production parameters and chemical compositions. In the aluminum samples, the hardness value ranged between 88 and 97 HB, while in the antimony samples, a Brinell hardness value between 88 and 90 was obtained. For the bismuth samples, a significant decrease in hardness was observed compared to the other samples, with measurement results, according to the ISO 6506-1:2014 [23] standard, ranging from 74 to 79 HB. Additionally, for yield and tensile strength, tensile tests were conducted on samples produced from the casting samples according to the BS EN ISO 6892-1:2019 [24] standard. According to the test results, the faucet charge with the best yield and tensile strength was again observed to be the third sample, which was the antimony-modified one. The obtained Brinell hardness test value ranges are presented in

Table 5. Brinell hardness values of the faucets.

Samples	Lead Rate (%Pb)	Brinell Hardness (HB)
1. Sample	0.190	88–90
2. Sample	0.194	94–97
3. Sample	0.190	88–90
4. Sample	0.194	86–90
5. Sample	0.197	77–79
6. Sample	0.247	74–77

.

All tensile and yield tests results are given in the following

Table 6. Tensile strength results (MPa).

Tests	1. Sample	2. Sample	3. Sample	4. Sample	5. Sample	6. Sample
Test 1	217.45	195.62	223.10	189.72	152.14	142.75
Test 2	218.63	195.88	224.02	190.70	154.10	141.18
Test 3	218.40	196.06	222.15	190.05	154.00	142.33
Test 4	218.24	196.58	222.72	191.14	153.13	142.80
Test 5	217.83	196.86	222.91	190.34	153.58	141.74
St. Deviation	0.47	0.51	0.68	0.55	0.79	0.69
Average	218.11	196.20	222.98	190.39	153.47	142.16

and

Table 7. Yield strength results (MPa).

Tests	1. Sample	2. Sample	3. Sample	4. Sample	5. Sample	6. Sample
Test 1	174.89	134.56	215.88	162.08	146.87	136.79
Test 2	174.36	134.50	215.26	161.97	148.33	137.28
Test 3	174.64	135.25	216.14	162.16	147.56	138.80
Test 4	175.25	136.28	214.20	163.00	149.07	138.00
Test 5	175.44	134.46	215.97	163.89	147.27	138.33
St. Deviation	0.44	0.78	0.79	0.82	0.88	0.81
Average	174.92	135.01	215.49	162.62	147.82	137.84

, respectively. The tensile test sample from the third sample, which contains 0.02% antimony, showed the best tensile and yield strength values. This result is similar within the results of the study of Kim et al. [10]. One of the stress–strain diagrams for the third sample (Sb 0.02%) sample is shown in Figure 3. In that figure the red line is the strees-strain curve and the symbols on the curve indicate the tensile strength results of the third sample.

The results obtained show that there is no direct relationship between the scrap rate and the mechanical properties. For this reason, while the scrap rate is negatively affected, it is an expected result that the mechanical properties will increase or decrease. The effect of alloy modification on mechanical properties is, of course, important, but the quality of sanitary products from the end-user’s perspective is much more closely related to their visual properties.

The addition of antimony to lead-free copper alloys significantly improves their crystal structure and metallurgical properties. It is thought that antimony dissolves into the copper matrix, creating stress that could enhance the alloy’s mechanical strength, as mentioned in A. La Fontaine et al.’s investigations [9]. Additionally, antimony may react with copper to form copper–antimony compounds (e.g., Cu3Sb), which could increase the alloy’s hardness and wear resistance. Changes in the crystal structure might disrupt the face-centered cubic (FCC) structure of copper, leading to the formation of more durable phases. These alterations are believed to result in a finer-grain structure during casting, potentially providing better casting characteristics and improving the overall performance and longevity of the material.

3.4. Optical Microscope Examinations

Sections of the production parts were taken, and microstructure analysis was performed using an optical microscope. This allowed for the identification of phase distributions and grain sizes within the alloy. The microstructure images of the aluminum, antimony, and bismuth alloy samples examined under the microscope are shown in Figure 4.

The microstructure of the antimony-containing samples, being homogeneous and fine-grained, ensured the production of high-quality products during the casting process and contributed to a low scrap rate. This homogeneous structure enhances the material’s strength and durability, demonstrating its potential for long-lasting performance in various applications. The hardness test results, with a range of 88–90 HB and the best yield strength, along with a production efficiency of 81%, support this indication of the part’s manufacturability.

On the other hand, the addition of bismuth (Bi) showed that while the alloy could tolerate the reduced lead addition in terms of machinability, its mechanical strength after casting was not as good as the antimony-containing samples. After the solidification process was completed and the mold was opened, the fractures observed in the sprue areas, particularly in the riser regions, as well as the microstructure images of the bismuth-containing parts, which displayed an irregular and non-homogeneous distribution, supported these results.

In the aluminum-supplemented samples, a tighter microstructure and higher strength were observed; however, this resulted in some brittleness in the parts. As a result, it was concluded that aluminum addition did not contribute to production efficiency as much as antimony addition.

4. Conclusions

This paper presents the results of experimental studies on the manufacturability of low-lead copper alloys using the low-pressure casting method. The results obtained from the conducted studies are satisfactory in terms of the high quality and high production efficiency of low-lead copper alloy faucet manufacturing. The studies have demonstrated the feasibility of using environmentally friendly low-lead alloys in faucet production. In this way, in meeting the requirements of international regulations, which will soon become mandatory within our and some other countries, an important step has been taken in this sector.

Among the alloying elements used in the study, antimony (Sb) in particular has emerged as an element that positively influences casting performance. The addition of antimony improved fluidity and significantly reduced shrinkage formation. This resulted in the improved surface quality of the parts and a low scrap rate. This result also supports some above-mentioned studies in the literature, such as faucet production. The homogeneous microstructure distribution observed in the antimony samples suggests that they could be an effective alternative for lead-free alloys. Also, antimony takes over the function of the lead element, increasing the machining capability and providing the necessary improvement in terms of mechanical properties.

The addition of bismuth (Bi) to the alloy did not positively affect the casting ability. It had a favorable impact on machinability, with its low hardness value. It compensated for the loss of machinability properties due to the reduction in the lead element. It was observed that in terms of post-casting mechanical strength, the bismuth-supplemented charges performed worse than those with antimony.

Although the addition of aluminum (Al) to the alloy led to increased microstructural strength, it did not provide satisfactory results in terms of production efficiency.

This study is important for enabling our country’s industry to meet both existing and forthcoming regulations, to respond to increasing foreign demand, and to acquire the ability to produce products that are less harmful to human and environmental health in our domestic industry. Additionally, by increasing export opportunities, this will contribute to supporting our country’s industry and economy.

The findings of this study show that lead-free copper alloys could be environmentally friendly and safe alternatives with respect to human health. This work can be developed to be used in highly complex low-pressure casting systems and intensive production environments. In particular, the effect of antimony on casting performance and its mechanical and microstructural properties post casting are expected to contribute significantly to future research and the development of lead-free alloys.