Adrina P. Silva

Inter-relation of Microstructural Features and Dry Sliding Wear Behavior of Monotectic Al–Bi and Al–Pb Alloys

AbstractnImmiscible Al-based alloys of monotectic composition have a particular feature of minority phases embedded into the Al-rich matrix. The disseminated particles may act as in situ self-lubricating agents due to their lower hardnesses compared with that of the Al-rich matrix, favoring good tribological behavior. There is a lack of systematic fundamental studies on the microstructural evolution of monotectic alloys connected to application properties. In the present investigation, the monotectic Al-1.2wt%Pb and Al-3.2wt%Bi alloys have been chosen to permit the effect of microstructural parameters on the wear behavior to be analyzed. Directional solidification experiments were carried out under transient heat flow conditions allowing a large range of cooling rates to be experienced, permitting a representative variation on the scale of the microstructure to be examined. Samples of the monotectic alloys having different interphase spacing, λ, have been subjected to microadhesive wear tests, and experimental laws correlating the wear volume with the microstructural interphase spacing and test time are proposed. It was found that microstructural features such as the interphase spacing and the morphology of the minority phase play a significant role on the wear process and that for the alloys examined λ exhibits opposite effects on the corresponding wear volume.

The Role of Si and Cu Alloying Elements on the Dendritic Growth and Microhardness in Horizontally Solidified Binary and Multicomponent Aluminum-Based Alloys

Horizontal directional solidification (HDS) experiments were carried out with Al-3wtpctCu, Al-3wtpctSi, and Al- 3wtpctCu-5.5wtpctSi alloys in order to analyze the interrelation between the secondary dendrite arm spacing (λ2) and microhardness (HV). A water-cooled horizontal directional solidification device was applied. Microstructural characterization has been carried out using traditional techniques of metallography, optical, and SEM microscopy. The ThermoCalc software was used to generate the phase equilibrium diagrams as a function of Cu and Si for the analyzed alloys. The effects of Si and Cu elements on the λ2 and HV evolution of the hypoeutectic binary Al-Cu and Al-Si alloys have been analyzed as well as the addition of Si in the formation of ternary Al-Cu-Si alloy. The secondary dendrite arm spacing was correlated with local solidification thermal parameters such as growth rate (VL), cooling rate (TR), and local solidification time (tSL). This has allowed to observe that power experimental functions given by λ2xa0=xa0Constant (VL)−2/3, λ2xa0=xa0Constant (TR)−1/3 and λ2xa0=xa0Constant (tSL)1/3 may represent growth laws of λ2 with corresponding thermal parameters for investigated alloys. Hall–Petch equations have also been used to characterize the dependence of HV with λ2. A comparative analysis is performed between λ2 experimental values obtained in this study for Al-3wtpctCu-5.5wtpctSi alloy and the only theoretical model from the literature that has been proposed to predict the λ2 growth in multicomponent alloys. Comparisons with literature results for upward directional solidification were also performed.

Microstructural development during transient directional solidification of a hypomonotectic Al–In alloy

Upward directional non-steady-state solidification experiments have been performed on a hypomonotectic Al–5.5u2009wt%In alloy. The alloy developed cellular as-solidified microstructure for tip growth rates, V L, higher than 0.95u2009mm/s. The casting regions associated with V Lu2009<u20090.95u2009mm/s were shown to be characterized by a microstructure formed by In droplets disseminated in the Al matrix. Tip growth rate and microstructural features, such as cell spacing and interphase spacing, have been experimentally determined. The experimental cell spacings have been compared with theoretical predictions furnished by the Hunt–Lu model. It was found that the experimental scatter lies below the minimum range of values theoretically predicted. Moreover, the experimental cell spacing evolution with V L is characterized by a −1.1 power law. The droplets’ interphase spacing, λ, is related to the growth rate by the Jackson–Hunt relationship (λ 2 V Lu2009=u2009constant).

On the growth of the minority phase during downward transient directional solidification of hypomonotectic and monotectic Al-Pb alloys

Al–Pb alloys were solidified under non steady heat flow conditions using a casting assembly in order to promote vertical downward directional solidification. The downward configuration enables the effects of gravity-driven convection on the final microstructure to be evaluated since the collective movement of Pb-rich particles downwards is favored, due to density differences between the two coexisting liquid phases. Investigations have been made of the obtained solidification structures. Growth rates (v) and cooling rates (

Analysis of the Cutting Temperatures along the Macrostructure of a Directionally Solidified Al-7wt%Si Alloy

mathop Tlimits^{cdot}

Examination of Thermal Parameters, Primary Dendritic Spacings and Microhardness of a Horizontally Solidified Al-Cu-Mg Ternary Alloy

) of the Al–Pb alloys solidified downwards were experimentally determined by the cooling curves recorded along the casting length. The monotectic structure was characterized by metallography and a microstructural transition has been observed in all cases. The microstructure was characterized by well-distributed Pb-rich droplets in the aluminum-rich matrix from the casting cooled surface up to a certain position in the casting, followed by a mixture of fibers and strings of pearls from this point to the bottom of the casting. The increase in alloy lead content delays the formation of fibers for alloys solidified downwards, which occurs for vxa0<xa00.48xa0mm/s and vxa0<xa00.15xa0mm/s for Al–0.9xa0wt%Pb and Al–1.2xa0wt%Pb alloys, respectively. Experimental power laws relating the interphase spacing, λ, to v and characterized by −2.0 and −6.5 exponents, were found to represent the growth of droplets and fibers, respectively, for both alloys solidified downwards.

Effect of Interfacial Heat Transfer Coefficient on Dendritic Growth and Microhardness during Horizontal Directional Solidification of an Aluminum-Copper Alloy

An experimental study has been carried out to evaluate the microstructural and microhardness evolution on the directionally solidified binary Al-Cu and multicomponent Al-Cu-Si alloys and the influence of Si alloying. For this purpose specimens of Al-6wt.%Cu and Al-6wt.%Cu-8wt.%Si alloys were prepared and directionally solidified under transient conditions of heat extraction. A water-cooled horizontal directional solidification device was applied. A comprehensive characterization is performed including experimental dendrite tip growth rates (VL) and cooling rates (TR) by measuring Vickers microhardness (HV), optical microscopy and scanning electron microscopy with microanalysis performed by energy dispersive spectrometry (SEM-EDS). The results show, for both studied alloys, the increasing of TR and VL reduced the primary dendrite arm spacing (l1) increasing the microhardness. Furthermore, the incorporation of Si in Al-6wt.%Cu alloy to form the Al-6wt.%Cu-8wt.%Si alloy influenced significantly the microstructure and consequently the microhardness but did not affect the primary dendritic growth law. An analysis on the formation of the columnar to equiaxed transition (CET) is also performed and the results show that the occurrence of CET is not sharp, i.e., the CET in both cases occurs in a zone rather than in a parallel plane to the chill wall, where both columnar and equiaxed grains are be able to exist.

Correlation between Thermal Parameters, Primary Dendrite Arm Spacing and Microhardness on a Directionally Solidified Ternary Al-6wt%Cu-8wt%Si Alloy

The aim of this research is to investigate the influence of solidification thermal parameters on the macrostructure of an Al-7wt%Si alloy during the horizontal directional solidification under unsteady-state heat flow conditions and its correlation with cutting temperatures. The solidification experiments were instrumented by thermocouples and an experimental approach was developed to quantitatively determine the solidification thermal parameters considered. The observation of the macrostructures has indicated that the columnar-to-equiaxed transition occurred in a sharp plane parallel to the chill wall and a higher average cutting temperature was obtained for the columnar structure.