technical review & Discussion
An Evaluation of Quality Parameters for High Pressure Diecastings R. Lumley, N. Deeva, M. Gershenzon; CSIRO Future Manufacturing Flagship, Victoria, Australia
Reviewer: As important to die casting results as gate veloc- ity, is terminal pressure and achieved final cavity pressure. These values are not given in the paper, but higher terminal pressure likely resulted from the higher gate velocity and was the reason for better performance in some experiments.
Authors: In the current results, we have found that the dif- ferences in quality due to gate velocity arise primarily be- cause of the dispersed shrinkage porosity observed on the fracture surfaces of the tensile samples. This is substantially reduced in the higher velocity material. The question then becomes “Which part of the shot pressure and velocity pro- file relates to this microstructural defect?” It is our opinion, and based on discussions with a colleague who has conduct- ed experiments examining these effects, that increasing melt velocity at the gate primarily influences shear of the metal at the point of restriction. From his experience and examina- tion of shot traces taken under differing conditions, the melt velocity only influences the impact (terminal) pressure (i.e. the instant at which the cavity is full), and not the maximum in-cavity pressure during intensification, which occurs after the cavity is full.
Reviewer: The results regarding hydrogen (degassed vs. not degassed) should not be a surprise—at HPDC pres- sures, hydrogen solubility in the liquid would be higher in the solidifying metal than during gravity casting, and any hydrogen pores that might have formed would, according to the gas laws, have been compressed to extremely small and insignificant size. While it can be argued that degassing also removed oxides, only removal of large films or dross would be significant because the extreme turbulence encountered as melt is sprayed at high velocity into the die cavity would rapidly react with any oxygen-bearing atmosphere or lubri- cants to create new finely divided oxides.
Authors: We essentially agree with the reviewers comment. The solubility of hydrogen in aluminium under pressure might reasonably be assumed to increase under pressure. Certainly, it has been proven that this occurs with Si, for example. However we have not currently seen documented proof of this effect for hydrogen, and the effect may not be significant. Irrespective of the solubility during solidifica- tion, even during cooling to room temperature, it follows that any hydrogen residual in the solid solution is non-equi- librium and likely to be forced out of solution, in the same way that Cu can precipitate onto grain boundaries as equi- librium Al2
Cu. That is, the hydrogen must precipitate out
as porosity if it was actually present previously in the solid solution. The only way in which this hydrogen would be re-
International Journal of Metalcasting/Summer 2011
tained would be to have much higher cooling rates than are achieved in diecasting, and even then, it might be expected that the hydrogen will still precipitate out of solid solution as porosity. Any hydrogen porosity present and oxide films must contribute to the equivalent defect fraction appear- ing on the fracture surface, but it is the largest oxides that are most visible and appear in conjunction with the lowest values of tensile properties (Figure 16). For the readers in- terest, an excellent representation of how rotary degassing can influence inclusion size and content is provided in ref 1, page 150. We believe that degassing mainly eliminates a proportion of the oxides that contribute to the equivalent de- fect fraction present on the fracture surface. In this regard, the Weibull modulus does show a significant change / reduc- tion in the flaw size distribution, since the value of modulus is substantially increased (from a value of 25 to a value of 44.7 for tensile strength).
Reviewer: 3. The comment that Cu (alloy 2) might cre- ate hard intermetallic domains does not seem reason- able when compared to the Si and beta type crystals al- ways present in 380 alloy HPDC components, but Cu does reduce ductility simply by increasing strength.
clearly of primary importance in affecting ductility in the cast material, the solidified Cu bearing phases are also extensively distributed within the solidified eutectic in the as-cast condition. As may be appreciated, there may be 1% Fe by weight in the casting, but 3-4% by weight of Cu. Of this, some is retained in solution in the aluminium grains during cooling, but much of it actually is observed to solidify in the eutectic, or subsequently precipitates out of the solid solution as coarse equilibrium Al2
Authors: Whereas the Si and β- Al5 Cu par- hard intermetallic phase.
ticles, forming on grain boundaries and other high free energy sites. This intermetallic phase also adversely in- fluences ductility. Increasing the content of any brittle particle which resides in the fracture path will result in lower ductility, whether it is β- Al5
FeSi, Si, or any other
Reviewer: Zn (alloy 3) in 380 at normal and increased levels does little more then increase the density of the al- loy (greater weight of metal in each casting). This was shown to be significant in past studies when lower cost 380 having 3% Zn proved to actually be more expen- sive because of the greater weight that was shipped, and the lower cost 380 did not offset that additional cost.
Authors: Zn is an interesting element in diecastings and we believe its role requires further investigation; it seems to impart both advantages and disadvantages. Many diecast- ers believe that at least 0.5% is required (this amount may always have been there in the alloy purchased), and higher Zn alloy (e.g. 2%) is generally cheaper, as mentioned. We have observed here that increased Zn content was appar- ently beneficial to castability, but only at the higher melt
55 FeSi crystals are
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