The Effect Of Stress On The Microporosity Evolution Of Asquenched Al-Cu Alloy

The typical aerospace aluminium alloy structural parts have the features of structural complexity,large dimension and hugely varying wall thickness.The integrative manufacturing can combine various geometric features by employing low-pressure casting,quenching,aging and finally machining.Having high efficiency and low cost,it has been widely applied in aerospace engineering.However,the presence of microvoids,segregation and other defects can't be avoided in the casting process.Research showed that small porosity ratio has a significant effect on the mechanical properties of casting.On the other hand,the existing research on microporosity did not provide a viable solution for the existing problems.Some researchers have ignored the intricate shapes of the contraction cavities,and their shapes were simplified as an ideal sphere or cylinder.Also,the characterization method was a 2D observation,which lacks quantitative characterization.Further,the microporosity was considered as a static shape,which ignored the effects of stress on its morphology evolution.Some researchers did not provide a comprehensive understanding on the features of casting microporosity in aluminum alloy,and only used the empirical model to describe the influence mechanism of microporosity on the mechanical behaviors.In this paper,the widely used aerospace Al-Cu alloy ZL205A was taken as experimental material,and the researches were carried out by using high precision densimeter and X ray computed tomography.The different low-pressure casting processes were conducted to investigate the influence of casting technology parameters on microporosity distribution of casting part.The spoon-like distribution law of microporosity in casting part was concluded.The influence of solidification sequence on microporosity distribution was revealed.In order to investigate the effect of stress and strain on the porosity ratio,the porosity was characterized by the density of the as-quenched Al-Cu-Mn alloy.The density measurement experiment was designed and the ZL205A samples with density ranging between 2.79 g/cm~3 and 2.82 g/cm~3 were tested to investigate the density variation patterns of tension and compression process.Finally,the linear relation of density and strain was established.This paper studied the morphological properties of the microporosity existing in as-quenched ZL205A by using the X-ray computed tomography.Meanwhile,the hypothesis that most of microporosity is caused by intercrystalline shrinkage was verified.The microporosity data of the stressed state(the strain is 6%)and unstressed state was characterized and compared.Hence it was concluded that the volumes of microporosity mainly ranged between 10~2 and 10~4?m~3.The results showed that microporosity with different volumes level were uniformly distributed in the composite.The quantity of microporosity decreased exponentially as the volumes of microporosity increased.The pores with larger volume had smaller proportion when compared with total pore quantity,but it had a significant contribution on porosity ratio.The volume ratio of the microporosity was affected by the tensile stress to a certain extent,and the quantity of microporosity with a volume greater than 10~4?m~3 increased obviously compared with the quantity of the microporosity under the unstressed state.By comparing the three-dimensional shapes of the micropores,it was found that there exist three kinds distinguished evolution modes of microporosity occurring when tensile stress affected the microporosity,microporosity expansion,coalescence or shrinkage.And the volume,sphericality and posture of the microporosity had a close relation with the evolution modes.When the volume was in the range of2*10~3?m~3~10~4?m~3,the microporosity coalescence took place often.Since contraction cavities have the features of big size and low sphericality,the microporosity expansion took place easier under stressed state.With the developement of the microporosity coalescence,the microporosities evolved into microcrack,which will evolve into the crack source of material fracture.