Investigation of the effects of secondary melting on microstructre and mechanical properties of zamak alloys

In industry, zamak 5 (3.7-4.3 Al, 0.7-1.25 CU, 0.02-0.06 mg, Zn) and zamak 3 (3.7-4.3 al, 0.05 cu, 0.02-0.06 mg, zn) alloys are used commonly. The faulty ones of the industrial products produced from these alloys can not be not fed directly to the hot chamber of the injection molding system (as painted, coated or assambled with non-zinc metals). They are re-melted (secondary) at temperatures up to 650°C in the recycling process and recycled ingots are produced. As a result of re-melting of the alloy at high temperatures, the values of some critical elements fall out of standart limits. In this study, these alloys were defined as "secondary melted". In this study, two secondary melted zamak alloys (notation: E1, E2), which were obtained at different times from the recycling workshop, were used. It was aimed to investigated the effects of secondary melting on microstructure and mechanical properties of standard zamak 3 and zamak 5 alloys. Chemical compositions of alloys were determined by optical emission spectrometer. In secondary melting alloys, aluminum and magnesium were found to be below the limit values, and in addition, the presence of undesired impurity elements was determined in the alloy. Parts which were cut from standard and secondary melted ingots were melted at 500°C in a graphite crucible and cast into a steel mold. The effects of secondary melting on mechanical properties can occur over a long period of time. In the literature, it was stated that the changes that take place in mechanical properties of zamak 3 and zamak 5 alloys in 24 hours at 105°C are similar to those ocur in 365 days at room temperature. In this study, the samples of each alloy were held for 24 hours at 80°C, 105°C and 130°C and the aging process was performed and changes in the mechanical properties that may occur after a long time at room temperature were attempted to be investigated. After casting and aging, the samples were subjected to three point bending tests, hardness measurements and microstructure examinations. When the bending strength values were compared, it was determined that the E1 and E2 samples obtained from secondary melting ingots had lower strength values after casting and after aging. It is thought that aluminum and magnesium in E1 and E2 alloys being below the limit values and the presence of impurity elements has a negative effect on the strength. In addition, in all alloys, hardness and strength values were found to decrease as a result of increasing aging temperature. After casting, bending strength of zamak 3 alloy was measured as 490MPa, its hardness was 88,73 HB10; after aging for 24 hours 130°C its strength was 414 MPa, hardness was 75,38 HB10. after casting bending strength of zamak 5 alloy was 457MPa, hardness was 104,13 HB10; after aging its strength was 387 MPA, hardness was 87,08 HB10. Bending strength of E1 alloy after casting wsa 444MPa, its hardness was 93,59 HB10; its strength after aging was 391 MPA, hardness was 82,40 HB10. Strength E2 alloy after casting was 383MPA, hardness was 76,80 HB10; its strength after aging was 325 MPA, hardness was 66,72 HB10. Hardness values of E1 alloy samples were lower than those of zamak 5 alloy, where as they were higher than those of zamak 3. The reason for this can be that the value of copper in E1 was higher than that in zamak 3, and iron was present as an impurity element in the alloy. After mechanical tests, the samples were examined with an optical microscope after grinding, polishing and etching. No apparent changes were observed with optical microscopy in the microstructure of the alloys related to secondary melting or aging. In the microstructure of E1 alloy, some particles that were not present in other alloys were determined, and in energy-dispersive X-ray spectroscopy (EDS) analyses, these particles were found to contain iron and aluminum and were thought to be iron -aluminum intermetallic compounds. After the optical microscope examinations, the polished surfaces and fracture surfaces of the samples were examined in scanning electron microscope (SEM). Fracture surface examinations showed that the structure was generally flat, presenting brittle fracture, but in some regions the fracture was ductile (with dimples). In EDS analyses, it was determined that the ductile phase was aluminum-rich α (alpha), and the brittle parts were zinc-rich η (eta) phase regions. As a result of this study, it was determined that the general properties of secondary melted zinc alloys were negatively affected due to their improper chemical compositions. For this reason, it can recommended in production operations to check the chemical composition before utilization of secondary melted alloys and to use these alloys in a controlled manner. Considering that secondary melted zinc alloys can be used in workpiece design, it was conclued that it is necessary to design with additional safety coefficient.