Microstructural Morphologies and Experimental Growth Laws during Solidification of Monotectic and Hypermonotectic AI--Pb Alloys

Al—Pb alloys with monotectic and hypermonotectic compositions were directionally solidified under unsteadystate heat flow conditions.The cooling curves recorded during solidification allowed solidification thermal parameters such as the cooling rate(T),growth rate(v) and thermal gradient(G) to be experimentally determined.Different microstructural patterns have been associated with the alloy solute content,i.e.,Al—1.2and 2.1 wt%Pb.A sequence of morphologies from the bottom to the top of the Al—1.2 wt%Pb alloy casting(monotectic) can be observed:Pb-rich droplets in the aluminum-rich matrix,followed by a region of microstructural transition formed by droplets and fibers and finally by a mixture of fibers and strings of pearls.A completely fibrous structure(without transition) has been observed along the entire Al—2.1 wt%Pb alloy casting(hypermonotectic).The interphase spacing(λ) was measured along the casting length,and experimental correlations between λ and experimental solidification thermal parameters have been established.Power laws with a —2.2 exponent expressing λ as a function of the growth rate,v,were found to better represent the fibrous growth of both Al—Pb alloys.Moreover,a single experimental law expressing λ as a function of both G and v was found to describe the fibrous growth of both the monotectic and the hypermonotectic alloys experimentally examined.     

Assessment of Tertiary Dendritic Growth and Its Effects on Mechanical Properties of Directionally Solidified Sn-0.7Cu-xAg Solder Alloys

Although commercial SAC107/207/307 alloys are being used as alternatives to traditional Sn-Pb solder alloys, there is a lack of studies emphasizing some metallurgical aspects, for instance, the different morphologies of Cu₆Sn₅ and Ag₃Sn intermetallic compounds (IMCs) as well as conditions for the launch of tertiary dendritic β-Sn branches and their effects on localized mechanical properties. A wide range of cooling rates during solidification is normally associated with quite different microstructural length parameters. Hence, Sn-0.7 wt.%Cu-x wt.%Ag (where x = 1.0, 2.0, and 3.0) alloys were directionally solidified under transient heat flow conditions, undergoing cooling rates varying from 0.1 K/s to 32.0 K/s. This experimental study encompasses: primary, secondary, and tertiary dendrite arm spacings (λ ₁, λ ₂, and λ ₃) associated with the evolution of the tip cooling rate ( $ \dot{T} $ ) during solidification; start and growth of tertiary dendrite branches; yield (σ _y) and ultimate tensile strengths (σ _u); elongation to fracture (δ); and the morphology of IMCs embedded in the Sn-rich phase. A single ratio between the cooling rate ( $ \dot{T} $ ) and the alloy silver content (C _0-Ag) of 0.45 seems to be the parametric factor associated with the beginning of the growth of tertiary dendritic branches. SAC307 is shown to be the only alloy examined having λ ₃ along the entire casting length. Despite the presence of some tertiary branches in part of the SAC207 alloy casting, for both the SAC107 and SAC207 alloys, σ _u and σ _y are shown to increase with decreasing λ ₂, while the opposite trend is exhibited by the SAC307 alloy. It seems that the well-defined array of tertiary branches in such alloy, and consequently the more complex dendritic network, allowed the strength to increase despite the associated increase in λ ₂ and the change in the morphology of the Ag₃Sn IMC from spheroidal to fibrous.     

Modeling of cutting forces in helical milling by analysis of tool contact angle and respective depths of cut

Knowledge of the behavior and magnitude of cutting forces is very important for correctly calculating cutting power and for obtaining tight tolerances and low levels of tool wear. In this way, the appropriate prediction of the force components collaborates with the correct choice of the cutting parameters and strategies. High oscillation of force values in helical milling increase the relevance of the analysis. In this context, present work describes an approach for modeling cutting forces in helical milling based on the analysis of tool contact angle and the respective depths of cut. From the model, it is possible to predict the behavior and magnitude of the force acting on the insert, which contributes to better process planning. The results indicated a good fit of the experimental values with the models, despite the observation of some errors, which occurred mainly due to the dynamics of the machine and the used approximations.     

The correlation between dendritic microstructure and mechanical properties of directionally solidified hypoeutectic Al-Ni alloys

Al-Ni hypoeutectic alloys were directionally solidified under upward transient heat flow conditions. The aim of the present study is to set up correlations between the as-cast microstructure and the resulting mechanical properties of these alloys. The dependence of primary and secondary dendrite arm spacing on the alloy solute content and on solidification thermal parameters is also analyzed. The results include transient metal/mold heat transfer coefficient, tip growth rate, cooling rate, dendrite arm spacing, ultimate tensile strength, yield tensile strength and elongation. Expressions relating dendrite spacing to solidification thermal parameters and mechanical properties to the scale of the dendritic microstructure have been determined. It was found that the ultimate tensile strength and the yield tensile strength increase with increasing alloy solute content and with decreasing primary and secondary dendrite arm spacing. In contrast, the elongation was found to be independent of both alloy composition and dendritic arrangement.     

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 (h _i), secondary dendrite arm spacing (λ₂), corrosion potential (E _Corr), corrosion rate (i _Corr), polarization resistance (R ₁), capacitances values (Z _CPE), ultimate tensile strength (UTS, σ _u ), yield strength (YS, σ _y ), and elongation. It is shown that σ _U decreases with increasing λ₂ while the CR increases with increasing λ₂, for both alloys experimentally examined. A combined plot of CR and σ _U 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.     

Evaluation of heat transfer coefficients during upward and downward transient directional solidification of Al–Si alloys

Aluminum alloys with silicon as a major alloying element consist of a class of alloys which provides the most significant part of all shaped castings manufactured. This is mainly due to the outstanding effect of silicon in the improvement of casting characteristics, combined with other physical properties such as mechanical properties and corrosion resistance. In general, an optimum range of silicon content can be assigned to casting processes. For slow cooling rate processes (sand, plaster, investment), the range is 5 to 7 wt%; for permanent molds, 7 to 9%; and for die castings, 8 to 12%. Since most casting parts are produced considering there is no dominant heat flow direction during solidification, it seems to be adequate to examine both upward and downward growth directions to better understand foundry systems. The way the heat flows across the metal/mold interface strongly affects the evaluation of solidification and plays a remarkable role in the structural integrity of castings. Gravity or pressure die casting, continuous casting, and squeeze casting are some of the processes where product quality is more directly affected by the interfacial heat transfer conditions. Once information in this area is accurate, foundrymen can effectively optimize the design of their chilling systems to produce sound castings. The present work focuses on the determination and evaluation of transient heat transfer coefficients from the experimental cooling curves during solidification of Al 5, 7, and 9 wt% Si alloys. The method used is based on comparisons between experimental data and theoretical temperature profiles furnished by a numerical solidification model, which applies finite volume techniques. In other words, the resulting data were compared with a solution for the inverse heat conduction problem. The necessary solidification thermodynamic input data were obtained by coupling the software ThermoCalc Fortran interface with the solidification model. A comparison between upward and downward transient metal/mold heat transfer coefficients is conducted.     

Influences of solute content, melt superheat and growth direction on the transient metal/mold interfacial heat transfer coefficient during solidification of Sn–Pb alloys

Several factors such as alloy composition, melt superheat, mold material, roughness of inner mold surface, mold coating layer, etc., can affect the transient metal/mold heat transfer coefficient, h_i. An accurate casting solidification model should be able to unequivocally consider these effects on h_i determination. After this previous knowledge on interfacial heat transfer, such models might be used to control the process based on thermal and operational parameters and to predict microstructure which affects casting final properties. In the present work, three different directional solidification systems were designed in such a way that thermal data could be monitored no matter what configuration was tested with respect to the gravity vector: vertical upward and downward or horizontal. Experiments were carried-out with Sn–Pb hypoeutectic alloys (5 wt.% Pb, 10 wt.% Pb, 15 wt.% Pb and 30 wt.% Pb) for investigating the influence of solute content, growth direction and melt superheat on h_i values. The experimentally obtained temperatures were used by a numerical technique in order to determine time-varying h_i values. It was found that h_i rises with decreasing lead content of the alloy, and that h_i profiles can be affected by the initial melt temperature distribution.     

Mechanical properties of Sn–Zn lead-free solder alloys based on the microstructure array

The aim of this study is to develop a comparative experimental study interrelating mechanical properties, solidification thermal parameters and microstructure characteristics of a hypoeutectic Sn–4 wt.% Zn, a hypereutectic Sn–12 wt.% Zn and a eutectic Sn–9 wt.% Zn solder alloys. A water-cooled vertical upward unidirectional solidification system was used to obtain the samples. It was found that a more homogeneous distribution of the eutectic mixture, which occurs for smaller dendritic spacings in hypoeutectic and hypereutectic alloys, increases the ultimate tensile strength. The resulting microstructure of the eutectic Sn-9 wt.% Zn alloy has induced higher mechanical strength than those of the Sn–4 wt.% Zn and Sn–12 wt.% Zn alloys. It was found that the eutectic alloy experiences a microstructural transition from globular-to-needle-like Zn-rich morphologies which depend on the solidification growth rate. It is also shown that a globular-like Zn-rich morphology provides higher ultimate tensile strength than a needle-like Zn-rich eutectic morphology.