Low Pressure Permanent Mold Research Articles

Producing Structural High Integrity Castings with Minimum Carbon Footprint with the Comptech-Rheocasting Process

Structural high vacuum die casting has been growing significantly over the past decades. It is typically done with special (primary type) alloys, special melt treatment practices, high vacuum, a carefully engineered die casting process applying many best practices in our industry. Structural die castings are growing in size and complexity and die casting machines are getting bigger and bigger - and those castings are becoming a significant challenge with current technologies. It was now found that Rheocasting - a process of preparing a semi-solid slurry (with 30-45% solid fraction) that is injected slower and in a laminar way into the cavity, can take structural die casting to completely new heights, enable the casting of parts that would otherwise be impossible to cast and at the same time reduce or eliminate some of the current problems of structural die casting. With Rheocasting, lower purity (secondary) aluminum alloys can be used and excellent properties can be achieved as cast and heat treated (T5, T6/7). Rheocasting has had a breakthrough in the telecom and other industries, but it has now been optimized to become an essential tool in the toolbox of making structural castings. This paper shows the developments that lead to the successful series production of a large (about 4’ long) hinge pillar for an Electric Vehicle (EV). It explains the challenges and how they were overcome, and some of the possibilities of the process regarding alloys, heat treatments and achievable properties. It also shows how Rheocasting can help die casters produce more complex (thin and thick walled) and very large castings with the highest integrity. It can replace structural low-pressure permanent mold (LPPM) castings and reduce wall thickness, improve tolerances and make them more economical. Rheocasting can enable die casters to enter this new market of structural castings including Gigacastings, produce parts previously thought impossible to cast (and on much smaller machines than thought), and expand the growing structural die casting market even faster and further.

Influence of different casting processes on high cycle fatigue behavior of Mg–10Gd–3Y–0.5Zr alloy

Abstract

Effect of Inclusions on the Tensile Properties Inside a LPPM A356 Casting

Contrary to metallurgical properties such as dendrite fineness or microporosity, the level of inclusions in a casting is difficult if not impossible to assess accurately. Consequently, it is not easy to quantify the detrimental effect of inclusions on tensile properties. In the present work, this has been attempted on an aluminum A356 step casting, the geometry of which offers a wide range of solidifying conditions. It was poured in low pressure permanent mold, a process which minimizes the variability of mold filling. The interaction between the melt gas content and inclusions in producing microporosity was highlighted by analyzing reduced pressure test samples. Radiographic analyses and die penetrant checks were performed to detect microshrinkage and inclusions in the castings. The effect of inclusions on tensile properties was assessed by separating tensile samples in which inclusions were observed in the fracture surface from those with a flawless fracture surface. It was thus possible to compare tensile properties with and without inclusions in the same casting. This procedure eliminated the variability introduced by the numerous additional factors bearing on the tensile properties such as the chemical composition, melt preparation and heat treatment. This distinguishes the present study from those previously carried out on the influence of inclusions on tensile properties.

Microstructure and Performance of Four Casting Processes for Magnesium Alloy AZ91

The performance of four different casting processes for magnesium alloy AZ91 were evaluated through microstructure characterization, mechanical testing, and SEM analysis of fracture surfaces. Passenger car control arms were cast by indirect squeeze cast, low pressure permanent mold (LPPM), T-Mag, and ablation processes. Samples were cut from twelve locations of the control arms for microstructure characterization. The microstructure, grain size, porosity distribution, and defect analyses were performed using optical microscopy and an image analyzer. Five different defects including microporosity, oxide film, sponge shrinkage, gas pore, and a crack-like defect were identified. The mechanical behavior was characterized using four-point bending (FPB) and tensile tests. The four casting processes were evaluated in terms of reliability using two-parameter Weibull statistics of the ultimate bending strength (UBS) determined from the FPB test samples. Metallographic analyses were performed on these samples, revealing noticeable microstructural differences between them, with some showing possible evidence of oxide films.

Simulation of defects and stresses in castings

For cast aluminum wheel both the technology of low-pressure permanent mold casting and the bending test were simulated. The shrinkage defects and residual stresses were predicted by computer methods. The total stress fields were examined taking into account the metal structure and the technological features. Non-uniform strength distribution was detected in the cast alloy. It was shown that the technological residual stresses affect the magnitude and distribution of stress concentrators in the part under load.

Distribution of the Quality Index in AlSiMg Castings Produced by Different Processes: Reality vs Prediction

The Quality Index is a useful tool allowing to assess the metallurgical quality of aluminium AlSiMg foundry alloys (AlSi7Mg03/AlSi7Mg06, or A356/357 in North America). Its value, calculated from the ultimate tensile strength and elongation, increases with the metallurgical quality of the casting, i.e. with finer dendrites, lower microporosity, lower iron content and cleaner metal. This Quality Index does not depend on the degree of temper applied during the heat treatment (aging time and temperature). This paper will briefly explain how solidification modeling can provide the local value of the Quality Index, via the secondary dendrite arm spacing and level of microporisity calculated from the predicted values of the local solidification time and solidus velocity. The validity of the predictions has been assessed against the properties of tensile specimens excised from parts of different geometries produced by a variety of casting processes, as reported in the literature. The case study of a race car bell housing poured by two processes, namely gravity sand casting (GSC) and low pressure die casting (LPDC, or LPPM for Low Pressure Permanent Mold in North America), will be the focus of the present paper. The Quality Index has been measured and predicted at 3 locations in the castings after a T6 temper was applied which resulted in a minimum elongation of 5% for both the GDC and LPDC bell housings. Metallographic examination and image analysis allowed to quantify the superior metallurgical quality of the LPDC housings when compared to their GDC competitors.

Energy Efficient Heat Treatment for Linerless Hypereutectic Al-Si Engine Blocks Made Using Vacuum HPDC Process

Heat treatment standards developed by the aluminum industry over the last several decades are often not well optimized when applied to components cast by high cooling rate processes such as High Pressure Die Casting (HPDC), Low Pressure Permanent Mold (LPPM), Squeeze Casting, etc. The inherently finer as-cast structures should not require long solution times for the effective dissolution of intermetallic phases and the adequate thermal modification of structural constituents. Hence, long and expensive T6 and T7 treatments should not be required. Heat treatment studies involving as-cast laboratory samples with SDAS = 13.6 μm (equivalent to a thick-section HPDC casting) were conducted. Traditional and modified solution and aging treatments were compared. These studies suggest that a reduction of up to 92% in thermal processing time is possible while maintaining and/or improving the cast component’s metallurgical characteristics including hardness (≥75 HRB), dissolution of secondary phases, and spheroidization of the eutectic Si as well as overall homogeneity. Vacuum HPDC of an actual hypereutectic Al-20%Si motorcycle engine block confirmed the potential for significantly reduced heat treatment times, energy consumption, and overall costs.

Determination of interfacial heat-transfer boundary conditions in an aluminum low-pressure permanent mold test casting

Great advances in casting modeling/simulation software have been made in the past several years. However, software predictions are only as accurate as the material and processing properties used as inputs. The process of low-pressure semipermanent mold casting is widely used by the automotive industry in the production of aluminum cylinder head castings. However, the process is highly sensitive to various casting parameters and many of the influences are not well understood. This report presents results from casting experiments and simulations using a specially designed casting, the “wing casting.” The casting experiments demonstrated the variation in temperature change in different parts of the casting for different processing parameters. Boundary conditions were determined for different areas on the mold surface and those parameters were discussed in terms of temperature sensitivity in the casting behavior. It was also demonstrated that boundary conditions play a major role in the thermal behavior in the casting during solidification. Finally, changes in the mold/casting surface geometry were found to have a large influence on the value of the boundary conditions.