Rheo-diecasting Research Articles

Rheo-diecasting process for semi-solid aluminum alloys

A novel one-step semisolid processing technique, the rheo-diecasting (RDC) process, was developed, which adapts in situ creation of semisolid metal slurry with fine and spherical solid particles followed by direct shaping of the slurry into a near-net shape component using the existing cold chamber diecasting process. The RDC process was applied to process A356 and A380 aluminum alloys. The resulting microstructures and mechanical properties of RDC products under as-cast and various heat treatment conditions were analyzed. The experimental results show that the RDC samples have an extremely low porosity, a fine and uniform microstructure throughout entire casting, and consequently much improved strength and ductility in the as-cast condition. The strength of RDC A356 alloy can be substantially improved under T5 and T6 heat treatments without loss of ductility.

A physical approach to the direct recycling of Mg-alloy scrap by the rheo-diecasting process

Due to the rapidly growing market for magnesium diecastings in the automotive industry in the past decade, there is a steady increase in magnesium scrap sourced from both end-of-life vehicles (old scrap) and the manufacturing processes (new scrap). In addition, the energy required to recycle magnesium is only 5% of that for extraction of primary magnesium from Mg-containing minerals. This makes magnesium recycling extremely beneficial for cost reduction, preservation of natural resources and protection of the environment. In this paper, a physical approach is presented to magnesium recycling using the rheo-diecasting (RDC) process. The RDC process was applied to process AZ91D alloy sourced from both primary alloy ingots and diecast scrap. The experimental results showed that the RDC process could be used to produce recycled AZ91D alloy with fine and uniform microstructure and very low level of porosity. The intermetallic compounds containing the impurity elements were fine and spherical particles distributed uniformly in the alloy matrix. No oxide particle clusters and oxide films were found in the RDC microstructure. The tensile properties of the recycled AZ91D alloy were comparable to those produced from the primary alloy ingots.

Effects of intensive forced melt convection on the mechanical properties of Fe containing Al–Si based alloys

The pickup of impurity Fe is an inherent engineering problem for the recycling of aluminium alloys, due to the detrimental effects of Fe containing intermetallic compounds on the mechanical properties. This work aims to understand the effects of intensive forced melt convection and alloying on the mechanical properties of Fe containing Al–Si based cast alloys. Varied Fe levels were introduced into widely used commercial Al–Si based cast alloys with different Mn contents. The role of intensive forced melt convection was investigated through comparative study of the microstructure properties relationship of alloys that are processed via rheo-diecasting (RDC) and conventional high-pressure diecasting (HPDC), with the former involving intensively shearing the melt in a slurry maker before diecasting. CALPHAD modelling of the thermodynamic properties of the multi-component alloys was carried out to clarify the role of alloying elements on the formation of different primary Fe containing intermetallic compounds. The experimental results show that intensive forced melt convection during solidification promotes the formation of α-AlFeMnSi compound with a compact morphology. It was also found that Mn as an alloying element assists the conversion of needle-shaped β-AlFeSi compound into more compact α-AlFeMnSi compound.

Study on Microstructures and Mechanical Properties of Rheo-Die Casting Semi-Solid A356 Aluminum Alloy

In this paper, with a newly self-developed rotating barrel rheomoulding machine(RBRM), microstructures and mechanical properties of rheo-die casting A356 alloy were studied. In order to clearly show the characteristic of rheo-die casting, liquid die casting and semi-solid casting were done too. The experimental results showed that microstructures of rheo-die casting were composed of solid grains, which were finer and rounder, and had fewer pores. In the three technologies, integrated mechanical properties of semi-solid rheo-die casting were the best.

Microstructure and Mechanical Properties of Fe-Containing Al-Alloys Processed by a Rheo-Diecasting Process

Al-Fe compounds are usually present in the as-cast microstructure of Al-alloys as large needles or plates. As such, they have a detrimental effect on the mechanical properties of Al-alloys containing Fe, either as an impurity element or as an alloying addition. However, Fe-containing Al-alloys also offer attractive physical properties, such as improved stiffness, wear resistance and thermal resistance. If the needle and plate morphology of the Al-Fe compounds can be modified to a more compact morphology, with refined particle size and uniform distribution, the mechanical properties of Al-Fe based Al-alloys can be substantially improved, and therefore, they will find wider applications in many engineering sectors. A new semisolid metal processing technology, rheodiecasting (RDC), has been developed for production of Al-alloy components with high integrity. The RDC process innovatively combines the dispersive mixing power of the twin-screw mechanism, for the creation of high quality semisolid slurry, with the high efficiency, low cost nature of the high-pressure diecasting (HPDC) process for component shaping. In this paper, we present our experimental results on the effects of intensive melt shearing on the size and morphology of Al-Fe compounds in A380 alloys, with different levels of Fe additions. The experimental results have shown that intensive melt shearing during solidification can effectively change the particle shape from the usual needles and plates, to an equiaxed morphology. Samples which have undergone with melt shearing, exhibit much improved strength and ductility compared to those with the same level of Fe addition, but without exposure to melt shearing.

Solidification Behaviour of 357 Al-Alloy under Intensive Forced Convection

Solidification behaviour of 357 Al-alloy under intensive forced convection in the rheo-die-casting (RDC) process, was investigated experimentally to understand the effects of the intensity of forced convection and shearing time on the nucleation and growth behaviour. It was found that under intensive forced convection, heterogeneous nucleation occurred continuously throughout the entire volume of the solidifying melt. All the nuclei could survive due to the uniform temperature and composition fields created by the forced convection. This has been named as ‘effective and continuous nucleation’. It is also found that the nuclei grow spherically with an extremely fast growth rate. This makes the primary solidification essentially a slow coarsening process, in which Ostwald ripening takes place by dissolution of the smaller particles. In addition, it was found that intensive forced convection suppresses partially the formation of the primary phase, promote nucleation of the primary particles, and hinders the particle growth.

Microstructure and mechanical properties of rheo-diecast AZ91D magnesium alloy

A new semisolid metal processing technology, rheo-diecasting (RDC) has been developed for production of Mg-alloy components with high integrity. The RDC process innovatively combines the dispersive mixing power of the twin-screw mechanism for creation of high quality semisolid slurry and the high efficiency, low cost nature of the high pressure die casting (HPDC) process for component shaping. AZ91D Mg-alloy was used to optimise the RDC process and to establish its advantages over both the HPDC process and other existing semisolid processing techniques. In this paper we present the RDC process for processing Mg-alloys and the resulting microstructure and mechanical properties of RDC AZ91D alloy. The solidification behaviour of the Mg-alloys in the RDC process and the co-relationships between microstructure and mechanical properties of the RDC AZ91D alloy are discussed. It was found that the RDC process is capable of producing Mg-alloy samples with close-to-zero porosity and a fine, uniform microstructure throughout the entire sample irrespective of the section thickness. Compared with those obtained by other existing processing techniques, the RDC samples have substantially improved or equivalent mechanical properties, with the tensile elongation showing more than 100% improvement.

Microstructural evolution of rheo-diecast AZ91D magnesium alloy during heat treatment

Microstructures and mechanical properties of an AZ91D magnesium alloy prepared with rheo-diecasting (RDC) were characterised in as-cast and various heat treatment conditions. The RDC alloy exhibited a unique microstructure featuring primary α-Mg globules uniformly distributed in the matrix of fine secondary α-Mg grains and β-Mg 17Al 12 intermetallic. Solution treatment at 413 °C caused microstructural evolution, involving four processes: dissolution of the β-phase; formation of a fine-grain structure; morphological change of the α globules into traditional grain shape; and grain coarsening. The RDC alloy exhibited a faster β-phase dissolution rate than a high-pressure diecast alloy. During aging at 216 °C, discontinuous precipitates were preferentially initiated at some of the grain boundaries and then continuous precipitates appeared in the grain interiors with accelerated age-hardening kinetics for the RDC alloy. A newly developed heat treatment, T x , carried out at a typical temperature and time of 365 °C and 2 h, produced an improved combination of mechanical properties compared to the conventional heat treatments studied in the present work.

Development of the rheo-diecasting process for magnesium alloys

The rheo-diecasting (RDC) process is an innovative one-step semisolid processing technique for manufacturing near-net shape components of high integrity directly from liquid Mg-alloys. The process innovatively adapts the well-established high shear dispersive mixing action of the twin-screw mechanism to the task of in situ creation of semisolid metal (SSM) slurry with fine and spheroidal solid particles followed by direct shaping of the SSM slurry into a near-net shape component using the existing cold chamber high pressure diecasting (HPDC) process. In this paper, we present the RDC process, and the resulting microstructures and mechanical properties of various Mg-alloys. The experimental results show that the RDC samples have extremely low porosity (typically 0.1–0.3%), fine and uniform microstructure throughout entire casting, and consequently much improved strength and ductility, compared with those produced by the HPDC process and other semisolid processes. In addition, the RDC process is more tolerant to alloy composition in terms of processability.

Effects of rheo-die casting process on the microstructure and mechanical properties of AM50 magnesium alloy

AM50 Mg alloy is usually processed by high-pressure die casting (HPDC) for applications demanding high ductility. However, this alloy has a high tendency of hot tearing, and is difficult to cast because of its large freezing range and low final solidification temperature. In this investigation, the rheo-die casting (RDC) process, a new semi-solid processing technology developed by BCAST at Brunel University, was used to overcome such casting difficulties and to improve mechanical properties. It was found that the RDC AM50 alloy exhibited much improved mechanical properties, especially for the elongation, over those of the same alloy produced by HPDC and other semi-solid processing technologies. The improved mechanical properties can be attributed to the elimination of large gas pores as a result of gas entrapment, substantial reduction of hot tearing, and fine, uniform microstructure, as a consequence of solidification under high shear rate and high intensity of turbulence in the RDC process.

Development of the Rheo-Diecasting Process for Mg-Alloys

Research on rheology of semisolid slurries suggests that an ideal semisolid slurry for semisolid metal (SSM) processing is one in which a suitable volume fraction of fine and spherical particles dispersed uniformly in a liquid matrix. Such ideal semisolid slurry can be obtained by enhancing the effective nucleation and promoting spherical growth during solidification. Experimental investigation and theoretical analysis of solidification under forced convection allowed us to identify the conditions for achieving enhanced effective nucleation and promotion of spherical growth. Such conditions can be summarised as uniform temperature and chemistry throughout the whole volume of liquid alloy, high shear rate and high intensity of turbulence during the solidification process. Based on such understandings, a new SSM processing technology, rheodiecasting (RDC), has been developed for the production of components with high integrity. AZ91D Mg-alloy was used to optimise the rheo-diecasting process and component production trials. The experimental results indicate that the rheo-diecast samples have close-to-zero porosity, fine and uniform microstructure and much improved strength and ductility. Rheo-diecasting process is particularly suitable for production of high-safety, airtight and highly stressed components in the automotive industry.

Solidification behaviour of AZ91D alloy under intensive forced convection in the RDC process

Rheo-diecasting (RDC) is a new semisolid processing technology for production of near net shape components. In this work, the solidification behaviour of AZ91D alloy under intensive forced convection in the RDC process was investigated experimentally to understand the effects of the intensity of forced convection, shearing time and shearing temperature on the nucleation and growth behaviour. It was found that under intensive forced convection, heterogeneous nucleation occurred continuously throughout the entire volume of the solidifying melt. All the nuclei could survive due to the uniform temperature and composition fields created by the forced convection. This has been named as continuous effective nucleation. It is also found that the nuclei grow spherically with an extremely fast growth rate. This makes the primary solidification essentially a coarsening process, in which Ostwald ripening takes place by dissolution of the smaller particles. Secondary solidification of the intensively sheared semisolid slurry takes place also through effective nucleation, but with dendritic growth. Increasing the intensity of forced convection enhances nucleation and promotes the formation of the primary phase during the secondary solidification in the shot sleeve. The final solidification microstructure is strongly dependent on the presence of turbulence rather than the shear rate.

Microstructure and Mechanical Properties of a Rheo-Diecast Mg–10Zn–4.5Al Alloy

High zinc content Mg–Zn–Al alloys have the potential to be used as high pressure die casting (HPDC) alloys for applications up to 150C. However, these alloys show high tendency to hot tearing and microporosity. Rheo-diecasting (RDC) is an innovative semisolid HPDC process, which can effectively eliminate the formation of primary magnesium dendrites and convert them into fine, globular particles. This very positive change in the morphology and distribution of the primary phase reduces the subsequent formation of various casting defects. In this study, a composition of Mg–10Zn–4.5Al was selected from the Mg–Zn–Al system and processed with both HPDC and RDC. It is shown that, samples produced by the RDC process exhibit substantially reduced hot cracks, few gas pores, and a nearly uniform distribution of fine, globular primary particles. These microstructural changes resulted in much improved strength and elongation, which are comparable to those of AZ91D, while the processing temperature is much lower. It is concluded that high zinc content Mg–Zn–Al alloys have good potential to be exploited as commercially useful alloys by the RDC process.