Near-net Shape Casting Research Articles

Determining casting defects in near-net shape casting aluminum parts by computed tomography

Three types of near-net shape casting aluminum parts were investigated by computed tomography to determine casting defects and evaluate quality. The first, second, and third parts were produced by low-pressure die casting (Al-12Si-0.8Cu-0.5Fe-0.9Mg-0.7Ni-0.2Zn alloy), die casting (A356, Al-7Si-0.3Mg), and semi-solid casting (A356, Al-7Si-0.3Mg), respectively. Unlike die casting (second part), low-pressure die casting (first part) significantly reduced the formation of casting defects (i.e., porosity) due to its smooth filling and solidification under pressure. No significant casting defect was observed in the third part, and this absence of defects indicates that semi-solid casting could produce high-quality near-net shape casting aluminum parts. Moreover, casting defects were mostly distributed along the eutectic grain boundaries. This finding reveals that refinement of eutectic grains is necessary to optimize the distribution of casting defects and reduce their size. This investigation demonstrated that computed tomography is an efficient method to determine casting defects in near-net shape casting aluminum parts.

Conventional and near net shape casting options for steel sheet

The conventional continuous casting (CCC) of steel, starting in the 1950s, has now become the dominant casting process, worldwide. It presently accounts for about 1.5 billion tonnes of steel cast per year. Nonetheless, given the inexorable forces favouring near net shape casting (NNSC) of semi-finished products, for reasons of economy of scale, process rationalisation and the protection of the environment, the question of how long CCC can continue to dominate in the casting of steel, must remain a question. Two NNSC candidates for steel are the twin roll casting (TRC) and horizontal single belt casting (HSBC) processes. While the TRC of steel sheet by ‘CASTRIP’ has now been operating commercially for some 12 years within NUCOR, producing low carbon steel sheets, HSBC has only just been commercialised, at Salzgitter’s Peine Plant, in Germany, under the name ‘Belt Cast Technology’. It had to await the strong demand for high strength high ductility steels for the auto industry. The McGill Metals Processing Centre has been involved in both casting processes since 1987, its research being aimed at addressing the various technical aspects and fundamental problems associated with these two NNSC processes for forming steel sheet material. This paper addresses the pros and cons of NNSC processes versus CCC, and the differences between TRC and HSBC, for future steel sheet production.

Al 박육 다이캐스팅 주물에서 기포결함에 미치는 주물두께, 사출속도의 영향

The Al die casting process has been widely used in the manufacturing of automotive parts when the process requires near-net shape casting and a high productive rate. However, porosity arises in the casting process, and this hampers the wider use of this method for the creation of high-durability automotive components. The porosity can be controlled by the shot condition, but, it is critical to set the shot condition in the sleeve, and it remains difficult to optimize the shot condition to avoid air entrapment efficiently. In this study, the 4.5 mm, 2.0 mm plate die castings were fabricated under various shot conditions, such as plunger velocities of 0.7 m/s ~ 3.0 m/s and fast shot set points of the cavity of -25%, 0%, 25%, and 50%. The mold filling behavior of Al melts in the cavity was analyzed by a numerical method. Also, according to the shot conditions, the results of numerical analyses were compared to those of die-casting experiments. The porosity levels of the plate castings were analyzed by X-ray CT images and by density and microstructural analyses. The effects of the porosity on the mechanical properties were analyzed by tensile tests and hardness tests. The simulation results are in good general agreements with the die-casting experimental results. When plunger velocity and fast shot set point are 1.0 m/s and cavity 25% position, castings had optimum condition for good mechanical properties and a low level of porosity.

The Effect of Natural Pre-Ageing on the Mechanical Properties of Rheo-High Pressure Die Cast Aluminium Alloy 2139

Near-net shape casting of wrought aluminium alloys has proven to be difficult due to hot tearing. The Council for Scientific and Industrial Research (CSIR) has successfully processed wrought aluminium alloy 2139 into plate castings using the Rheo-high pressure die casting process (R-HPDC). Alloy 2139 is a Ag-containing aluminium alloy from the Al-Cu-Mg 2xxx series family. The addition of Ag enhances the age hardening response through the formation of co-clusters that act as precursors to the formation of plate-like Ω precipitates. These co-clusters typically form during natural ageing and 12-24 h of natural pre-ageing is normally specified before artificial ageing in Ag-containing Al-Cu-Mg alloys. The T6 hardness and tensile properties of R-HPDC 2139 alloy were investigated with and without natural pre-ageing. It is shown that there is no significant difference in both peak hardness and tensile properties in R-HPDC alloy 2139 with and without natural pre-ageing. The possible precipitation phenomena in both cases are discussed.

The Mechanical Properties of Rheo-High Pressure Die Cast Al-Mg-Si-(Cu) 6xxx Series Alloys

Near-net shape casting of wrought aluminium alloys has proven to be difficult due to a tendency towards hot tearing during cooling. Rheo-high pressure die casting (R-HPDC), has been shown to be an effective method of producing near-net shape wrought aluminium alloy castings. Limited information is available regarding the mechanical properties of age-hardenable wrought Al-castings produced by semi-solid metal forming. The purpose of this study is to investigate the effects of chemical composition and natural pre-ageing on the hardness and mechanical properties of rheo-HPDC 6xxx series Al-Mg-Si-(Cu) alloys in the T6 temper condition. The effects of the addition of Cu, as well as the (Mg+Si) content and Mg:Si ratio of the alloys are quantified. Alloys that are included are Cu-free 6004 and 6082, as well as Cu-containing 6013, 6111 and 6066. It is shown that the addition of Cu and excess Si result in higher hardness and strength. Natural pre-ageing has a significant effect (positive for 6004 and negative for the others) on the T6 properties. Good strength values can be achieved, but ductility is dependent on factors such as hot tearing during casting and incipient melting during solution heat treatment.

Fusion Welding of Rheocast Semi-Solid Metal (SSM) Processed Aluminium Alloy 7017

Near-net shape casting of wrought aluminium alloys has proven to be difficult due to a tendency towards hot tearing during cooling. Rheocasting, or semi-solid metal (SSM) processing followed by high pressure die casting (HPDC), has recently been shown to be an effective alternative to conventional die casting, yielding near-net shape wrought aluminium alloy castings with less risk of hot tearing. This casting process involves pouring the liquid metal into a processing cup, which is then transferred into a coil for induction stirring and simultaneous forced air cooling. When the metal reaches the semi-solid casting temperature, the resultant slurry is transferred to a high pressure die casting machine and cast to near-net shape. This modifies the as-cast microstructure, yielding a more globular primary phase and results in mechanical properties in the -T6 condition closely approaching those of wrought material in the same condition. Little information is currently available on the response of SSM-HPDC material to welding. This project investigated the influence of autogenous laser and gas tungsten arc welding on the microstructure and mechanical properties of aluminium 7017 after rheocasting. It is possible to successfully weld this material without solidification or liquation cracking. The effect of welding on the rheocast microstructure in the heat-affected zone and weld metal was shown, and the hardness and tensile properties of the resulting joints in the as-welded condition were tested and related to the microstructures achieved.

Physical Simulation at Hot Deformation

Today the numerical simulation of hot deformation processes is very advanced. But it requires mathematical models for metalphysical processes as for microstructure development, which take place during the deformation. Until now such models were developed for many steel grades and non-ferrous materials. For new steels as multi-phase steels laboratory investigations are required, in order to determine the optimal processing technologies of these materials. This applies also to the modelling. So far it is impossible, to calculate sole by mathematical solutions the manifold parameters of metalphysical processes and microstructure, for this reason laboratory trials and simulations are needed implicitly. Even for well known materials such procedures can be essential and useful. Using the multi-functional simulation system Gleeble HDS-V40 it is shown, which possibilities a physical simulation offers today. Starting with the annealing conditions, followed by microstructure development up to cooling, selected examples reflect the results of property development during hot deformation processes. The differences between conventional deformation after re-heating and deformation after direct-charging will be presented. The last-mentioned concept offers in its combination of near-netshape casting and direct charging special benefits, especially saving of energy.

Improvement of bloom casting and high reduction breakdown process

In May 1998, Muroran Works installed a near-net shape casting and compact high reduction (NCR) process in its steelmaking plant. The No.3 continuous casting machine was rearranged to cast middle cross section blooms, and a reheating furnace and a 2-passes high reduction breakdown mill were newly equipped. No.3 CCM casts square 220 mm mould size blooms and the breakdown mill reduces their cross section to square 162 mm. This combined process improved the quality of the cast billets and achieved high productivity.

Recent advances in near-net shape flat steel manufacturing

Near-Net Shape Casting (NNSC) has been one of the major paradigms of process development in the steel industry in the last ten years. It has also made possible the development of flat product mini-mills, in conjunction with electric arc furnace steelmaking and in-line rolling on compact hot-strip mills. More development is still to come, with the adoption of NNSC casting in integrated mills and the emergence of strip casting.

A front tracking model for the rapid solidification of dendritic alloys

A new front-tracking model for the rapid solidification of dendritic alloys is described. The model uses a 1-D control volume finite difference scheme, incorporating external extraction and internal redistribution of heat, nucleation characteristics, crystal growth kinetics, and redistribution of solute. The mushy zone is modelled using a new solid fraction relationship which has been designed to both conserve solute and minimise computational requirements. Applications of the model are given for various alloys—carbon steel, aluminium-iron, stainless steel—under various process conditions—planar flow casting, near-net shape casting of thin strip. These examples demonstrate the versatility of the model.

The effects of binders and heating temperatures on the properties of preforms

The use of binders in fabricating ceramic fiber preforms is essential when producing metal-matrix com-posites via squeeze casting or liquid metal infiltration. The binder is not only used to stabilize the shape of the preform, but also to increase its strength. A strong preform is needed to make a net shape or near-net shape casting so that many post-machining processes can be reduced or eliminated. However, very lit-tle attention has been paid to preform fabrication, subsequent processing, and its characterization. This work focuses on the relation between preform processing (selection of binder, heat treatment of the pre-form, etc.) and the properties of the resulting preforms. Silica and phosphate binders were used to make short alumina fiber preforms. The use of binders and the heat treatments used were correlated to the properties of the resulting preforms. The results showed that the heat treating temperature, as well as the amount of binder, is relevant to the resulting properties of the preform. Correlations among the use of binders, heat treatment condition, composition of the binders, and the properties of the preforms were investigated.