Keywords

1 Introduction

The advancement of technology forces the application of modern engineering materials of increasingly better utility properties. Due to increasing market competition, ones expects the quality of products and the effectiveness of the production process to increase. This leads to a search for such solutions regarding the realised processes, which on one hand will allow for ensuring better, fulfilling the clients’ expectations utility features, and on the other hand, will allow for cost limitation. There is an obvious need to apply such tools, which in a complex way will allow for limiting the risk of not fulfilling the requirements stated by the clients and the producers. Surely enough, on the first stage the vital thing is to check if the new processes give the chance for ensuring the stable conditions during the production time [1–5].

Zn–Al–Cu cast alloys are characterised by good technological features, the main problem in the production technology of these alloys is the susceptibility to gassing and oxidation as well as creating an inhomogeneous thick-grained structure [1, 3, 6–9].

Zinc cast alloys are in the possession of similar features to Al or Cu cast alloys. The differences are connected with the lower temperature of melting and higher density [2].

In order to shape the features of the Zn–Al–Cu cast alloys, one should implement the modifiers, which in turn give to the liquid alloys the properties allowing for obtaining the modified structure in the solid state. The effectiveness of the modification is estimated mainly by the analysis of the obtained mechanical and utility properties as well as the level of the structure fragmentation. For the estimation of the modified Zn–Al–Cu alloys one may also apply the analysis of the cooling curve, the basis of which was developed by W. Long. The structure obtained in the modified Zn–Al–Cu alloys highly depends on the cooling rate [1–3].

In order to improve the mechanical properties of the cast alloys, apart from the heat treatment, one may also apply the modification operation, which causes the change within the morphology and diminishes the inter-phase α + β eutectic distance as well as the fragmentation of the micro-structure. Due to the reasons mentioned above, various modifiers are applied [1–3].

Nowadays, one applies the modification by strontium and antimony, since they are long-term-activity modifiers. The effect of strontium activity is still maintained after numerous re-meltings of alloy, which in turn enables the modified alloys to already be manufactured in the steel mills. For the modification of the cast alloys, rare earth elements are increasingly being used [1–3, 7].

The Zn alloys, especially those with high Al concentration, are prone to casting shrinkage, which may be eliminated by implementation of the following casting additions: Sr, Ca, Li, Na, Be; in this case, the following additions are meaningless: Ti, Zr, Sb, B and the rare earth elements. The good castability as well as the filling of the mould are decreased by the rare earth elements, which as Titanium cause susceptibility to hot-cracking [2, 3].

Properly run chemical modifications as well as application of the right cooling of the moulds lead to the improvement of the utility and quality of the produced moulds. Therefore, the knowledge concerning: the changeability of the mould’s structure simultaneously with the change of cooling rate or change of the chemical content towards adding the modifiers to the liquid metal, is of high significance [3, 8–11].

In the result of this research, concerning the influence of the addition of rare earth elements, one has stated that they cause the following: the deoxidating process during melting in the open furnace, the fragmentation of the micro-structure as a result of implementing plenty of crystallisation nuclei and finally the hardening of the precipitation. Addition of the rare earth elements enables the application of the material within the elements working in the increased temperature, since it causes the improvement of the mechanical properties in the increased temperature range (above 300 °C) [3, 8–11].

2 Materials and Experimental Procedure

For statement of the interdependence between the chemical composition and the structure of the Zn–Al–Cu zinc cast alloy modified Sr and Ti–B, see Table 1. The casts were made in the resistance furnace with chamote—graphite crucibles. During the procedure the protective atmosphere was used in form of argon. The alloys were cast into metal moulds.

Table 1 Chemical composition of Zn–Al–Cu zinc alloy
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In order to determine the relationship between the crystallization kinetics of the alloy, and the chemical composition, microstructure and properties of the zinc alloy, the casting were further modified with cerium and the following tests were performed:

  • thermo-derivative analysis of the investigated Zn alloys tested before and after modification with different cooling rates in the temperature range of TLiq and TSol 0.09 °C s−1 as well as 0.13 °C s−1. Study of the effects of cerium addition was performed by means of a crystallization process simulation with UMSA using cylindrical specimens melted in a graphite crucible. For temperature measurement a chromel-alumel thermocouple of the K type was applied with a reaction time of 250 ms,

  • macrostructure by light microscopy,

  • wear resistance testing with the ball on plate method. The tribological test consisted of the following steps: changeover the tribotester for testing the wear resistance in a reciprocating mode “ball on plate”, attachment of the sample to the device table, setting the previously selected parameters (load, speed of linear movement of the sample, the wear distance setting, read-out frequency of the coefficient of friction) on the basis of experimental tests, performing the test, measurement of the wear resistance on the cross-section of the sample and determination of the mechanical cross-sectional area, performance of the final report,

  • corrosion resistance. The corrosion resistance investigations in salt spray were carried out according to the guidelines contained in ISO 9227:2006: Corrosion tests in artificial atmospheres—Salt spray tests. Before testing, the samples were weighed. Corrosion resistance test have consisted of the following steps in a salt spray test: preparation of 5 % saline solution, setting the samples at the angle of 15° in a salt spray chamber, starting a test images after 2, 6, 24, 48 and 96 h, drying the samples after test, cleaning the samples with rinsing water and a re-drying in an air stream,

  • assessment of the stability of the realised processes of Zn–Al–Cu alloy modifications with application of X bar and R bar control charts for different properties of Zn–Al–Cu, Zn–Al–Cu–Sr and Zn–Al–Cu–Ti–B alloys. Variables were obtained by collecting the samples after the modification process. The samples had the same sample size in the assessment of the specific properties (corrosion resistance, hardness, wearability). On every chart both the central line and upper and lower control limits statistically identified were marked. The charts were used to identify occurrences of assignable causes in the situation, when a plot point is outside the chart’s control limits or there is a trend in points.

3 Results and Discussion

The results of the metallographic research, carried out with the usage of an optical microscope, have proved that the cast zinc-aluminium-copper alloys are of the α′ solid solution character, posing the precipitation of aluminium and eutectic grains α′ + η, morphology of which depends on the mass concentration of the modifiers or the alloy additions represented by Sr, Ti–B. The micro-structure of the Zn–Al–Cu alloy is characterised by globular precipitations of the α′ phase (Fig. 1). Modification of the Zn–Al–Cu alloy with strontium causes the change within the morphology of the α′ precipitations, which obtain the dendritic shape (Fig. 2). Modification of Ti–B does not cause changes in the morphology of the eutectic phase. However, it causes the fragmentation of the α′ phase precipitations as well as the α′ + η eutectic phase (Fig. 3).

Fig. 1
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Microstructure of the Zn–Al–Cu alloy cooled at the rate of 0.09 °C*s−1

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Fig. 2
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Microstructure of the Zn–Al–Cu–Sr alloy cooled at the rate of 0.09 °C*s−1

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Fig. 3
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Microstructure of the Zn–Al–Cu–Ti–B alloy cooled at the rate of 0.09 °C*s−1

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Modification of the analysed zinc alloys with strontium, titanium as well as boron is synonymous to the partial modification of the microstructure, also the α′ + η eutectic, what has been proven by the metallographic research and the changes are shown on the diagrams of the thermo derivative analysis (Fig. 4).

Fig. 4
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Cooling curves and derivative curves for alloys: Zn–Al–Cu, Zn–Al–Cu–Sr and Zn–Al–Cu–Ti–B cooled with the rate of 0.2 °C*s−1

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The conducted research of corrosion resistance have indicated its increase only in the Sr modified sample. The research hasn’t proven the increase of corrosion resistance of the Ti–B modified alloy (Fig. 5).

Fig. 5
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Corrosion measurement results of the investigated Zn–Al–Cu alloys

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In Zn–Al–Cu alloys modified both with Sr and Ti–B one has observed the increase of hardness in comparison with the original alloy (Fig. 6).

Fig. 6
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Hardness measurement results of the investigated Zn–Al–Cu alloys

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Wear resistance research has revealed wear resistance improvement both in Sr and Ti–B modified alloys (Fig. 7).

Fig. 7
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Hardness measurement results of the investigated Zn–Al–Cu alloys

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To analyse if the processes of Zn–Al–Cu alloys modification can fulfil the stability conditions even in the laboratory conditions the X bar and range charts have been prepared, for both the Zn–Al–Cu–Sr and Zn–Al–Cu–Ti–B alloys. In both cases variables have reflected the results of corrosion resistance, hardness and wearability resistance measurements. For preparation of quality control charts it was necessary to calculate: central line, upper control line and lower control line for X bar chart as well as central line and upper control line for range chart.

For the application of the X bar and range charts and the assessment of process stability based on corrosion measurement of the Zn–Al–Cu–Sr alloy (Fig. 8) in the laboratory conditions one has calculated the following values: central line for X bar chart (CL = 15.02), upper control line for X bar chart (UCL = 15.24), lower control line for X bar chart (LCL = 14.79), central line for range chart (CL = 0.12), upper control line for range chart (UCL = 0.39).

Fig. 8
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X bar and R control charts for corrosion resistance of Zn–Al–Cu–Sr alloy

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The application of the X bar and range charts and the assessment of process stability based on hardness measurement of the Zn–Al–Cu–Sr alloy (Fig. 9) in the laboratory conditions has required the calculation of the following values: central line for X bar chart (CL = 35.39), upper control line for X bar chart (UCL = 36.07), lower control line for X bar chart (LCL = 34.70), central line for range chart (CL = 1.19), upper control line for range chart (UCL = 2.52).

Fig. 9
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X bar and R control charts for hardness of Zn–Al–Cu–Sr alloy

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To assess the process stability based on the hardness measurement of the Zn–Al–Cu–Sr alloy (Fig. 10) in the laboratory conditions one has calculated the following values: central line for X bar chart (CL = 137.76), upper control line for X bar chart (UCL = 145.73), lower control line for X bar chart (LCL = 129.79), central line for range chart (CL = 4.24), upper control line for range chart (UCL = 13.85).

Fig. 10
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X bar and R control charts for wearability of Zn–Al–Cu–Sr alloy

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4 Conclusion

Changes in the α phase (Al) morphology of the cast zinc alloy can be the effect of the modification of the alloy with strontium. It is caused by the changes in crystallization kinetics. The base for heterogeneous nucleation of the alloy in this case is aluminium.

A confirmation of these changes is the shape of the cooling curve of the modified alloy in comparison with the original one. The increase of hardness of investigated alloy of about 7 % is the result of the addition of strontium. In areas out of the heat centre, where a shrinkage cavity pipe occurs, the new alloys are also characterized by low porosity. The microstructure of the mould gets more uniform without segregation of its compounds, when the shrinkage cavity pipe reaches the outside form of the melting system.

Furthermore, modification process of Zn–Al–Cu alloys—with formation of Zn–Al–Cu–Sr and Zn–Al–Cu–Ti–B alloys—realised and verified in the laboratory conditions can be defined as a stable one. Potential variables are of natural character and there are no variations resulting from special causes. Basing on the analysis’ results it can be stated that modification process—realised according to the planed suggested parameters—can assure the fulfilment of specified requirements made to the modified Zn–Al–Cu alloys even in the laboratory conditions.