Papers by Keyword: Hot Tearing

Abstract: MPI and CANMET, both members of the Light Metal Alliance, launched a research project to study hot tearing in aluminum alloys. In this project, a reliable test method to quantitatively evaluate hot tearing was developed, and the processing variables affecting hot tearing tendency were also investigated. This paper summarizes the outcomes of this work from the following aspects: 1) Critical literature survey; 2) Experimental set-up, test methodology and procedures; 3) Effects of mold temperature and pouring temperature; 4) Effect of grain refinement; 5) Thermomechanical simulation of hot tearing.

Abstract: Wrought aluminium alloys are prone to hot tearing when cast into near-net shapes. This problem can be overcome by the novel casting technique of rheo-processing combined with high pressure die casting. An industrial engine mounting bracket is produced by rheo-process commercial 7075 with the patented CSIR-RCS and subsequent high pressure die casting. Section thickness changes and constraining geometry make this a difficult component to rheocast. X-ray radiography is used to evaluate hot tearing over the component and is correlated to piston injection shot profile velocities. Gross hot tearing is significantly reduced by a higher injection velocity but turbulent flow entraps air. Faster injection allows more time for flow before final solidification.

Abstract: Using finite element method, a numerical simulation is presented for engine block casting. The filling process analysis provides an accurate initial temperature field to the thermal stress and hot tearing investigations; meanwhile, it is useful in prediction of cooling shut. The thermal stress distribution after solidification of the casting is solved. To assess the hot tearing, the RGD criterion is adopted. Both hot tearing indicator and thermal stress can be used to estimate the susceptibility of hot tearing. The results can offer a reference to casting parameter design for engine block.

Abstract: The effect of elements such as Ti, Al, Hf, Zr and B on hot tearing susceptibility during directional solidification was explored by tube-like samples with thickness of 1.0 mm, 1.5mm and 2.0mm in wall. Results of SEM of elements on solidification of casting Ni-based superalloys were presented. The influential extent among the investigated elements is found to be: the largest is Ti, then Zr, B and Al, and Hf is the smallest. Titanium promotes the formation of (γ+γ´) eutectic, increases sizes of the eutectic. Increasing Al content results in the size of γ+γ´ eutectic and the number of γ´. Increasing Zr content promotes γ+γ´ eutectic.Varying B content shows little microstructural evolution.

Abstract: Hot tear is one of the main defects during the aluminium casting process, and it usually occurs during solidification. In this work, the low frequency electromagnetic field was applied to study the effects of low frequency electromagnetic field on hot tear during the conventional hot-top casting process. The results show that under the effect of the low frequency electromagnetic field, the structure of the ingot is refined, the flow direction is changed, and the depth of the sump becomes shallow. All these factors can decrease the hot tear susceptibility.

Abstract: Grain refinement and hot tearing are important key factors affecting the quality of castings. There have been substantial advances in the understanding of both of these phenomena over the last two decades. The paper discusses strategies for obtaining the lowest cost grain refiner addition and provides an explanation for how the refinement of equiaxed grains leads to a reduction in hot tear susceptibility. However, it also provides a warning that adding more grain refiner may not be better for reducing hot tear susceptibility. Alloy factors affecting hot tearing are also discussed. Finally, a list of six key considerations is provided to help casthouse and foundry engineers when trying to optimise grain refinement and reduce hot tearing.

Abstract: Aluminium-Zinc-Magnesium-Copper, 7075, alloy offers both high strength (comparable with those of alloy steels) and very good ductility which makes this alloy group suitable for many automotive, aerospace and defence applications, however the alloys are difficult to cast because of their tendency for Hot Tearing. The Semi-Solid Metalforming (SSM) casting technique allows for near net shape casting using High-Pressure Diecasting (HPDC). The lower forming temperature reduces the problem of Hot Tearing. The CSIR has developed a rheocasting process which prepares semi-solid slurries from molten metal which can then be formed using HPDC. Since 7075 is a wrought alloy there is limited information on the foundry practice for this alloy. The aim of this study is to determine the foundry practice for 7075 aluminium alloy. Melting trials and optical emission spectroscopy on, AA7075 was conducted to study the effect of metallurgical melting parameters and compositional variations. 7075 Alloy was melted using a resistance melting furnace and compositional variation in the crucible for varying holding times was evaluated to determine the degree of segregation of key alloying elements. Results have shown some compositional variations.

Abstract: Hot tearing, or hot cracking, is one of the most severe solidification defects commonly encountered during casting. It is such a complicated phenomenon that a full understanding is still not yet achieved, though it has been extensively investigated for decades. Most contributions are still based on qualitative characterisations. The purpose of this work is to develop a method that can quantitatively evaluate and investigate hot tearing behaviour. The principle is based on contraction stress/force measurements. The measured contraction force has been proven to be able to evaluate the hot tearing susceptibility as a more straightforward and quantitative index. By analyzing the contraction force curve, information can be obtained for both the initiation and the propagation of the hot tear. With this method, the influence of mould temperature and Al content on hot tearing behaviour of Mg-Al binary alloys has been investigated. The contraction force curves also indicate that the liquid refilling plays an important role during the hot crack propagation. With a lower cooling rate and higher onset temperature of hot tear, the remaining liquid is more favourable to refill the initiated hot crack, and consequently interrupts the propagation of cracks or possibly completely heals the cracks.

Abstract: The mechanisms of hot tearing are generally understood; inadequate feeding initiates the tear and further thermal deformation propagates the tear. However, a reliable experimental methodology/apparatus to quantitatively measure and characterize hot tearing is not available for the casting industry. In this study, a hot tearing apparatus with a load cell and LVDT developed at CANMET-MTL was used to measure the load and contraction in the mushy zone of an Al-Cu alloy. The onset of hot tearing can be determined from the load curve, its first derivative and cooling curve. The linear solidification contraction of the bar is measured. Alloy 206, which is an alloy that is quite prone to hot tearing was evaluated by the apparatus; results are given and discussed.

Abstract: The semi-solid forming (SSF) of aluminum alloys offers many advantages over conventional casting processes. Nevertheless, the semi-solid forming is still far from its full commercial potential and mainly used in specific niche markets. The market positioning requires simple, low cost, and versatile SSF processes that are capable of processing a wide range of alloy composition including wrought alloy compositions. However, wrought alloys must be adapted to obtain the desired semi-solid processing ability and proper mechanical properties. The processing of these attractive alloys with the SEED process offers the possibility to better target specific applications and customers’ needs. In the present paper, the alloy development of AA-6061 aiming to minimize the hot tearing propensity during forming process is reported. An overview of the industrial SEED process used to produce the semi-solid AA-6061 feedstock is presented. The mechanical properties of the cast parts subjected to a specific heat treatment were evaluated. As part of the joint effort between Alcan International Limited and the National Research Council of Canada (NRC), the fatigue results obtained from the semi-solid AA-6061 die cast parts will be also reported.