Murat Tiryakioğlu

Bifilm Defects in Ni-Based Alloy Castings

The Ni-base superalloys, which are normally melted and cast in a vacuum, entrain their surface-oxide film during turbulent pouring of the melt; unfortunately at this time, this process is universally practiced for investment castings of these materials. The entrained film becomes a bifilm crack automatically, so that cast alloys have a large population of cracks that controls their failure behavior. The problems of the growth of single crystals and the welding of polycrystalline alloys are reviewed to illustrate the central role of bifilms in the cracking of turbine blades, the heat-affected zones of welds, and the reliability of properties. It has been demonstrated that improved gravity pouring systems can reduce these problems significantly, but only countergravity filling of molds is expected to result in defect-free castings. Recent cases in which turbine blades failed in service are examined, and the central role of bifilm defects in these failures is discussed.

Quality Index for Aluminum Alloy Castings

Tensile testing along with nondestructive methods has been traditionally used to assess the quality of cast Al alloys. However, clear guidelines on what remedial steps can be taken in case of low quality indicated tensile data are not available to foundry engineers. Quality indices in the literature were developed as a measure to quantify quality to assist foundry engineers in their pursuit of higher quality. A brief review of those quality indices developed previously is provided. The quality indices can be grouped into two: (1) those with no physical meaning and (2) those that measure what fraction of the expected tensile performance is achieved. The quality index developed by the authors is based simply on the ratio of measured elongation to maximum achievable elongation, portrayed as a percentage of what is achievable. This quality index is recommended over the others. In principle the approach is applicable to all alloys. Step-by-step guidelines on its use and interpretation are provided.

Statistical distributions for the size of fatigue-initiating defects in Al–7%Si–0.3%Mg alloy castings: A comparative study

Statistical distributions used in the literature for size distributions of fatigue-initiating defects are reviewed. Seven distributions from the literature are fitted to two datasets for the fatigueinitiating defects in A356 aluminum alloy castings, reported previously by Yi et al. (Mater. Sci. Eng. A, A432 (2006) 59). The results of the goodness-of-fit tests indicate that only lognormal, Gumbel and General Extreme Value distributions provide acceptable fits to both datasets. Weibull, Frechet and Generalized Pareto distributions do not provide good fits to the two datasets. Based on mathematical statistics, however, it is recommended that only Gumbel or General Extreme Value distributions be used for the size of fatigue-initiating defects. The effect of using the correct statistical distribution for the size of defects on the accuracy of the fatigue life estimates is demonstrated.

Weibull Analysis of Mechanical Data for Castings II: Weibull Mixtures and Their Interpretation

The interpretation of Weibull probability plots of mechanical testing data from castings was discussed in Part 1 (M. Tiryakioğlu, J. Campbell: Metall. Mater. Trans. A, 41 (2010) 3121-3129). In Part II, details about the mathematical models of Weibull mixtures are introduced. The links between the occurrence of Weibull mixtures and casting process parameters are discussed. Worked examples are introduced in five case studies in which six datasets from the literature were reanalyzed. Results show that tensile and fatigue life data should be interpreted differently. In tensile data, Weibull mixtures are due to two distinct defect distributions, namely “old” and “young” bifilms, which are a result of prior processing and mold filling, respectively. “Old” bifilms are the predominant defect and result in the lower distribution, whereas “young” bifilms results on the upper distribution. In fatigue life data, Weibull mixtures are due to two failure mechanisms being active: failure due to cracks initiating from surface defects and interior defects. Surface defects are predominant and interior defects lead to fatigue failure only when there are no cracks initiated by surface defects. In all cases, only the mutually exclusive Weibull mixture model was found to be applicable.

Toughness after Interrupted Quench

We discuss data from a range of heat-treatable aluminum alloys, showing both yield strength and fracture toughness vs time at temperature of interrupted quench. Drop in toughness occurs at much shorter hold time than drop in strength. Concurrently the fracture becomes more intergranular. When later the yield strength falls, fracture becomes more transgranular, and toughness may rise. We attribute this pattern to two mechanisms: 1) Early quench precipitates nucleated on grain and/or subgrain boundaries grow to size sufficient to initiate fracture under tension, long before they withdraw significant solute from subsequent age-hardening. 2) Later quench precipitates nucleated on dispersoids and/or dislocations withdraw solute relatively uniformly, reducing matrix yield strength while increasing matrix ductility. We propose that quantitative modeling of change in strength and toughness with change in quench, requires multiple C-curves for multiple types of quench precipitates, and nonlinear relation of toughness to amount of boundary quench precipitate.

The Effect of Structural Quality on Fatigue Life in 319 Aluminum Alloy Castings

Tensile and fatigue life data for 319 aluminum alloy from seventeen datasets reported in four independent studies from the literature have been reanalyzed. Analysis of fatigue life data involved mean stress correction for different R ratios used in fatigue testing, inclusion of survival (runout) data along with failure data, as well as volumetric correction for Weibull distributions for different specimen sizes used in these studies. Tensile data have been transformed into the structural quality index, QT, which is used as a measure of the structural quality of castings. A distinct relationship has been observed between the expected fatigue life and mean quality index. Moreover, fatigue strengths at 104 and 106 cycles have been found increase with quality index, providing further evidence about the relationship observed between structural quality and fatigue performance. Empirical equations between Basquin parameters and structural quality index have been developed. The use of the comprehensive methodology to estimate fatigue life is demonstrated with an example.

Evaluation of the Effects of Rotary Degassing Process Variables on the Quality of A357 Aluminum Alloy Castings

The effects of rotary degassing process variables on the melt and casting quality have been investigated using reduced pressure test results and quality index calculations from tensile data. The results showed that the effectiveness of the rotary degassing process of Al alloys is highly dependent on the combination of rotational speed and the gas flow rate, and that the wrong combination of these factors may result in no improvement or even degradation in quality of castings. For the first time, it has been found that the effectiveness of the pouring and filling system to produce high-quality castings can be characterized numerically. This new method of quantifying the casting system is introduced as a new quality improvement tool for materials and process engineers.

The effect of casting shape and size on solidification time: a new approach

The limitations of Chvorinovs Rule for predicting the solidification time of a casting are discussed and previous studies intended to modify Chvorinovs Rule in order to express the effect of the size and shape of castings on solidification time are reviewed. A new model that introduces a shape factor is presented that predicts the solidification times of castings as well as or better than Chvorinovs Rule, particularly for more complex shapes. The validity of the new model is compared with the conventional Chvorinovs Rule by a statistical analysis of twelve sets of data obtained from the literature.

Pore Formation During Solidification of Aluminum: Reconciliation of Experimental Observations, Modeling Assumptions, and Classical Nucleation Theory

An in-depth discussion of pore formation is presented in this paper by first reinterpreting in situ observations reported in the literature as well as assumptions commonly made to model pore formation in aluminum castings. The physics of pore formation is reviewed through theoretical fracture pressure calculations based on classical nucleation theory for homogeneous and heterogeneous nucleation, with and without dissolved gas, i.e., hydrogen. Based on the fracture pressure for aluminum, critical pore size and the corresponding probability of vacancies clustering to form that size have been calculated using thermodynamic data reported in the literature. Calculations show that it is impossible for a pore to nucleate either homogeneously or heterogeneously in aluminum, even with dissolved hydrogen. The formation of pores in aluminum castings can only be explained by inflation of entrained surface oxide films (bifilms) under reduced pressure and/or with dissolved gas, which involves only growth, avoiding any nucleation problem. This mechanism is consistent with the reinterpretations of in situ observations as well as the assumptions made in the literature to model pore formation.

MASS AND HEAT TRANSFER DURING FEEDING OF CASTINGS

Feeder models that incorporate mass transfer from feeder to casting are reviewed, and assumptions made during their development are discussed. A new approach that incorporates a superheat model from the literature is introduced. The new approach is based on the equality of solidification times of feeder and the feeder-casting combination. The superheat mode is used to adjust the solidification times of feeder and casting to account for heat transfer.