Wislei R. Osório

Modeling dendritic structure and mechanical properties of Zn-Al alloys as a function of solidification conditions

Abstract In the present article, some important trends have been shown regarding the relationship between mechanical properties, microstructure and solidification variables of Zn–Al alloys castings. Some theoretical dendritic growth models, expressing secondary spacings as function of tip growth rate or local solidification time, have been tested against experimental data obtained during unsteady-state solidification. Based on these dendritic models, on analytical expressions describing the position of solidus and liquidus isotherms in the unidirectional solidification of binary alloys and on experimental results concerning tensile testing of casting samples, expressions have been developed permitting a correlation between ultimate and yield strength, dendrite secondary spacings and solidification processing variables.

Effect of dendritic arm spacing on mechanical properties and corrosion resistance of Al 9 Wt Pct Si and Zn 27 Wt Pct Al alloys

It has been reported that the mechanical properties and the corrosion resistance (CR) of metallic alloys depend strongly on the solidification microstructural arrangement. The correlation of corrosion behavior and mechanical properties with microstructure parameters can be very useful for planning solidification conditions in order to achieve a desired level of final properties. The aim of the present work is to investigate the influence of heat-transfer solidification variables on the microstructural array of both Al 9 wt pct Si and Zn 27 wt pct Al alloy castings and to develop correlations between the as-cast dendritic microstructure, CR, and tensile mechanical properties. Experimental results include transient metal/mold heat-transfer coefficient (hi), secondary dendrite arm spacing (λ2), corrosion potential (ECorr), corrosion rate (iCorr), polarization resistance (R1), capacitances values (ZCPE), ultimate tensile strength (UTS, σu), yield strength (YS, σy), and elongation. It is shown that σU decreases with increasing λ2 while the CR increases with increasing λ2, for both alloys experimentally examined. A combined plot of CR and σU as a function of λ2 is proposed as a way to determine an optimum range of secondary dendrite arm spacing that provides good balance between both properties.

Mechanical properties as a function of thermal parameters and microstructure of Zn–Al castings

Abstract The imposition of a wide range of operational conditions in foundry and casting process generates, as a direct consequence, a diversity of solidification structures. Structural parameters such as grain size and interdendritic spacings are highly influenced by thermal behavior of the metal/mould system during solidification, consequently imposing a close correlation between the described system and the resulting microstructure. The mechanical properties of an alloy depend on the solidification microstructural arrangement. Under this circumstance, grain size, interdendritic spacings, casual porosities, segregated products and other phases will define the mechanical behavior of the alloy, represented by stresses and/or strains. Expressions correlating the mechanical behavior with microstructure parameters are very useful for a previous planning of the solidification conditions in terms of a determined level of mechanical resistance which is intended to be attained, i.e. to settle a way of programming the microstructure and the mechanical properties as well. Particularly, the literature in this field presents relationships between the material yield strength and the grain size, such as the Hall–Petch’s equation. The aim of the present work is to investigate the influence of heat transfer on solidification microstructure of Zn–Al alloys and the correlation with mechanical properties. Experimental results include transient metal/mould heat transfer coefficients, secondary dendrite arm spacings and ultimate and yield strengths as a function of solidification conditions imposed by the metal/mould system.

Dendritic Microstructure Affecting Mechanical Properties and Corrosion Resistance of an Al-9 wt% Si Alloy

It has been reported that the mechanical properties and corrosion resistance of metallic alloys depend strongly on the solidification microstructural arrangement. The correlation of corrosion behavior and mechanical properties with microstructure parameters can be very useful for planning solidification conditions in order to achieve desired final properties. The aim of the present work is to investigate the influence of dendritic microstructural parameters of an Al-9 wt.% Si alloy on mechanical properties and corrosion resistance. The experimental results establish correlations between secondary dendrite arm spacings (λ2) and ultimate tensile strength (σ u ), yield strength (σ y ), corrosion potential (E Corr ), and corrosion rate (i Corr ).

Corrosion behavior of hypoeutectic Al-Cu alloys in H2SO4 and NaCl solutions

The aim of this work was to evaluate the electrochemical behaviour of hypoeutectic Al-Cu alloys immersed in two different solutions containing sulphate and chloride ions, respectively. The influence of Al 2 Cu associated to the dendritic arm spacing on the general corrosion resistance of such alloys is analysed. The typical microstructural pattern was examined by using scanning electron microscope. The corrosion tests were performed in both 0.5 M sulphuric acid and 0.5 M NaCl solutions at 25 °C by using an electrochemical impedance spectroscopy (EIS) technique and potentio-dynamic polarization curves. Equivalent circuits by using the ZView software, were also used to provide quantitative support for the discussions. It was found that as the Cu content increased ( i.e ., increasing the Al 2 Cu fraction), a higher susceptibility to the corrosion action in the NaCl solution is detected. In contrast, the tests carried out in the H 2 SO 4 solution resulted in similar corrosion rates for the three different hypoeutectic alloys.

Microstructural and Hardness Evaluations of a Centrifuged Sn-22Pb Casting Alloy Compared with a Lead-Free SnAg Alloy

A great preoccupation with replacing the traditional Sn-Pb alloy with a Pb-free alloy (“green alloy”) is recognized. There are industrial sectors that demand metallurgical improvements to attain certain unsoundness and adequate properties as a function of imposed operational parameters. In this experimental investigation, two distinctive centrifuged casting alloys (i.e., Sn-2 wt pct Ag and Sn-22 wt pct Pb) are compared. It is found that centrifuged castings have similar microstructure constituents, although distinctive cooling rates and solute contents are considered. It is also found that Ag3Sn intermetallic particles are responsible for attaining similar tensile strength, since more dislocations between Ag3Sn particles and the Sn-rich phase are provided. In order to replace the Sn-Pb alloys with a successor alloy containing sustainability and environmental aspects associated with castability and to guarantee the desired properties, it seems that a green alloy (Pb free) with intermetallic particles finely and homogeneously distributed provides an interesting benefit to various industrial applications.

Interrelation of wettability–microstructure–tensile strength of lead-free Sn–Ag and Sn–Bi solder alloys

A comparative investigation on the wettability and tensile strength of a Sn–2Ag, a Sn–40Bi and the traditional eutectic Sn–Pb solder alloys was carried out. The wettability is represented by thickness of covered layer (TCL) and spread area (SA) while the mechanical behaviour by the ultimate tensile strength (UTS). It is shown that the TCL of studied alloys decreased with the increase in the dipping temperature. It is also shown that TCL and SA have opposite behaviour with respect to the cooling rate. The Sn–Bi solder alloy has lower SA when compared with those of the Sn–Ag solder when similar cooling rates are considered. The Sn–Bi solder exhibits the best UTS/SA combination for dendritic spacings between 25 and 27 µm, associated with cooling rates ∼2°C s−1, 2× lower than those of the Sn–Ag alloy. Besides, the Sn–Bi alloy has shown SA >70∼80% associated with higher UTS (∼80 MPa) as compared with the other alloys examined.

Effects of Microstructural Arrangement on Hot Corrosion Resistance of a Pb-Sb Alloy for Battery Grids

It is well known that the solution characteristics, such as pH and temperature may affect the corrosion mechanism and the corrosion behavior. Lead acid batteries manufacturers have provided modifications into the grid project in order to decrease battery grid weight as well as to reduce the production costs, and to increase the battery life-time cycle and the corrosion-resistance. The performance of lead-acid batteries in automotive applications can significantly be affected by temperature variation. The aim of this study was to evaluate the effects of the microstructural morphology of a Pb-6.6wt%Sb alloy under conditions of hot corrosion. A water-cooled unidirectional solidification system was used to obtain coarse and fine dendritic microstructures. Electrochemical impedance spectroscopy (EIS) diagrams, potentiodynamic polarization curves and an equivalent circuit analysis were used to evaluate the corrosion behavior of fine and coarse dendritic samples in a 0.5M H2SO4 solution in three different working temperatures. It was found that independently of the working temperature, samples with finer dendritic microstructures provide better corrosion resistance than coarser ones, which is an indication that the former microstructural pattern may provide a higher battery life-time in severe temperatures than a coarser one.

The Role of Macrostructural and Microstructural Morphologies on the Corrosion Resistance of Zn and a Zn-4% Al Alloy

Different structural morphologies may develop due to a wide range of the operational conditions that may exist during casting. It is well known that corrosion resistance and mechanical properties depend on solidification structures. The aim of this study is to investigate both the influence of columnar and equiaxed structures of zinc as-cast samples and of dendritic microstructural array of a Zn-Al alloy on the corrosion resistance. In order to obtain columnar and equiaxed structures, both a vertical upward solidification apparatus and a permanent steel mold casting assembly were used. The corrosion resistance was analyzed using electrochemical impedance spectroscopy and Tafel extrapolation. Corrosion tests were conducted in a 3% (vol.) NaCl solution at room temperature. It was found that coarser macrostructures tend to provide higher corrosion resistance for both columnar and equiaxed morphologies. On the other hand, finer secondary dendrite arm spacing is conducive to a better corrosion behavior of a Zn-4% Al alloy.

Experimental analysis of corrosion resistance on columnar to equiaxed transition region of as cast structures of Al–Cu alloys

Abstract In the present article, in order to investigate the corrosion resistance tendency on both sides of the columnar to equiaxed transition (CET), some samples of Al–Cu alloys (5 and 8 wt-%Cu) were prepared on the columnar and on the equiaxed sides of the casting and subjected to corrosion tests. The corrosion resistance has been analysed by both the electrochemical impedance spectroscopy technique and Tafel extrapolation method conducted in a 3%NaCl solution at room temperature. It was found that both columnar and equiaxed morphologies at the CET region, for each alloy examined, have attained similar experimental electrochemical impedance and polarisation results. The secondary dendrite arm spacings λ 2 on both columnar and equiaxed sides of the CET region are very similar in any case experimentally examined and the resemblance on the corrosion resistance of columnar and equiaxed structures has been attributed to the similarity of such microstructural parameter. The corrosion resistance has decreased with increasing alloy Cu content. The experimental evidence has indicated that this can be associated with smaller dendritic arm spacings.