As-cast Dendritic Microstructure Research Articles

Effect of Widmanstätten η phase on tensile shear strength of resistance spot welding joints of A286 superalloy

This work aims to study the effect of Widmanstätten η phase on tensile shear strength (TSS) of resistance spot welding joints (RSW) of A286 superalloy subjected to post-weld high temperature aging treatment. The tensile shear test specimens were welded in the solution treated condition, and then subjected to six different post-weld aging treatments (at an aging temperature of 840 °C for six different aging times). The as-cast dendritic microstructure of the weld nugget and the austenite equiaxed-grain microstructure of the base metal have a different response to the post-weld high temperature aging treatment. Growth of Widmanstätten η phase in the weld nugget is faster than in the base metal. While in the base metal the Widmanstätten η phase precipitates into the grain, in the weld nugget precipitates at the interdendritic region because of the segregation of Ti towards the last-to-solidify interdendritic regions (studied by performing SEM/EDX analysis). Although the hardness increases with the presence of Widmanstätten η phase, the increase in hardness does not necessarily imply an increase in the experimental TSS.

Experimental and Numerical Studies on Recrystallization Behavior of Single-Crystal Ni-Base Superalloy.

The recrystallization (RX) behavior of superalloy during standard solution heat treatment (SSHT) varies significantly with deformation temperature. Single-crystal (SX) samples of Ni-base superalloy were compressed to 5% plastic deformation at room temperature (RT) and 980 °C, and the deformed samples were then subjected to SSHT process which consists of 1290 °C/1 h, 1300 °C/2 h, and 1315 °C/4 h, air cooling. RT-deformed samples showed almost no RX grains until the annealing temperature was elevated to 1315 °C, while 980 °C-deformed samples showed a large number of RX grains in the initial stage of SSHT. It is inferred that the strengthening effect of γ’ phases and the stacking faults in them increase the driving force of RX for 980 °C-deformed samples. The RX grains nucleate and grow in dendritic arms preferentially when the microstructural inhomogeneity is not completely eliminated by SSHT. A model coupling crystal plasticity finite element method (CPFEM) and cellular automaton (CA) method was proposed to simulate the RX evolution during SSHT. One ({111} <110>) and three ({111} <110>, {100} <110>, {111} <112>) slip modes were assumed to be activated at RT and 980 °C in CPFEM calculations, respectively. The simulation takes the inhomogeneous as-cast dendritic microstructure into consideration. The simulated RX morphology and density conform well to experimental results.

Friction stir modification of GTA 7075-T6 Al alloy weld joints: EBSD study and microstructural evolutions

As known, mechanical properties of gas tungsten arc welded 7075 Al alloys are not desirable and some techniques should be utilized in order to refine the microstructure and hence to improve the mechanical properties of weld joints. In this research work, the microstructure of gas tungsten arc welded 7075 Al alloy was modified by friction stir processing. Evaluation of the tensile strength of the welded joints showed that the tensile strength of the welded joint (228MPa) increases up to 320MPa after friction stir processing. In addition, electron backscattered diffractometry (EBSD) was used in order to study the microstructure and grain boundary character evolutions during arc welding and friction stir processing. It was revealed that as-cast dendritic microstructure of gas tungsten arc welded joint completely disappears during friction stir processing and very fine equiaxed grains are formed in welded joints. Analysis of EBSD data showed that friction stir processing of gas tungsten arc welded joints leads to increase of specific boundaries from 0.7% up to 7.8%. In addition, fraction of high angle boundaries increases after friction stir processing which is resulted from dynamic recrystallization occurring during friction stir processing.

Influence of heat treatment on the microstructure and wear behavior of end-chill cast Zn–27Al alloys with different copper content

The aim of this paper was to study the effect of heat treatment on the microstructure and wear behavior of Zn–27Al alloys with different copper content. In order to study the relationship between microstructure features and wear behavior, the alloys prepared by an end-chill cast apparatus and then heat treated. Heat treatment procedure involved solutionizing at temperature of 350 °C for 72 h followed by cooling within the furnace to room temperature. Microstructural characteristics of as-cast and heat-treated alloys at different distances from the chill were investigated by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction. Wear tests were performed using a pin-on-disk test machine. To determine the wear mechanisms, the worn surfaces of the samples were also examined by SEM and EDS. Results showed that heat treatment led to the complete dissolution of as-cast dendritic microstructure and formation of a fine lamellar structure with well-distributed microconstituents. Moreover, addition of copper up to 1 wt% had no significant change in the microstructure, while addition of 2 and 4 wt% copper resulted in formation of e (CuZn4) particle in the interdendritic regions. The influence of copper content on the wear behavior of the alloys was explained in terms of microstructural characteristics. Delamination was proposed as the dominant wear mechanism.

Role of as-cast dendritic microstructure in recrystallization of a Ni-based single crystal superalloy

The role of as-cast dendritic microstructure in the recrystallization of a nickel-based SX superalloy DD6 was investigated. SX alloys were deformed at 980 °C using a Gleeble 1500D and subsequently annealed at lower temperatures under an inert argon atmosphere. Recrystallization initially nucleates in the dendritic arms and rapidly grows. In the interdendritic regions, nucleation was observed in the form of a large amount of annealing twinning. Recrystallization can occur in eutectics before it is completed in the interdendritic regions. Fine grains with plenty of twinning grains remained in this region. It was difficult for grain coarsening to occur as a result of the abundant twinning. Finally, the mechanism of the nucleation and growth of the annealing twinning was discussed.

Rheological and Microstructural Study of A356 Alloy Solidified under Magnetic Stirring

Results on the rheological behavior of an A356 alloy compressed between parallel plates in the semisolid state are presented in this work. The tests were performed on cylindrical specimens machined from billets cast while being subjected to a rotating permanent magnetic field. These samples did not exhibit the conventional microstructure of dendrites, and were tested with different solid fractions. The tests were performed either at a constant load or at a constant displacement rate. A power law equation was derived to describe the rheological behavior of the material when tested at constant load, however, it was not possible to establish a mathematical relationship to describe the flow on the semisolid material when the tests were made at constant displacement rate. It can be concluded from these results that the rheological behavior of the alloy is affected by the morphology of the phases present in the microstructure, in such a way that a globular microstructure, with a shape factor near the unit, behaves as a fluid while deformed in the semisolid state, a behavior that is not witnessed in an as-cast dendritic microstructure.

Mechanical Properties of Equal-Channel Angular Extrusion-Processed Al-20Zn Alloy

The mechanical properties mainly tensile properties, impact toughness and high-cycle fatigue properties, of two-phase Al-20Zn alloy subjected to severe plastic deformation (SPD) via equal-channel angular extrusion (ECAE) using route A up to 2 passes were studied. The ECAE almost completely eliminated as-cast dendritic microstructure including casting defects such as micro porosities. A refined microstructure consisting of elongated micro constituents, α and α+η eutectic phases, formed after ECAE via route A. As a result of this microstructural change, mechanical properties mainly the impact toughness and fatigue performance of the as-cast Al-20Zn alloy increased significantly through the ECAE. The rates of increase in fatigue endurance limit are approximately 74 % after one pass and 89 % after two passes while the increase in impact toughness is 122 %. Also the yield and tensile strengths of the alloy increase with ECAE. However, no considerable change occurred in hardness and percentage elongation of the alloy. It was also observed that the ECAE changed the nature of the fatigue fracture characteristics of the as-cast Al-20Zn alloy.

Mechanical Properties of Equal-Channel Angular Extrusion-Processed Al-20Zn Alloy

The mechanical properties mainly tensile properties, impact toughness and high-cycle fatigue properties, of two-phase Al-20Zn alloy subjected to severe plastic deformation (SPD) via equal-channel angular extrusion (ECAE) using route A up to 2 passes were studied. The ECAE almost completely eliminated as-cast dendritic microstructure including casting defects such as micro porosities. A refined microstructure consisting of elongated micro constituents, α and α+η eutectic phases, formed after ECAE via route A. As a result of this microstructural change, mechanical properties mainly the impact toughness and fatigue performance of the as-cast Al-20Zn alloy increased significantly through the ECAE. The rates of increase in fatigue endurance limit are approximately 74 % after one pass and 89 % after two passes while the increase in impact toughness is 122 %. Also the yield and tensile strengths of the alloy increase with ECAE. However, no considerable change occurred in hardness and percentage elongation of the alloy. It was also observed that the ECAE changed the nature of the fatigue fracture characteristics of the as-cast Al-20Zn alloy.

Flow response of a severe plastically deformed two-phase zinc–aluminum alloy

A two-phase zinc–aluminum alloy (Zn–8 wt.% Al) has been subjected to severe plastic deformation via equal channel angular extrusion (ECAE). The alloy was successfully extruded at homologous temperatures around 0.5 T m through various strain paths and magnitudes. Multi-pass ECAE processing following different routes led to the elimination of the as-cast dendritic microstructure and formed a structure of elongated, ribbon-shaped phases. Monotonic tensile tests were conducted at room temperature along the longitudinal axis of the ECAE samples in addition to the directions parallel and perpendicular to the long axis of the elongated hard eutectoid phase particles in order to reveal the effect of microstuctural morphology on the anisotropic flow response. The flow strength levels increased significantly after the first ECAE pass, and then saturated at a slightly higher value after the subsequent passes in route B C. An average increase of about 50% in ultimate tensile strength and about 100 times increase in elongation to failure were achieved after eight ECAE passes following route B C, as compared to the as-cast values. Despite the relative chemical homogenization between the hard and soft phases, the size and distribution of the hard phase in the matrix are found to be the dominant factor controlling the flow response of the present two-phase zinc–aluminum alloy after ECAE. The hard phase size, morphology, and distribution were also found to control the anisotropy in the flow strength and elongation to failure of the ECAE processed samples. Notable flow softening with increasing number of ECAE passes, a general observation for the ECAE processed Zn–Al alloys with Al content more than 12%, was lacking in the present alloy which was attributed to the hardening effect of the fine eutectoid particles in the eutectic matrix overcoming the softening effect of deformation-induced chemical homogenization.

Microstructural Evolution and Mechanical Response of Equal-Channel Angular Extrusion-Processed Al-40Zn-2Cu Alloy

Microstructural evolution, tensile properties, and impact toughness of an aluminum-zinc-copper (Al-40Zn-2Cu) alloy subjected to repetitive equal-channel angular extrusion (ECAE) up to four passes following either route A or route BC were investigated. The experimental results reveal that the ECAE eliminated as-cast dendritic microstructure along with casting defects such as microporosities almost completely. The ECAE-processed samples consisted of mostly elongated microconstituents via route A and equiaxed microconstituents via route BC. The high stresses imposed in ECAE lead to the fragmentation of the copper-rich θ phase into smaller particles with significant fragmentation occurring in the first pass and additional breaking in the subsequent passes in both routes. The ECAE processing simultaneously increased both the strength and ductility of the alloy as compared to the as-cast state, regardless of the processing route and number of passes. The deformation behavior of as-cast Al-40Zn-2Cu alloy has changed from brittle to ductile mode after ECAE due to the microstructural refinement, deformation-induced homogenization, and reduction of porosities. The limited impact toughness of as-cast alloy was significantly improved by multipass ECAE, especially in route A.

Microstructural evolution and mechanical properties of Al–40 wt.%Zn alloy processed by equal-channel angular extrusion

The Al-based Al–40 wt.%Zn alloy was subjected to multi-pass equal-channel angular extrusion (ECAE) via route-A and route-B C. Before and after ECAE processing, microstructural evolution, the tensile properties, impact toughness and fracture behavior of the alloy were investigated. ECAE processing caused to elimination of the as-cast dendritic microstructure and formed a structure consisting of elongated, ribbon shaped α-phase via route-A and mostly equiaxed α-phase via route-B C. ECAE processing also caused plastic instability as necking at early onset of deformation. As a result of more effective mechanical mixing via route-B C, softening and necking occurred more apparently. The tensile and yield strength of the alloy increased just after first pass and then slightly decreased with increasing number of passes. On the other hand, its elongation to failure and impact toughness increased with increasing number of passes in both routes. The increase obtained via route-A is more pronounced in both properties. Fracture behavior of the as-cast alloy changed from brittle to ductile mode after multi-pass ECAE. Elimination of dendritic as-cast structure with reduction of porosities and deformation-induced homogenization by the effect of ECAE processing increased the ductility and impact toughness of the alloy and caused formation of a fracture surface consisting of micro-voids and dimples which indicates of ductile fracture. Attained experimental results indicate that multi-pass ECAE processing is very effective in improving the tensile elongation and impact toughness of binary Al–40 wt.%Zn alloy.

Design of mechanical properties of a Zn27Al alloy based on microstructure dendritic array spacing

The present work focuses on the influence of the as-cast dendritic microstructure of a ZA27 alloy (Zn–27 wt%Al) on tensile mechanical properties. A low carbon steel chill was used in a unidirectional solidification experimental set-up in order to permit a wide range of dendritic spacings to be obtained along the casting. Experimental results include transient metal/mold heat transfer coefficients ( h i ), tip growth rate ( V L), secondary dendrite arm spacing ( λ 2), ultimate tensile strength ( σ U) and yield strength ( σ y) as a function of solidification conditions imposed by the metal/mold system. Experimental laws relating σ U and σ y with secondary dendrite arm spacing are proposed. It was found that both tensile properties increase with decreasing λ 2. A predictive theoretical dendritic growth model has been compared with the present experimental observations. Expressions correlating tensile properties, dendritic spacing and solidification thermal variables have been established. Such expressions permit the control of as-cast microstructures by manipulating casting variables, such as the cooling rate and the tip growth rate and can be used as an alternative way to design mechanical properties.

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.

Improvements in the metallography of as-cast AZ91 alloy

Evaluation of the grain structure and grain size of cast magnesium–aluminum alloys is usually performed after grain-boundary-enhancing treatments which involve tedious heat treatment at high temperature. In this work, a rapid and simple etching method is described for metallographic preparation of AZ91 alloy, revealing grains without altering the as-cast dendritic microstructure and the presence of secondary phases. The etching reagent, composed of acetic acid, alcohol and water, has not been reported previously. After etching and drying, a crackled film containing fine stripes is found at the surface of the substrate. The optical contrast between grains under polarized light is attributed to form birefringence induced by the stripes, preferentially aligned according to the orientation of underlying grains. The composition of the crackled film was evaluated and its characteristics are presented. It was found that a minimum concentration of aluminum is necessary in the magnesium-rich matrix for the formation of the crackled film. Likewise, beyond a maximum concentration of aluminum, the surface film does not crack.

Fatigue studies of high-palladium dental casting alloys: Part I. Fatigue limits and fracture characteristics.

The fatigue limits and fracture characteristics for a Pd-Cu-Ga alloy and a Pd-Ga alloy were studied. The alloys were cast into tensile test bars with gauge diameter of 3 mm and gauge length of 15 mm, and the surfaces of the castings were neither air-abraded nor polished after removal from the investment. Specimens were prepared from all-new metal (not previously melted), a combination of 50% new metal and 50% old metal (previously melted one time) and 100% old metal. The cast bars were subjected to heat treatment simulating the complete firing cycles for dental porcelain, and fatigued in air at room temperature under uniaxial tension-compression stress at 10 Hz and a ratio of tensile stress amplitude to compressive stress amplitude (R-ratio) of -1. The alloy microstructures and fracture surfaces were examined with a scanning electron microscope (SEM). Results showed that the fatigue limits at 2 x 10(6)cycles of the Pd-Cu-Ga and Pd-Ga alloys were approximately 0.20 and 0.15 of their 0.1% yield strength (YS) in tension, respectively. The fatigue resistance for specimens from both alloys containing 50% old metal and 50% new metal was comparable to that of specimens containing all-new metal, although this decreased dramatically for Pd-Cu-Ga alloy specimens containing all-old metal. The fatigue resistance of the Pd-Cu-Ga alloy subjected to heat treatment simulating the porcelain firing cycles was not adversely affected by remnants of the original as-cast dendritic microstructure that remained in the relatively large test specimens. A longer heat treatment than recommended by the manufacturer for the porcelain firing cycles is needed to completely eliminate the as-cast dendritic structure in these specimens. The Pd-Cu-Ga alloy exhibited superior fatigue resistance to the Pd-Ga alloy, which has an equiaxed-grain microstructure and lower yield strength.