A New Tensile Test for Aluminum Alloys in the Mushy State: Experimental Method and Numerical Modeling

A fairly simple experimental setup has been designed for testing the resistance of the mushy zone of alloys during solidification under tensile conditions. It has been used to study the effect of coalescence among the solid grains at a late stage of solidification. The experimental approach involves both tensile-strength measurements and scanning electron microscope (SEM) observations of fracture surfaces. Complementary information can be obtained by numerical modeling of this solidification process. The latter takes into account heat flow in the sample, rheology of the mushy alloy, liquid feeding, and porosity formation. All of the available information indicates that the transition from a granular mushy alloy to a coalesced solid-skeleton behavior starts for a solid fraction of approximately 92pc

Two-Phase Modeling of Hot Tearing in Aluminum Alloys: Applications of a Semicoupled Method

Hot tearing formation in both a classical tensile test and during direct chill (DC) casting of aluminum alloys has been modeled using a semicoupled, two-phase approach. Following a thermal calculation, the deformation of the mushy solid is computed using a compressive rheological model that neglects the pressure of the intergranular liquid. The nonzero expansion/compression of the solid and the solidification shrinkage are then introduced as source terms for the calculation of the pressure drop and pore formation in the liquid phase. A comparison between the simulation results and experimental data permits a detailed understanding of the specific conditions under which hot tears form under given conditions. It is shown that the failure modes can be quite different for these two experiments and that, as a consequence, the appropriate hot tearing criterion may differ. It is foreseen that a fully predictive theoretical tool could be obtained by coupling such a model with a granular approach. These two techniques do, indeed, permit coverage of the range of the length scales and the physical phenomena involved in hot tearin

A new tensile test for aluminum alloys in the mushy state: Experimental method and numerical modeling

A fairly simple experimental setup has been designed for testing the resistance of the mushy zone of alloys during solidification under tensile conditions. It has been used to study the effect of coalescence among the solid grains at a late stage of solidification. The experimental approach involves both tensile-strength measurements and scanning electron microscope (SEM) observations of fracture surfaces. Complementary information can be obtained by numerical modeling of this solidification process. The latter takes into account heat flow in the sample, rheology of the mushy alloy, liquid feeding, and porosity formation. All of the available information indicates that the transition from a granular mushy alloy to a coalesced solid skeleton behavior starts for a solid fraction of approximately 92 pct

Two-Phase Modeling of Hot Tearing in Aluminum Alloys: Applications of a Semicoupled Method

Hot tearing formation in both a classical tensile test and during direct chill (DC) casting of aluminum alloys has been modeled using a semicoupled, two-phase approach. Following a thermal calculation, the deformation of the mushy solid is computed using a compressive rheological model that neglects the pressure of the intergranular liquid. The nonzero expansion/compression of the solid and the solidification shrinkage are then introduced as source terms for the calculation of the pressure drop and pore formation in the liquid phase. A comparison between the simulation results and experimental data permits a detailed understanding of the specific conditions under which hot tears form under given conditions. It is shown that the failure modes can be quite different for these two experiments and that, as a consequence, the appropriate hot tearing criterion may differ. It is foreseen that a fully predictive theoretical tool could be obtained by coupling such a model with a granular approach. These two techniques do, indeed, permit coverage of the range of the length scales and the physical phenomena involved in hot tearing

HOW DOES COALESCENCE OF DENDRITE ARMS OR GRAINS INFLUENCE HOT TEARING ?

Hot tearing, a severe defect occurring during solidification, is the conjunction of tensile stresses which are transmitted to the mushy zone by the coherent solid underneath and of an insufficient liquid feeding to compensate for the volumetric change. In most recent hot tearing criteria, one of the critical issues is the definition of a coherency point which, in low-concentration alloys, corresponds to the bridging or coalescence of the primary phase. A coalescence model has been developed recently using the concept of the disruptive pressure in thin liquid films.[1] It has been shown that large-misorientation grain boundaries, which are characterized by an interfacial energy, γgb, larger than twice the solid-liquid interfacial energy, γsl, solidify at an undercooling ΔTb = (γgb - 2γsl)/(Δsfδ), where Δsf is the entropy of fusion and δ the thickness of the diffuse interface. When γgb < 2γsl (e.g., low-angle grain boundaries), dendrite arms coalesce as soon as they impinge on each other. Using such concepts and a back-diffusion model, the percolation of equiaxed, randomly oriented grains has been studied in 2D : it is shown that the grain structure gradually evolves from isolated grains separated by a continuous interdendritic liquid film, to a fully coherent solid with a few remaining wet boundaries. The implication of such findings for the hot cracking tendency of aluminum alloys are discussed

Effects of iron-rich intermetallics and grain structure on semisolid tensile properties of Al-Cu 206 cast alloys near solidus temperature

The effects of iron-rich intermetallics and grain size on the semisolid tensile properties of Al-Cu 206 cast alloys near the solidus were evaluated in relation to the mush microstructure. Analyses of the stress–displacement curves showed that the damage expanded faster in the mush structure dominated by plate-like β-Fe compared to the mush structure dominated by Chinese script-like α-Fe. While there was no evidence of void formation on the β-Fe intermetallics, they blocked the interdendritic liquid channels and thus hindered liquid flow and feeding during semisolid deformation. In contrast, the interdendritic liquid flows more freely within the mush structure containing α-Fe. The tensile properties of the alloy containing α-Fe are generally higher than those containing β-Fe over the crucial liquid fraction range of ~0.6 to 2.8 pct, indicating that the latter alloy may be more susceptible to stress-related casting defects such as hot tearing. A comparison of the semisolid tensile properties of the alloy containing α-Fe with different grain sizes showed that the maximum stress and elongation of the alloy with finer grains were moderately higher for the liquid fractions of ~2.2 to 3.6 pct. The application of semisolid tensile properties for the evaluation of the hot tearing susceptibility of experimental alloys is discussed

Combination therapy with oral treprostinil for pulmonary arterial hypertension. A double-blind placebo-controlled clinical trial

Rationale: Oral treprostinil improves exercise capacity in patients with pulmonary arterial hypertension (PAH), but the effect on clinical outcomes was unknown. Objectives: To evaluate the effect of oral treprostinil compared with placebo on time to first adjudicated clinical worsening event in participants with PAH who recently began approved oral monotherapy. Methods: In this event-driven, double-blind study, we randomly allocated 690 participants (1:1 ratio) with PAH to receive placebo or oral treprostinil extended-release tablets three times daily. Eligible participants were using approved oral monotherapy for over 30 days before randomization and had a 6-minute-walk distance 150 m or greater. The primary endpoint was the time to first adjudicated clinical worsening event: death; hospitalization due to worsening PAH; initiation of inhaled or parenteral prostacyclin therapy; disease progression; or unsatisfactory long-term clinical response. Measurements and Main Results: Clinical worsening occurred in 26% of the oral treprostinil group compared with 36% of placebo participants (hazard ratio, 0.74; 95% confidence interval, 0.56–0.97; P = 0.028). Key measures of disease status, including functional class, Borg dyspnea score, and N-terminal pro–brain natriuretic peptide, all favored oral treprostinil treatment at Week 24 and beyond. A noninvasive risk stratification analysis demonstrated that oral treprostinil–assigned participants had a substantially higher mortality risk at baseline but achieved a lower risk profile from Study Weeks 12–60. The most common adverse events in the oral treprostinil group were headache, diarrhea, flushing, nausea, and vomiting. Conclusions: In participants with PAH, addition of oral treprostinil to approved oral monotherapy reduced the risk of clinical worsening. Clinical trial registered with www.clinicaltrials.gov (NCT01560624)