Relationships between secondary dendrite arm spacing and mechanical properties of Zn-40Al-Cu alloys

Three ternary monotectoid-based Zn-40Al-(1, 2, 3%) Cu alloys were produced by permanent mould casting at different pouring and mould temperatures. The average cooling rate for each alloy was determined. Structure of the alloys was examined using optical and electron microscopes and their hardness, tensile strength, percentage elongation and impact energy were measured. As a result of these investigations the relationships between structure and mechanical properties of the alloys were determined.It was observed that the secondary dendrite arm spacing of the alloys decreased with increasing cooling rate and their hardness, tensile strength, percentage elongation and impact energy increased. Correlation of experimental results showed that the hardness, tensile strength, percentage elongation and impact energy of the alloys could be related to their secondary dendrite arm spacing using straight line equations.     

Berechnung der Wöhler‐Linie für Aluminium‐Gusslegierungen aus dem statischen Zugversuch und dem Dendritenarmabstand

Calculation of the S‐N curve for cast aluminium alloys based on static tensile test and dendrite arm spacing A fatigue life model based on fracture mechanics was developed in order to calculate the S‐N curve for cast aluminium alloys due to the characteristic static tensile test values (0,2 % yield strength, tensile strength, elastic modulus) and to the secondary dendrite arm spacing of the casting structure.     

Development of solidification microstructure and tensile mechanical properties of Sn-0.7Cu and Sn-0.7Cu-2.0Ag solders

Non modified and Ag-modified eutectic Sn-0.7Cu solder alloys were directionally solidified under transient heat flow conditions. The microstructure of the Sn-0.7Cu alloy has been characterized and the present experimental results include the cell/primary dendrite arm spacing (λ₁) and its correlation with: the tip cooling rate ( $\mathop T\limits^{ \bullet }$ ) during solidification, ultimate tensile strength (σ_u) and elongation to fracture (δ). Distinct morphologies of intermetallic compounds have been associated with the solidification cooling rate for both alloys examined. For the Sn-0.7Cu alloy, cellular regions were observed to occur for cooling rates lower than 0.9 K/s, being characterized by aligned eutectic colonies. On the other hand, the alloy containing 2.0 wt %Ag enabled the launch of tertiary branches within the dendritic arrangement. The comparison of results allows stating that finer solder microstructures are shown to be associated with higher ultimate tensile strengths (σ_u) for both alloys although a more complex microstructure was found for the SAC alloy. In contrast the elongation (δ) exhibited opposite tendencies. The growth of coarse Ag₃Sn fibers and platelets within interdendritic regions seems to contribute for the reduction on ductility observed for the SAC alloy.     

Effects of neodymium rich rare earth elements on microstructure and mechanical properties of as cast AZ31 magnesium alloy

Abstract

The effects of neodymium rich rare earth elements [RE(Nd)] on microstructure and mechanical properties of as cast AZ31 magnesium alloy were investigated. The microstructures of as cast AZ31–xRE(Nd) alloys display a dendrite configuration, and the secondary dendrite spacing of the α-Mg phase was decreased with the increasing Nd content. The addition of RE(Nd) resulted in the formation of Al₂Nd and Mg₁₂Nd phases. Mechanical properties were improved significantly due to grain refinement and precipitation of intermetallic phases. When the amount of RE is 1·0 wt-%,The as cast AZ31 alloy reached its maximum tensile strength of 249 MPa at room temperature, yield strength of 169 MPa and elongation of 9·0%.     

Correlation between ultimate tensile strength and solidification microstructure for the sand cast A357 aluminium alloy

Many studies have demonstrated a relationship between secondary dendrite arm spacing (SDAS) and the mechanical behaviour of cast aluminium–silicon alloys, both for tensile and fatigue strength. SDAS is related to the solidification time and can be predicted, with a good approximation, by finite-element simulation. However, other microstructural features can affect the tensile behaviour of cast aluminium alloys such as size and morphology of the eutectic Si particles, grain size, composition and morphology of the intermetallic compounds. The present investigation was aimed at finding valuable relationships between ultimate tensile strength and the previously mentioned microstructural parameters for the sand cast A357 aluminium alloy. The microstructural characterization was carried out by optical microscopy and image analysis on more than about 2500 micrographs. Starting from the microstructural parameters and taking into account the material hardness, a relationship able to predict the ultimate tensile strength of the alloy, with an error less than 5%, was found. This relationship can be used to evaluate the local values of the UTS in complex cast components knowing only the hardness and the microstructural parameters, even in positions where the extraction of tensile specimens is not possible.     

The influence of post‐heat treatments on the tensile strength and surface hardness of selectively laser‐melted AlSi10Mg

To improve the mechanical properties of cast aluminium alloys several post‐heat treatments are known. However, these treatments cannot directly be transposed to additively via selective laser melting manufactured aluminium alloys, e. g., aluminium‐silicon‐magnesium (AlSi10Mg). Therefore, this study aims to determine suitable post‐heat treatments to optimise the mechanical properties of SLM‐built AlSi10Mg specimen. The influence of various post‐heat treatment conditions on the material characteristics was examined through hardness and tensile tests. The findings indicate that the Vickers hardness and ultimate tensile strength could not be improved via secondary precipitation hardening, whereas the fracture elongation shows a value which is distinctly higher than the values of a comparable cast alloy. Solution annealing at 525 °C reduces the hardness and the ultimate tensile strength by about 40 % and increases the fracture elongation three times. A subsequent precipitation hardening allows recovery of 80 % of the as‐built hardness, and 90 % of the previous ultimate tensile strength combined with maintaining an improved fracture elongation of about 35 % compared to the respective as‐fabricated condition.     

Dendrite coarsening during solidification of hypo- and hyper-eutectic Al-Cu alloys

Dendrite coarsening during cooling at a constant rate was compared at various stages of solidification with that during isothermal holding for Al-Cu alloys of hypo- and hypereutectic compositions. For each specimen, the undercooling for the initial dendrite formation and the time elapsed after it were measured directly. The dendrite arm spacing was shown to be determined solely by the latter, and the dendrite structure was therefore coarsening-controlled from the early stage of solidification. The rate of coarsening in terms of the dendrite arm spacing during solidification at a constant cooling rate was same as that during isothermal holding in all the alloys tested. Numerical values of the fractional rate of solidification were evaluated for the hypo-eutectic compositions and the results show that the rate of dendrite coarsening does not depend on the fractional rate of solidification. Aluminium dendrites show structural coarsening with progressive solidification in the same way as during isothermal holding. CuAl₂ dendrites show curved boundaries after isothermal holding whereas those cooled at a constant rate are faceted.     

Low‐temperature annealing and graphitizing of white‐solidified low‐alloy cast irons

Hypoeutectic iron‐carbon and iron‐carbon‐silicon model alloys as well as conventional cast irons GJL‐250_mod and EN‐GJS‐600‐3 have been produced and processed by different solidification techniques, i. e. mold casting, electron beam surface remelting and melt spinning. The white‐solidified alloys exhibit different degrees of microstructural refinement indicated by a secondary dendrite arm spacing of 0.3 μm–12 μm. The effects of microstructural refinement and silicon content on the hardness as well as on coarsening of cementite and graphitizing at temperatures of 540 °C to 670 °C have been investigated. The hardness of the as‐solidified alloys increases with decreasing secondary dendrite arm spacing and increasing silicon content. High silicon content effectively retards coarsening of pearlitic cementite, and thus is beneficial to retain the hardness at small thermal load. On the downside, high degree of microstructural refinement and high silicon content promote and accelerate graphitizing at temperatures > 600 °C. The results are discussed in terms of the applicability of a recently developed two‐step surface treatment for cast irons, i. e. electron beam remelting followed by nitriding.     

Tensile behaviour in 30Ni-30Cu-40Mn-based alloys for a dental application

It was possible to examine the tensile behaviour in experimental 30Ni-30Cu-40Mn-based alloys which were modified by alloying additions of aluminium, indium and tin. Namely, the experimental alloys were developed on the basis of crack formation in a commercial nickel-based alloy and microstructural features in nickel-based alloys investigated. The addition was done by substituting only the manganese content (40 wt%) to 35 and 40 wt%. The results indicated that both changes of tensile strength and elongation were obtained with castability values above 94%. Comparison of tensile properties in experimental Ni-Cu-Mn-based alloys studied here showed that the addition of aluminium to the alloys was appropriate to obtain results similar to those for commercial alloys, indicating that the refining of dendrite arm spacing was obtained by aluminium addition.     

Microstructure and mechanical properties of directionally solidified Al–Si eutectic alloys with and without antimony

Abstract

Hardness H, interjlake spacing λ, and tensile properties are reported for Al–12·7Si and Al–12·7Si–0·2Sb (all wt-%) eutectic alloys directionally solidified at growth velocities of up to 250 μm s^?1 and under temperature gradients in the liquid of up to 12·9 K mm^?1. The hardness is related to interflake spacing by the equation H=H^o+Kλ^?0·2, where H^o is the initial hardness of the alloy. This behaviour contradicts previous results, which suggest that a Hall–Petch relationship is followed. The tensile properties are shown to follow similar behaviour, confirming that hardness shows the same dependence as proof stress on interflake spacing. However, the nature of the relationship depends on the Si morphology and caution should be exercised in using hardness or interflake spacing to indicate proof stress.

MST/1585     

Effect of microstructural variables on tensile behaviour of A356 cast aluminium alloy

Abstract

The effects of microstructural variables, including secondary dendrite arm spacing (SDAS), the size of primary α phase, the aspect ratio of eutectic Si particle and the thickness of eutectic wall structure, on tensile behaviour of A356 cast aluminium alloy, were quantitatively identified using linear regression analysis method. For systematic microstructural control of A356 specimen, directional solidification method was used with different solidification rates of 5, 25, 50 and 100 μm s^?1 respectively. The linear regression analysis suggests that each microstructural variable affects tensile strength and tensile elongation of A356 cast aluminium alloy in a similar fashion. The change in tensile behaviour with varying microstructural variables in A356 cast aluminium alloy is discussed based on fractographic and micrographic observations.     

冷却速度对Sn-Ag无铅焊料微观组织和机械性能的影响

研究了不同冷却速度下无铅焊料Sn-3．5％(质量分数)Ag合金的微观形貌(冷却速度从0．08K／s到10^4K／s)。结果表明焊料合金中二次枝晶间距随冷却速度增加而逐渐减小，且符合公式：d=atf^n，其中d为二次枝晶间距，tf是冷却时间，a和n是由材料和其成分所决定的常数，通过计算得到对于Sn-3．5％(质量分数)Ag合金其a为3．7，而n为0．43。维氏硬度测试结果表明：快速冷却条件能使焊料合金晶粒细化，其中作为强化相的金属间化合物Ag3Sn分布更加细密，从而能使整个合金机械性能得到提高。     

Complex alloy powders produced by different atomization techniques: relationship between heat flow and structure

Pre-alloyed powders of Managing 300, IN-100, and MAR-M-509 alloys made by different atomization techniques are compared with respect to chemistry, size, morphology and microstructure. Depending on the process employed and on the size and morphology of the powders produced, measured oxygen contents and secondary dendrite arm spacings vary from 40 ppm to 2000 ppm and 2 to 12μm, respectively. For the Maraging 300 alloy the relationshipd=39.8ε ^?0.30 between secondary dendrite arm spacing,d, and local average cooling rate,ε, is established. A simple heat flow analysis is presented which permits calculation of heat-transfer coefficients and solidification times of powders using this relationship. For example, a heat-transfer coefficienth=3.9 × 10^?3 cal cm^?2 sec^?1 ° C^?1 is calculated during solidification of steam atomized coarse powders of Maraging 300 alloy.     

AZ91镁合金的凝固冷却速度与二次枝晶间距的定量关系研究

通过设计制作一套可获得不同凝固冷却速度的镁合金熔炼与浇注装置,采用多通道连续温度记录仪,获得了对应不同凝固冷却速度下的AZ91镁合金试样.用定量金相分析之截线法测定了各个试样的二次枝晶间距,并对不同凝固冷却速度下的组织特征做了简要分析.用数学回归的方法得到了AZ91镁合金的凝固冷却速度与二次枝晶间距的定量关系式.     

Guidelines for establishment of correlations between mechanical properties and microstructure in Al–Si alloys

Abstract

This paper reports an analysis of the accuracy and sensitivity of ultimate tensile strength and strain to failure to changes in microstructure constituents in Al–Si alloys ranging from 7 to 18%Si. The influence of amount of constituents, namely dendrites, eutectic, silicon particles and intermetallics as well as their geometrical features, namely secondary dendrite arm spacing, silicon eutectic thickness and intermetallics thickness on both ultimate tensile strength and strain to failure is evaluated. This study provides information that will be useful in the establishment of robust correlations between mechanical properties and metallurgical features, since they are highly dependent on their sensitivity and accuracy.     

Effect of telmperature and strain rate on tensile behaviour of M250 maraging steel

Abstract

The effect of temperature and strain rate on the 0·2% yield strength, ultimate tensile strength, and percentage elongation of M250 maraging steel was investigated under uniaxial tensile conditions in the temperature range from 25 (room temperature) to 550°C and strain rate range 10^?4–10^?1 S^?l. Up to 400°C the steel shows essentially strain rate insensitive behaviour with a gradual decrease in the 0·2% yield strength and ultimate tensile strength. The elongation remains constant at all strain rates up to 300°C. Fractographic analysis indicates that the increasing strain rate induces strain constraint resulting in an increased dimple size. An elongated structure was observed at temperatures above 400°C. X-ray diffraction reveals the presence of reverted austenite in the specimens tested at 550°C.

MST/3263     

Effect of periodic melt shearing process and cooling rate on structure and hardness of Al–0.7Fe aluminum alloy

In this investigation, the effect of periodic melt shearing process and cooling rate on an Al–Fe alloy was studied. Microstructural examinations were conducted by X-ray diffraction, optical and scanning electron microscopy coupled with energy dispersive spectrometry (EDS). Experimental results suggest that shearing above melting point can refine the alloy structure and improve the morphology of intermetallic phases and; the level of refinement is increased by increase of shearing time. Also, increasing cooling rate through reduced sections, decreases grain size and secondary dendrite arm spacing (SDAS) and improves the microstructure of Al–Fe alloy. The results of hardness test showed that increasing cooling rate and time of melt shearing reduce the hardness of Al–Fe alloy.     

Effects of high magnetic fields on solidification microstructure of Al–Si alloys

The effects of high magnetic fields on the solidification microstructure of Al–Si alloys were investigated. Al–7.2 wt%Si and Al–11.8 wt%Si alloys were solidified in various high magnetic fields at different cooling rates. The secondary dendrite arm spacing (SDAS) of the primary Al dendrites and the lamellar spacing (LS) of the eutectics were measured. It was found that the application of a high magnetic field could decrease the SDAS of the primary Al dendrites in Al–7.2 wt%Si alloys and the LS of the eutectics in Al–11.8 wt%Si alloys. The effects of the high magnetic field on the SDAS decreased with increasing cooling rate. The decrease in the SDAS and LS can be attributed to the decrease of the solute diffusivity in the liquid ahead of the solid/liquid interface during the growth of the dendrite and eutectic. This decrease is caused by the high magnetic field which can damp the convection and avoid its contributions to the diffusion.     

Directional pressure quench casting of aluminium alloys

Abstract

A novel method of casting is described in which liquid metal in permeable moulds is subjected to high ambient pressure and quenching simultaneously. A small pressure quenching chamber was used and shapes of two different geometries cast into sand moulds. Two non-heat treatable alloys, LM6M and LM21M, comprised the raw material. The results show that increasing ambient pressure alone up to a value of 2 MPa (20 bar) can reduce porosity of castings, has a limited effect on tensile strength, and little effect on dendrite arm spacing. Casting under pressure with quenching increased tensile strength by up to 30% compared with conventional castings and virtually eliminated porosity. The dendrite arm spacing is also considerably reduced. Pressure quench casting is a potentially inexpensive means of improving the mechanical and microstructural properties and integrity of a wide range of aluminium alloys made by existing sand casting methods. For heat treatable alloys it offers the possibility of solution treating in the mould, thus eliminating one operation in a production process.

MST/3099     

Effects of carbon content on mechanical properties of 5%Mn steels exhibiting transformation induced plasticity

Abstract

A series of highly ductile, high strength steels exhibiting transformation induced plasticity due to retained austenite was developed by varying the carbon content in the range 0·01–0·4 wt-% in 5 wt-%Mn based steel. For up to 0·l%C the mechanical properties are insensitive to cooling rate after intercritical heating, but afurther increase in carbon content causes a large sensitivity to the cooling rate, owing to carbide precipitation occurring during slow cooling. By suppressing this carbide precipitation with water quenching after the intercritical holding, an excellent combination of tensile strength (1580 MN m^?2) and uniform elongation (21%) was attained at 0·3%C in this series.

MST/1964