Effects of cellular growth on fatigue life of directionally solidified hypoeutectic Al-Fe Alloys

Effects of cellular growth on fatigue life of directionally solidified hypoeutectic Al-Fe Alloys

Pryscilla Liberato Ribeiro^I; Bismarck Luiz Silva^II; Wanderson Santana da Silva^I; José E Spinelli^II,*

^IDepartment of Materials Engineering, Federal University of Rio Grande do Norte - UFRN, CEP 59072-970, Natal, RN, Brazil

^IIDepartment of Materials Engineering, Federal University of São Carlos - UFSCar, CEP 13565-905, São Carlos, SP, Brazil

ABSTRACT

Al-Fe hypoeutectic alloys are a family of casting alloys characterized by cell growth, low cost and appreciable formability. It is well known that fatigue strength is a requirement of prime importance considering the nature of load typically observed during operations involving the risers used in oil extraction. The aim of this study is to examine the influence of cell size and its intercellular phase distribution on the fatigue life (N_f) of the directionally solidified Al-0.5, 1.0 and 1.5wt% Fe alloys. A water-cooled vertical upward unidirectional solidification system was used to provide the castings. Microscopy light and SEM microscopy were used. It was found that fatigue life decreases as cell spacing (λ _c) increases. Smaller cell spacing allows a homogeneous distribution of Al-Fe fibers to happen within the intercellular regions, which tends to improve the mentioned fatigue property. Hall-Petch type correlations [N_f= N_f0+A(λ_c^-1/2)-B(λ_c^-1); where A and B are constants] seems to be able to encompass the fatigue life variation along the Al-Fe alloys.

Keywords: solidification, Al-Fe alloy, fatigue life

1. Introduction

Al-Fe based alloys can be used in a variety of applications, e.g. packaging, architectural sheet and lithography sheet¹. Al-Fe alloys always motivate commercial interest of the aluminum manufacturing industry once iron is a dominant impurity in commercial grades at significant levels (0.2-1.0 wt %). It is also considered a major contamination concerning purer grades of aluminium alloys.

Under equilibrium solidification conditions, any iron in excess of its solid solubility limit of about 0.04 wt %Fe, forms a eutectic of Al-rich α primary phase and an intermetallic Al₃Fe. Figure 1 depicts the partial phase diagram of the Al-Fe system emphasizing both liquidus lines and eutectic plateau. However, during non-equilibrium solidification typical of direct chill (DC) castings considerably different cooling rates occur from the ingot surface and the ingot center, causing the formation of metastable Al_mFe and Al₆Fe intermetallic phases in addition to the stable Al₃Fe phase².

The as-cast structures obtained from DC casting operations may be also characterized by high levels of inverse macrosegregation, which is a consequence of an enrichment of iron content near the surface of the Al-Fe castings³. It is well known that changes in the alloy chemistry and cooling rate during solidification can affect not only the cellular/ dendritic growth but also the distribution of eutectic mixture and intermetallic particles. The effects of such modification can be especially important on mechanical strength and corrosion resistance^4-10, which means that such properties are governed by either the scale of the microstructure or by size and distribution of intermetallic particles. The dendrite or cell fineness can be even more important in the prediction of mechanical properties than grain size. Goulart et al.^9,10 performed an investigation regarding to the solidification structure development of three hypoeutectic Al-Fe alloys, which were directionally solidified under unsteady-state heat flow conditions. Considering the same cooling rate and tip growth rate, the cellular spacing (λ₁) was found to be independent on the alloy iron content. The experimental cell spacing variation with cooling rate () and tip growth rate (V_L) was characterized by λ₁=31()^-0.55 and λ₁=17(V_L)^-1.1 experimental power laws, respectively.

One of the rare studies correlating mechanical properties with microstructural features of Al-Fe alloys can be found in Mondolfo¹¹, who stated that formability of Al-Fe alloys depends mainly on size and distribution of intermetallics, since coarse Al₃Fe particles tend to crack and produce notches that reduce formability and fatigue resistance. On the other hand, finely dispersed Al₆Fe does not have the same effect. The effects of the size and distribution of the Al₆Fe fibers on fatigue properties of Al-Fe alloys deserve a deep investigation to be fully understood. In addition, Vinogrdov¹² has stated that small grain size will have a positive effect on the yield and tensile strength while generally improving fatigue strength by retarding the crack initiation. The enhanced resistance to crack initiation is considered as a result of high strength of fine grains that act as potential obstacles for dislocation movement, and thereby the formation of microcracks is inhibited. On the other hand, according to Motyka et al.¹³ increase in strength under static load is not always accompanied by improved fatigue behaviour. This was proved by the results found for the 442 aluminium alloy subjected to rapid solidification and plastic consolidations, which led to increase on the static mechanical properties but a reduction on its fatigue strength.

The influence of casting-related porosity on the fatigue life of cast aluminium components is always a concern especially regarding to design aspects on road and rail vehicle engineering. For example, if the degree of porosity is increased from 0 to 8 according to ASTM E155, fatigue strength is reduced by about 17% for the age-hardened Al7Si-0.6Mg alloy¹⁴.

Some recent studies have pointed out the effect of microstructure, particularly of the scale of the dendritic array on the resulting tensile mechanical properties^4-8. All the investigations have showed that if the dendrite arm spacing is decreased, the mechanical properties of the cast alloy may be improved. Goulart et al.¹⁵ have carried out a study on the effects of iron content and cooling rate on the cell growth of hypoeutectic Al-Fe alloys and further on the development of tensile test parameters like ultimate tensile strength (σ_u), yield tensile strength (σ_y) and elongation (δ). It was shown that the microstructural arrangement strongly affects σ_u, σ_y and δ. Hall-Petch type relationships were proposed as follows corresponding to the ultimate tensile strength: σ =57.6+113.2(λ₁=61.2+152.2(λ₁^-1/2) and σ_u=62.8+170.2(λ₁^-1/2) for Al-0.5wt%Fe, Al-1.0wt%Fe and Al-1.5wt%Fe alloys, respectively.

The goal of present study is to develop experimental expressions, which correlate the fatigue life with the cell spacing for Al-Fe hypoeutectic alloys. Further, the work deals with both the analysis of microstructure features especially on the scale of the Al₆Fe fibers within eutectic mixture and the aspects of the final fracture surfaces.

2. Experimental Procedure

A solidification system was designed in such a way that the heat is directionally extracted only through a water-cooled bottom made of low carbon steel (SAE 1020), promoting vertical upward directional solidification. More details about such casting assembly can be found in previous articles^8,9. A stainless steel split mold was used having an internal diameter of 60 mm, height 157 mm and a 5 mm wall thickness. The lateral inner mold surface was covered with a layer of insulating alumina to minimize radial heat losses. The bottom part of the mold was closed with a thin (3 mm) carbon steel sheet. The initial melt temperatures (T_p) were standardized at 10 ºC above the liquidus temperature (T_Liq).

Experiments were performed with Al-Fe hypoeutectic alloys (0.5, 1.0 and 1.5 wt pct Fe). The chemical compositions of metals that were used to prepare these alloys are presented in Table 1.

Each cylindrical ingot was subsequently sectioned (in a longitudinal way) along its vertical axis, ground and etched with an acid solution to reveal the macrostructure (Poulton's reagent: 5mL H₂O; 5mL HF-48%; 30 mL HNO₃; 60 mL HCl). Transverse specimens were cut from the castings, as indicated in Figure 2a, and machined for fatigue testing following the dimensions shown in Figure 2b.

The fatigue tests were performed with 2000-2150 rpm frequency, under room temperature at using a WPM flexural and rotation fatigue machine (UBM model, Germany). This machine permits a variation from 2000 rpm (for higher loads) to 6000 rpm (for minor loads). All experiments were performed under a constant load of 4 Kg, which corresponds to flexural stress of 27.4MPa. The fatigue tests were run according to the standard DIN50113.

As the values of ultimate (UTS) and yield (YTS) tensile strength are well known for these alloys¹⁵, a reference value concerning the flexural stress (27.4MPa) was adopted as 40% of the UTS value or 60% of the YTS obtained for the Al-0.5wt%Fe alloys, which are lower typically than the UTS and YTS values obtained for the Al-1.0 and 1.5wt%Fe alloys. The value imposed for the fatigue tests conducted in the present study is quite reasonable since the literature¹⁶ indicates that fatigue strength limit is, in practice, around 50-60% of the UTS value for a given alloy. Figure 3 shows a schematic representation of the mentioned equipment as well as a detail of the specimen assemblage on the machine axis.

After performing the fatigue tests for each alloy, undamaged specimens from the broken fatigue samples were extracted, polished and etched (a solution of 0.5%HF in water) for metallography. An image processing systems Olympus, GX51 (Olympus Co., Japan) was used to acquire the images so that cell morphology and size could be proved as stated by Goulart et al.^9,10 on previous studies. The broken samples were also used in order to examine the fracture surfaces concerning both general aspects and specific features. These surfaces were analyzed by use of a scanning electron microscopy (SEM, Philips, XL-30, Netherlands).

3. Results and Discussion

Columnar macrostructures have prevailed along the directionally solidified castings of the experimentally examined Al-Fe alloys. Figure 4 depicts the macrostructures obtained for the three examined alloys. Columnar-toequiaxed transitions can be observed for the Al-1.0 and 1.5wt% Fe alloy, which means that increase on Fe content induces equiaxed grains to be formed, blocking the columnar development. In the present study only the columnar grain zones have been examined.

Figure 5 shows several aligned microstructures obtained for each examined alloy and for distinct positions along the castings. These positions were the same adopted to perform fatigue tests. It can be clearly observed that an arrangement of cells has prevailed in all examined samples solidified under unsteady-state conditions. The non-equilibrium Al₆Fe rod-like intermetallic is usually connected to the high cooling rates range during solidification. According to Goulart et al.^9,10,15 the water-cooled solidification setup used in the present study is able to produce as-cast structures with prevalence of metastable Al₆Fe particles. Increase of about 3 times in the cell spacing value can be seen if compared the first positions with the positions closer to the top of the three castings. This behavior is translated from the variation of the solidification thermal parameters, as for example the tip cooling rate, which decreases as solidification progresses.

In the range of the hypoeutectic compositions, the solidification microstructure is composed of a cellular matrix rich in aluminum (α) surrounded by a eutectic mixture α + β, where α is rich in aluminum and β is enriched by the intermetallic Al₆Fe, with a predominance of a rod-like morphology. Goulart et al.¹⁷ have found that the experimental development of interphase spacing and its distribution range as a function of growth rate for the Al-1.5wt%Fe alloy follows the classical growth law for eutectics: λ_c=1.6(v)^-1/2, where v means eutectic growth rate. It can be inferred from this expression that if the cell spacing diminishes the interphase spacing is also decreased.

It can also be observed that increase Fe content resulted in a higher Al₆Fe fibers density, with thicker intercellular walls. This Al₆Fe seems to operate as a reinforcement of the ductile Al-rich matrix during loading.

The data acquisition system, in which temperature readings are collected at a frequency of 0.1 s considering different positions along the casting, permits accurate determination of the slope of the experimental cooling curves. The cooling rate was determined by considering the thermal data recorded immediately after the passing of the liquidus front by each thermocouple. Tip cooling rate experimental dependences on cellular spacings is shown in Figure 6, where average spacings along with the standard variation are presented. The lines represent empirical power law which fit the experimental points. It can be observed that the cell spacing is not significantly influenced by the alloy solute content, which is evidenced by single experimental laws representing cellular spacing as a function of tip cooling rate.

Figure 7 shows the experimentally determined fatigue life for the three hypoeutectic Al-Fe alloys as a function of λ _c. Hall-Petch type relationships are able to encompass the fatigue life variation along the Al-Fe alloys once high values of correlation coefficients, R², may be inferred from the regressions. A single experimental relationship is able to encompass the experimental scatters obtained for both Al-1.0 and 1.5wt%Fe alloys.

 

Smaller cell spacing allows a homogeneous distribution of Al-Fe fibers to happen within the intercellular regions, which tends to increase the fatigue life. This can be especially observed in the case of alloys with higher solute content such as 1.0 and 1.5wt%Fe (Figure 7b). In the case of the diluteAl-0.5wt%Fe alloy a less expressive variation was observed concerning fatigue life. In general, the fatigue life increases with both increasing Fe content and decreasing λ_c.

The coarse Al₆Fe fibers (associated with coarse λ _c) tend to crack reducing fatigue life. The fatigue life becomes quite constant and seems to be independent of solute content if considered λ _c values higher than 15 µm (Figure 7c).

On the general point of view the fatigue fracture surface showed a complex aspect. The fatigue fracture surfaces are available in Figure 8. Figure 8a shows typical fatigue grooves observed in the fine cells (white arrows). Further, it is possible to identify in Figure 8b the radial marks initiated at some of the fatigue nuclei (indicated by the large grey arrow) together with some signs of cast porosity (indicated by large black arrows on the right side). At last, in Figure 8c it is possible note some nucleus surrounded by an intense deformation zone (indicated by the large white arrow on the figure bottom).

The level of porosity was not controlled in the present investigation. Taylor et al.¹⁸ have stated that after 0.4wt%Fe further iron additions provoke increase on porosity level. The same was noted in the present results with higher porosity being formed for higher iron content even though porosity fraction has not been determined. It seems that even higher porosity level was not able to avoid fatigue life increase as iron content is increased. Porosity is obviously a drawback concern on engineering applications even though it may be tolerated even in safety parts¹³.

Some more details of fatigue surface are shown in Figure 9. Some signs of groove growth can be observed within the intercellular regions (indicated by small white arrows). In Figure 9b is possible to note the subsurface nucleus of fatigue process in a cell zone with some intercellular shrinkage.

 

4. Conclusions

The directional solidification experiments with the hypoeutectic Al-Fe alloys associated with the performance of systematic flexural rotational fatigue tests permit the following conclusions to be drawn:

• A complete cellular growth was characterized by optical images along the three produced Al-Fe alloys castings. The solidification microstructure is composed of a cellular matrix rich in aluminum (α) surrounded by a eutectic mixture α + β, where α is rich in aluminum and β is enriched by the intermetallic Al₆Fe, with a predominance of a rod-like morphology. In all examined alloys, the cell spacing varied roughly from 7µm to 20µm from the first positions (finer) to the top of the casting (coarser);

• Higher Al₆Fe fibers density can be observed for Al-1.0 and 1.5wt%Fe alloys, which contributed to the higher fatigue values associated with such alloys compared with the Al-0.5wt%Fe alloy. The fineness of cells can also increase the fatigue life values due to the homogeneous distribution of Al-Fe fibers within the intercellular regions.

Acknowledgements

The authors acknowledge the financial support provided by FAPESP (The Scientific Research Foundation of the State of São Paulo, Brazil); CNPq (The Brazilian Research Council; grant 481292/2012-8); ANP (Petroleum National Agency).

Received: October 31, 2013

Revised: January 13, 2014

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Publication Dates

• Publication in this collection
  28 Feb 2014
• Date of issue
  June 2014

History

• Received
  31 Oct 2013
• Accepted
  13 Jan 2014