Mechanical properties according to special alloys used to casting

This article is focus on different alloys used to casting same part ,,COVER” 5 different aluminum alloys were studied for the best mechanical properties. Mechanical properties were evaluated in the same place from casting. Customer had special requirements on mechanical test. Suitable product was not governed by the mechanical properties obtained from standard tensile test. This special test was determined about correct or non-correct product. Mechanical properties were studied for better information about alloys for manufacturer. Measured mechanical properties were verified on tensile test similar as customer test.


INTRODUCTION
Everything was started with customer´s question: What we can do for bigger strength of this casting? Technology was got documentations for casting and requirements on mechanical properties. The first idea was changed topological shape of casting. Calculation was showed problem after change shape of casting. Construction of casting was stayed the same during test of different alloys in this experiment. The second idea was tested different types of alloy.

Type of casting
The casting was constructed as shaft cover on airport area. Cover must be resisted extremely strength conditions. Design of cover is showed on picture Figure 1. Customer had very specific requirements for ductility and on the other hand high strength by standard EN124-3-F900. [1] This combination of features was not easy. The strength was increasing with decreasing ductility and conversely. Casting was designed with AlSi7Mg0.3 T6 in the beginning. Specific requirements were for safety during heavy load. 90 tones was critical value for safe in short time. Large number of destroyed casting during test was the reason for something change. Manipulation in production was not easy with weight around 20 kilos. Checking to inside defects on X-ray was not possible. Problem was in massive thickness of casting. Attempt to change of shape was not successful with help by topological optimalization.
Foundry feasibility was evaluated with MAGMA software. The easiest way for the better mechanical properties was found the alloy with the best result during testing.

EXPERIMENT
Basic alloy of casting ,,COVER", was changed during same optimalization production parameters. ,,COVER", was prepared for test as standard production process. Castings were casted and heat treatment was happened after finishing of parts. Different alloys had different heat treatments than standard was recommended. Test groups were elected for better comparing between alloys.
• AlSi7Mg0.3 T5, • AlSi7Mg0.6 T6, Types of alloy were chosen with suitable for low pressure die casting technology. Basic choice was aluminum alloy. One of alloy was chosen for its mechanical properties. Alloys on base AlSi and AlZn were often used in foundry. Theoretical mechanical properties by standard ČSN EN 1706 for casting produced by LPDC technology are showed in Table 1.

Fractography
Analysis of cracks was made on the start production by alloy AlSi7Mg0.3 with T6. It was done for exclusion defect with alloy and inside impurities. Sample Figure 2 was evaluated in cooperation with the university. Fracture initiation was showed there. Sample was without essential impurities which could caused to fracture.

Figure 2 Sample for analysis
This was constituted fracture, probably initiated by a higher value of the internal stress in the casting. Several defects Figure 3 were observed on the fracture surface by Electron microscope TESCAN VEGA 3 with detector BSE.

Figure 3
Ductile fracture with part of brittle fracture of intermetallic particles The fracture was ductile of eutectic α(Al)-Si with part of brittle fracture of intermetallic particles α(Al)FeMgSi and α(Al)FeMnSi by SEM and EDS analysis. This particles were not initiation of fracture. The fracture was passed through the area of the radius. Combinations of radius and high surface roughness could caused to contributed to the formation of cracks and fracture. Test specimens was removed from 3 different places of casting Figure 4. Places for samples were elected with casting options. It was not easy because ,,COVER", had thin-walled ribs. Specimens were machining according to standard ČSN EN ISO 6892-1. [4] Samples were sent to acreditate testing laboratory. Measure machine was Zwick/Roell Z250 with loadcell 150 kN and extensometer -MultiXtens. Very surprising was result ductility alloy AlZn10Si8Mg. Alloy by standard had value around 6 %, but tensile test was showed very low values of ductility. It could be caused to some defects in specimens. Other values were more or less corresponded with standard.

Special customers test
Mechanical properties by the tensile test were information for foundry. Other castings, which were produced in this experiment were tested on special customer machine for approval for use. It was very easy test. Finish parts were saved into pressure machine Figure 6 and were tested with measurement reports. The final strength was 600 kN during 200 seconds on the testing part.

Figure 6
Customer testing machine and measurement report on alloy AlCu5NiCoSbZr T6 Result could be only correct or non-correct. Correct part is showed on the right side Figure 7. Non-correct part is on left side. AlSi7Mg0.6 with T6 heat treatment had the best result on customer test. All of parts from different alloys had larger or smaller cracks.

Figure 7
Non correct part and correct part after customer test

CONCLUSIONS
This article was focused on change type of alloys and influence on mechanical properties. Four different alloys was tested. Mechanical properties on standard tensile test were appropriated by standard in most alloy. Big experience for foundry was used to new alloy EN AC-AlCu5NiCoSbZr for LPDC. This alloy could be used for this casting with a little bit change of technology, but mechanical properties were not good for this application. Customer did not have specified requirements on values of mechanical properties. A suitable product did have to priority passed customer test with load of 90 tons.