Today, tool life in high pressure die casting (HPDC) is of growing interest. A common agreement is that die life is primarily decided by the thermal load and temperature gradients in the die materials. Conformal cooling with the growth of additive manufacturing has raised interest as a means of extending die life. In the current paper, conformal cooling channels’ performance and effect on the thermal cycle in high-pressure die casting and rheocasting are investigated for conventional HPDC and semisolid processing. It was found that conformal cooling aids die temperature reduction, and the use of die spray may be reduced and support the die-life extension. For the die filling, the increased temperature was possibly counterproductive. Instead, it was found that the main focus for conformal cooling should be focused to manage temperature around the in-let bushing and possibly the runner system. Due to the possible higher inlet pressures for semisolid casting, particular benefits could be seen.

Semisolid casting can provide excellent castings, but the nature of the pore-forming mechanisms has not been properly clarified. In the current communication, it was suggested that hydrogen precipitated during slurry making might have a decisive role in the formation of both gas and shrinkage porosity. Intensive stirring at the end of the slurry making process may act as a degassing step. Without the intense shearing, structures of primary slurry particles form around the hydrogen pores, strongly affecting pore formation and feeding during the intensification stage.

Metal casting is an industrially important manufacturing process offering a superior combination of design flexibility, productivity and cost-effectiveness, but has limitations due to filling related defects. Several semisolid casting processes are available capable of casting at a range of solid fractions to overcome this. The current communication aims to review the filling front behaviour and give a new perspective to the gate design in semisolid processing compared to conventional high-pressure die-casting. It is shown that solid fraction and gate widths are critical to avoid instability and spraying.

The effects of the stirring phase during slurry formation on the defect’s content and the influence on the high-temperature fatigue behaviour of a rheocast AlSi7Mg alloy were investigated. Single or double-stir step was used during RheoMetal processing. Metallographic and image analysis techniques were performed to quantitatively examine the defect’s changes occurring with different stirring methods. Samples were cross-sectioned along the gauge length, and the pores content was measured. Uniaxial fatigue tests were performed at high temperature (150 °C) with the staircase method, applying a stress ratio of R = −1. Fracture surfaces were examined using a scanning electron microscope to determine the fatigue crack initiation sites, type and size of the defects. Shrinkage porosity has more probability of acting as a crack initiator than gas porosity or oxides. The double-stir step effectively reduces the size of pores compared to the conventional technique; it reduces porosity to less than half if compared to the single-stir process. Shrinkage pores distribution show a higher population of smaller pores for the double-stir process. According to the Dixon-Mood formula, the mean fatigue strengths are comparable. The capability of the double-stir to reduce porosity is evident, even having no significant effect on the high-temperature fatigue life.

Semi-solid metal (SSM) casting has been widely used in automotive industries to reduce the weight. In RheoMetal^TM process which is one of the variations of SSM, the slurry fabrication can be finished within 30 second and can the slurry making process can be integrated into a high pressure die casting (HPDC) route without significant adjustments, making the process a promising alternative for industrial application. However, the application of SSM is still limited due to the semi-solid deformation-induced casting defects, such as macrosegregation and large pores. Due to the short stirring duration, the inefficient stirring is the leading cause of defects formation. Another critical issue in the RheoMetal^TM process is the oxidation during the stirring process, which results in the increase of oxides in the castings, reducing the mechanical properties.

This study aims to investigate the ductility and the fatigue performance of SSM castings. The main focuses were on the role of the stirring and oxides. The quenched slurry was analyzed to evaluate the effect of the stirring on the particle distribution in the slurry, and its effect on the formation of pores and segregation was discussed. To investigate the oxidation during the slurry making process, two alloys with different Mg content were cast. Scanning electron microscopy (JEOL JSM-7001F SEM) equipped with Energy-dispersive X-ray spectroscopy (EDS) was used to identify the oxides on the fracture surface. Tensile test and fatigue test accompanied with direct current potential drop (DCPD) were performed to investigate the ductility and fatigue performance, respectively.

The results suggest that the intensive stirring can avoid the formation of the large clusters, making the particle distribution homogeneous in the slurry. The Mg contents determined the types of the oxides formed in the slurry making process. For alloys with sufficient Mg, the oxides would be MgAl₂O₄, which exist as small films with numerous cracks, while a large oxides film will be formed in the case of low Mg content.

The combined influence of porosity and oxides was concluded. In the 42000 alloy, because of the low Mg content, the ductility was dominated by the large oxide films. In contrast, in the Magsimal 59 alloy, the presence of small oxides (less than 0.2 mm in majority) leads to the influence of oxides on the elongation negligible. However, a good correlation was obtained between the largest pores and ductility.

The fatigue test shows that the surface liquid segregation (SLS) determined the fatigue strength under cyclic bend loading, due to its higher hardness. The effect of the inner pores on the fatigue performance was negligible, as the maximum stress was applied on the surface.

Aluminum castings have been widely used in the automotive industry to reduce the vehicle's weight. However, the existence of casting defects significantly limits its application. The most common and detrimental defects in aluminum castings are porosity and oxides. The formation of the pores comes from the solute hydrogen and volumetric reduction during the solidification process, resulting in the gas and shrinkage pores, respectively. The oxides can be introduced by either the operation during the process or the originally existing oxides in the melt. To reduce these defects, optimizing the casting process and controlling the melt quality is essential.

In this work, the Semisolid Metal (SSM) process was used as it can significantly reduce the formation of shrinkage pores. The main focuses were on the influence of stirring intensity on the formation of casting defects and, thus, the resultant mechanical properties. In addition, to control the original melt quality, particularly the amount of oxides, efforts were made to develop proper methods for the melt quality assessment.

The results show that the slurry-making process, mainly through stirring intensity, can affect the casting defects significantly. On the one hand, the increasing stirring intensity can distribute the primary Al particles more homogeneously, reducing the pores in terms of size and number by increasing the permeability during the solidification process. On the other hand, the increasing stirring intensity can affect the size of oxides differently, depending on the composition, for instance, the Mg content.

For the alloys with sufficient Mg, the oxides would be MgAl2O4, which are small films with numerous cracks. Under intensive stirring, the oxides can break down into small oxide particles. As a result, intensive stirring can improve ductility by reducing the formation of big pores. However, the oxides would mainly be Al2O3 films for alloys with low Mg content. In this case, the current stirring intensity is insufficient to break the oxide films. Instead, the increased stirring has introduced more oxide films into the melt. Consequently, in the casting with intensive stirring, the increasing oxide films dominated the ductility rather than the reduced porosity.

The SSM castings exhibit better bending fatigue properties than the casting using the traditional high-pressure die casting (HPDC) process. This improvement is mainly due to the formation of the harder surface liquid segregation (SLS) layer on the SSM casting surface. Furthermore, compared with the standard SSM process, the castings using intensive stirring (hereinafter referred to as the modified SSM process) show similar but more reliable fatigue properties. This reliable fatigue property can be attributed to eliminating the big internal pores through intensive stirring, which results in local stress concentration and significantly reduces fatigue performance. Besides, due to the gradient stress distribution in the bending loading, the surface defects play a significant role in the fatigue properties. With the increase of the specimens’ thickness, the failure mechanisms changed.

The shrinkage pores in the reduced pressure test (RPT) test play a significant role in the accuracy of melt quality assessment. A good correlation between the bifilm index (BI)/ density index (DI) and hydrogen content is observed for the RPT samples without significant shrinkage pores. In addition, the correlation between the BI and elongation is also strongly affected by the clusters of shrinkage pores due to the conflict between the definition of the BI and the influence of clusters of shrinkage pores on the ductility. Based on this, we proposed an optimized BI where the clusters of shrinkage pores were treated as single pores, increasing the reliability of the correlation between the BI and elongation. 

Treatment of the slurry is important during RheoMetalTM casting. In this work, semi-solid slurries were prepared under different stirring intensities, using two types of stirrers: a naked rod (for regular stirring) and a rod with two blades (for intensive stirring). Tensile tests were performed, investigating fracture surfaces, as well as metallographic samples. The results show that intensive stirring produces castings with finer primary particles and a more homogeneous microstructure. On the other hand, more faceted Fe-rich phases are found along the α-Al grains boundary, due to the dissolution of Fe from the stirrers. Moreover, for intensive stirring castings, the porosity found on the fracture surfaces are smaller, while more second (intermetallic) phases, especially Fe-rich phases, are observed. Consequently, the castings with intensive stirring show worse ductility. Finally, a quantitative analysis was made regarding ductility, affected both by porosity and the presence of Fe-rich phases.

The surface liquid segregation (SLS) layer in semisolid casting presents higher hardness than the surface of specimens cast using high-pressure die casting (HPDC). Bending fatigue tests showed that semisolid castings present better fatigue properties at higher stress, and this improvement disappears when the applied stress is lower than a critical load. This is because HPDC and SSM castings share the same surface deformation at low stress. More significant deformation is observed for HPDC castings when the stress exceeds the critical load. The presence of surface defects enlarges the difference in deformation at high stress and reduces the critical load.

Two alloys containing different Mg contents have been used to study the combined effect of stirring and oxidation on microstructure and ductility. The results show that intensive stirring can sufficiently disperse the alpha-Al particles and enable better liquid feeding during solidification and consequently reduce the porosity. The morphology of the oxides is determined by the amount of both Mg and stirring. With lower Mg content, the oxides present as oxide films, which can be broken up during stirring. In alloy with higher Mg content, the oxides exist as particles with numerous cracks, and the particle size increases slightly after stirring. In the Magsimal 59 alloy, due to the presence of large clusters of pores in the fracture surface, the influence of the small oxide particles on the ductility is negligible. In contrast, in the 42000 alloy, large oxide films on the fracture surface are correlated with the ductility.

The influence of Na-Based flux on the melt quality assessment was studied using Density index (DI), bifilm index (BI) and tensile test. The melt was prepared with scrap and ingot with and/or without the addition of Na-based flux. The results show that the addition of Na-based flux decreases the accuracy of the correlation between BI/DI and the hydrogen content due to the increased shrinkage pores. The addition of flux promotes the formation of shrinkage pores by either altering the eutectic solidification behavior or generating more and bigger oxide films which leads to insufficient feeding. Moreover, the addition of flux improves the ductility of specimens by modifying the eutectic phase rather than improving the melt quality.

Bifilms have been proposed to be critical for forming casting defects. The reduced pressure test (RPT) and bifilm index (BI) have been widely studied and used in industry for melt quality assessment. However, the BI remains challenged in predicting the mechanical properties. This work investigated the usage of RPT for elongation prediction by analyzing the melt in two foundries. The results showed that the BI could be valid when the bifilms exist as fully inflated pores. In reality, bifilms present as pores with various morphology and distribution, generating local stress concentrations under tension. Consequently, the parameter areaeff, which considers the pores’ morphology and distribution, was proposed, and the resultant f eff showed a good correlation with the elongation of the castings. These results guide the foundry to predict the elongation of the final castings and give directions for further research on the bifilm defects.