Solidification of iron-rich intermetallic phases and their effects on tensile properties in aluminum-copper 206 cast alloys

The Al-Cu 206 cast alloys have been widely used in automotive and aerospace industries due to the high strength and good elevated temperature properties. However, this family alloys have an extremely low upper limit for the iron content (usually less than 0.15 wt. %) because the presence of more Fe can cause a great loss of the mechanical properties, particularly the ductility. With the increasing use of the recycled aluminum alloys, the requirement for extremely low iron contents has become a main concern in terms of the manufacturing technique and cost. Therefore, manufacturing premium castings with higher iron contents has become a great challenge.;In this study, the solidification behavior of the iron-rich intermetallics and the effect of alloy composition, cooling rate and solution heat treatment on the iron-rich intermetallics were systematically investigated in 206 cast alloys at 0.15, 0.3 and 0.5 wt. % Fe. The effect of the iron-rich intermetallics on the tensile properties was also evaluated. An optical microscope, a scanning electron microscope and a transmission electron microscope were used to observe the microstructures and analyze the volume fraction of the iron-rich intermetallics as well as the fracture surface. The solidification sequences of 206 cast alloys at 0.15∼0.5 wt. % Fe were well established. The experimental results in the present thesis are divided into four parts.;In the first part, the iron-rich intermetallics in 206 cast alloys at 0.15 wt. % Fe were studied. It was found that Chinese script α-Fe and platelet-like β-Fe can precipitate and coexist in the finally solidified alloy and the individual addition of either Mn or Si promotes the formation of α-Fe and hinders the occurrence of β-Fe. The critical cooling rate to effectively suppress the formation of β-Fe depends on the alloy composition. A casting process map is established to correlate the Mn and Si contents with cooling rate for the 206 cast alloys.;In the second part, the iron-rich intermetallics in 206 cast alloys at 0.3 wt. % Fe were investigated. Platelet β-Fe and Chinese script α-Fe were observed in the solidified samples. Both the α-Fe and β-Fe phases can nucleate on the oxide films. In addition, α-Fe can also nucleate on Al6(FeMnCu) and Al3Ti particles while the earlier formed α-Fe phase can also nucleate the later formed β-Fe phase. In addition, Either Si or Mn favors the transformation of β-Fe into the α-Fe phase. At a combination of both high Mn and high Si, almost all β-Fe platelets can be converted into Chinese script α-Fe. For a cast Al-4.5Cu-0.3Fe alloy, 0.3% Mn and 0.3% Si are required to completely suppress the β-Fe phase.;In the third part, the iron-rich intermetallics in 206 cast alloys at 0.5 wt. % Fe were studied. In addition to the two typical platelet β-Fe and Chinese script α-Fe phases, two extra phases, i.e. Chinese script Alm(FeMn) and platelet Al3(FeMn) were experimentally observed in the solidified alloys for the first time in the 206 cast alloys. Alm(FeMn), α-Fe and Al3(FeMn) are all possible as dominant iron-rich intermetallic phases. The individual addition of Si favors the formation of α-Fe but inhibits the precipitation of β-Fe while the individual addition of high Mn promotes the formation of Al 3(FeMn). The combined addition of both Si and Mn enhances the formation of predominate α-Fe. Furthermore, the formation temperature of each iron-rich intermetallic phase decreases and the stable iron-rich intermetallic is gradually replaced by the metastable phase with increasing cooling rate. There exists a threshold cooling rate to obtain the predominant Chinese script Alm(FeMn) or α-Fe phases.;Finally, the effect of iron-rich intermetallics on the tensile properties of the 206 cast alloys was performed. It was found that the tensile strengths linearly decrease with increasing iron content but higher strength are obtained for the alloys with dominant Chinese script iron-rich intermetallics than those with dominant platelet ones at similar iron levels. The 206 alloys above an iron level of 0.15% are hard to meet the minimum ductility (7%) in artificial overaging treatment (T7). However, the iron content limitation can be extended to 0.3%, or even to 0.5% to meet the 7% elongation in natural aging treatment (T4) condition under well controlled Mn and Si contents, providing the great potential to cast premium 206 alloys at high iron levels.