Effect of cooling rate on air gap formation in aluminium alloy permanent mould casting

Abstract

Displacements of the casting surface and the mould surface at the casting/mould interface were experimentally measured during the solidification of aluminium alloys in a permanent mould. Temperatures of the casting and mould surfaces at this interface were also recorded and correlated with displacement measurements. Four different commercial Al–Si alloys were investigated at varying cooling rates. These results are compared with available data on the effect of cooling rate on solid fraction evolution and consequently strength development during solidification. The temperature of the casting surface at the moment of air gap initiation was found to decrease with increasing cooling rate, although this relationship was confirmed at the 95% confidence level for only one of the alloys, AC601, for which sufficient data points were available. The solid fraction at the casting surface at gap initiation in this alloy is shown not to change with cooling rate. In all hypoeutectic alloys, the gap formed before the solid fraction at the casting surface reached 1·0 at slow cooling rates. For the near eutectic alloy BA401 it occurred at almost 1·0. Casting surface contraction rates following gap formation are also presented both as a function of time and casting surface temperature. It is shown that contractions predicted using the linear thermal expansion coefficient provide a reasonable approximation.     

Determination of the metal/die interfacial heat transfer coefficient and its application in evaluating the pressure distribution inside the casting during the high pressure die casting process

Abstract

High pressure die casting (HPDC) experiments were conducted on a 650 t cold chamber die casting machine to study the interfacial heat transfer behaviour between casting and die. A 'step shape' casting and two commercial alloys namely ADC12 and AM50 were used during the experiments. Temperature and pressure measurements were made inside the die and at the die surface. The metal/die interfacial heat transfer coefficient (IHTC) was successfully determined based on the measured temperature inside the die by solving the inverse heat transfer problem. The IHTC was then used as the boundary condition to determine the 3-D temperature field inside the casting. Based on the predicted temperature distribution, the pressure distribution inside the casting was evaluated by assuming that the transferred pressure from the plunger tip of the injection side to the casting is primarily influenced by the solid fraction of the casting. Reasonable agreement was found between the determined pressure values and the measured pressures at the die surface of the casting.     

Fracture toughness of ceramic moulds for investment casting with ice patterns

Abstract

Ice patterns can be used to make ceramic investment moulds for metal castings. Owing to the characteristics of ice, the ceramic mould must be made at subzero temperatures and consequently, requires a different formulation than shells built at room temperature. Success of this process depends greatly on the fracture toughness of mould materials. The present paper describes the experimental results of fracture toughness of mould materials processed from different compositions. The Taguchi method was used to reduce the trial runs. The parameters considered included the ratio of fibre containing fused silica and aluminosilicate powders, the volume of binder and the volume of catalyst. The microstructure and green fracture surface of test bars were also examined to understand the underlying mechanism. While conducting the four point bend test on ceramic mould samples, some samples had exceedingly low strengths appearing as outliers in the Weibull analysis. Examination of these low strength ceramic samples improved understanding of failure of mould materials. Sound moulds have been made for the investment casting process with ice patterns based on the analysis of experimental results. The casting of an M8 bolt is used to demonstrate that metal castings of complex geometry can be fabricated using ice patterns. The measured tolerances are within the required tolerance range.     

The Influence of Processing Variables on the Dimensional Integrity of Spheroidal-graphite Iron Castings

Abstract

This investigation was carried out to identify the major factors and their degree of influence on the dimensional accuracy of spheroidal-graphite iron castings produced in chemically- bonded sand moulds. Test castings were poured into furan-resin-bonded zircon sand and silica sand moulds and sodium silicate/ CO₂ bonded silica sand moulds. A comparison of casting sizes with those of the mould cavity into which they were poured showed considerable scatter and overlap. From these data the size that each casting would have had, had it solidified without graphite formation, was calculated and found to depend on mould cavity size for each type of mould. By isolating the differences in casting size due to graphite it was possible to identify the influencing factors. Thus castings poured into furan-resin-bonded zircon sand have the highest contraction and their size depends primarily on the amount of graphite present. The dimensions of castings poured into silica sand moulds show more variation and depend not only on the amount of graphite present and the structure of the metal but also on the thermal expansion of the silica sand moulds.     

Sensitivity study of IHTC on solidification simulation for automotive casting

Abstract

Interface heat transfer coefficient values between the mould/metal interfaces need to be precisely determined in order to accurately predict the thermal histories at different locations in automotive castings. Thermo-mechanical simulations are carried out for Al–Si alloy casting processes using a commercial code. The simulation results are verified with experimental data from the literature. Sensitivity studies show that the choice of the initial value of the interface heat transfer coefficient (IHTC) between chill/metal as well as the sand mould/metal interfaces has a marked effect on the cooling curves. In addition, having chosen an initial value of the IHTC, the analyses also show differences in the solidification rate of the casting alloy near the sand/metal and chill/metal interfaces, upon further cooling. The gap formation, which results in a change in IHTC from the initial value, does not affect the cooling curves in the vicinity of the sand/metal interface due to lower thermal conductivity of sand. However it is found to have a considerable effect in the chill/metal interfacial regions due to higher thermal conductivity of the chill. Based on these studies we recommend initial IHTC values of 3000 and 7000 W m^–2 K^–1 for sand/metal and chill (steel)/metal interfaces respectively, for application in casting simulations.     

Fabrication of SiC particle dispersed spheroidal graphite cast iron composite by vacuum assisted casting and its microstructure

Abstract

SiC particle preforms were infiltrated with spheroidal graphite cast iron melt by vacuum assisted casting in the sand mould, and spheroidal graphite cast iron composites in which the particles were dispersed in the surface region were fabricated. Although the melt infiltration was not accomplished when the melt was poured under atmospheric pressure, the infiltration was accomplished by the vacuum assisted casting when the SiC particle volume fraction and preform thickness were optimised. When the Si content of the cast iron was 2˙5 mass%, the phase consisting of mainly Fe₃Si was formed at the particle/matrix interface due to the reaction between the cast iron melt and the particles during the infiltration. The matrix of the composite consisted of fine spheroidal graphite particles, ferrite, pearite and chill crystal. Although the increase in the Si content suppressed the reaction and chill, no infiltrated area was observed in the composite.     

Analysis of Ohno continuous casting process using one-dimensional steady state heat transfer model

Abstract

The Ohno continuous casting process is being used regularly in the unidirectional crystallisation of high purity copper and silver wires for electronics applications. Many experimental investigations have been carried out on both fundamental and practical aspects of the process. This paper deals with the feasibility of a one-dimensional heat transfer approach for the analysis of the effect of the crucial processing parameters. The results on the effects of varying the parameters based on the calculation of the temperature profiles along the axis of the cast rod were in accord with the experimental results in the literature, indicating that the one-dimensional approach can be used as an analytical tool for the process.     

Influence of die casting process parameters on castability and properties of thin walled aluminium housings

Abstract

The objective of this study is to clarify effects of high pressure die casting process parameters for castability and mechanical properties. So the optimal die casting conditions for producing for thin walled Al component was conducted computational solidification simulation and actual die casting with three different venting systems, four straight, checker and full checker vent. Furthermore, the die casting process parameters, such as die controller temperature, high injection speed and die open time, were experimentally evaluated. The results of computational solidification simulation were found that the control of process parameters could lead to soundness of surface and no defect and improvement of mechanical properties. As increasing the high injection speed from 2˙0 to 4˙0 m s^?1 and die temperature, the castability was increased. The full checker venting system had best castability with good surface quality among the three cast specimens.     

Compression behaviour of semisolid YL112 die casting aluminium alloy following isothermal heat treatment

Abstract

The semisolid compression deformation behaviour of YL112 die casting aluminium alloy with the non-dendritic and dendritic structures has been compared in tests using a Gleeble-1500 thermomechanical simulator. The non-dendritic structure was obtained by isothermal treatment at 570°C for 120 min. In tests up to compressive strains of 0·8, the semisolid compression stress of the alloy with non-dendritic structure initially increases rapidly with increasing strain, then decreases, before reaching a plateau value. Under different deformation temperatures and deformation rates, the maximum compressive stresses are obtained in all cases at strain values of ～0·07. The semisolid deformation stress decreases with increasing deformation temperature and increases with increasing deformation rate. The compression behaviour of the two types of alloy differs. The semisolid compression stress of the alloy with dendritic structure is higher than that of the alloy with non-dendritic structure; and for strains >0·07, the semisolid compression stress increases and decreases with increasing strain for alloys with dendritic and non-dendritic structures respectively.     

Study on ac-PMIG welding of AZ31B magnesium alloy

Abstract

Welding of AZ31B magnesium alloy is carried out using alternating current pulsed metal inert gas (ac-PMIG) welding with 1·6 mm diameter of filler wire. Typical current waveform is used to make sure arc given an accurate energy input into filler wire. The arc characteristics, metal transfer forms, microstructure and mechanical property of ac-PMIG welding of AZ31B magnesium are investigated. The results show that a stable welding procedure and continuous joints can be obtained easily under a wide range of welding parameters. The most important factors for ac-PMIG welding are negative electrode (EN) ratio and pulse rework current, which give an accurate energy input into filler wire. The grain in fusion zone is much finer and more uniform, and grain size does not grow significantly in the heat affected zone compared with base metal. The average ultimate tensile strength of weld beads is 97·2% of base metal.     

Effect of controlled forced convection on macrosegregation and structure in direct-chill casting of an aluminium alloy

Abstract

Effects of upward or downward forced flow in the centre of the sump of a 195 mm Al-alloy billet on structure and macrosegregation were studied. Introduction of forced flow in the liquid part of the sump resulted in some changes in the structure and dramatic changes in macrosegregation. Upward flow coarsened the structure and increased negative centreline segregation. Downward flow made the structure finer and more uniform across the billet, and suppressed the macrosegregation. Strong downward flow produced positive macrosegregation in the billet. The results are discussed in terms of macrosegregation mechanisms.     

Effect of solidification velocity on weld solidification process of alloy tool steel

Abstract

In order to clarify the effect of solidification velocity on the weld solidification process of alloy tool steel during the welding, the information about microstructure evolution was obtained by the concurrent experiments of liquid tin quenching and time resolved X-ray diffraction technique using intense synchrotron radiation. It was found from the experiments that the solidification mode was transferred from an FA to an A mode at the high solidification velocity. The effect of solidification velocity on the phase selection during solidification between the primary δ-ferrite and γ-austenite was theoretically proved by the Kurz, Giovanola and Trivedi (KGT) model. It is thus explained that the solidification cracking susceptibility of the weld metal of alloy tool steel was enhanced due to the δ to γ transition of the primary phase.     

镁合金薄板快速铸轧过程有限元仿真研究

为了研究铸轧工艺参数对AZ31镁合金薄板快速铸轧过程温度场和热应力场的影响,基于铸轧区板坯的对称性建立了纵截面1/2的二维几何模型;选择了基于热弹塑性增量理论的热应力控制方程;采用大型通用有限元分析软件ANSYS对镁合金快速铸轧过程中的铸坯温度场和热-应力场进行了仿真分析,并就不同工艺参数(浇注温度、接触界面换热系数、铸轧速度)对铸坯温度和应力的分布及其相变区的影响进行了研究。仿真结果增强了对镁合金快速铸轧过程相变区温度变化和热裂产生机制的理解,为快速铸轧工艺参数的优化提供了依据。     

工艺参数对镁合金压铸件热裂纹倾向性的影响

研究了工艺参数对压铸镁合金AM60B薄壁件产生热裂纹倾向性的影响。结果表明,在本试验条件下浇注温度为680℃、压射速度3.5m／s和压射比压58.7MPa时,薄壁压铸件产生热裂纹的倾向较小。     

Effects of casting speed on microstructure and segregation of electromagnetically stirred Aluminum alloy in continuous casting process

Recently, a semi-solid metal processing has been acknowledged as a cost-effective technique to be able to manufacture high quality product for the transportation industry.In this study a hypo-eutectic Al alloy was fabricated by means of an electromagnetic stirrer in continuous casting process and the microstructural change during solidification due to a fluid flow by electromagnetic stirring was examined.Due to the forced fluid flow during solidification a dendritic phase of primary α phase of Al alloy was turned into a globular phase, which can make the Al alloy get a thixotropic behavior in the semi-solid region.In order to establish the quantitative relationship between microstructure and the process parameters, the morphology shape, a silicon distribution and a size of primary α phase were observed according to casting speed in continuous casting machine.The primary α phase was turned into the degenerate dendrites approaching a spherical configuration with increasing casting speed.The fine-grained and equiaxed microstructure appeared at higher casting speed.A segregation behavior of Si element was declined with increasing casting speed and a very uniform distribution of Si element was observed on the billet at a casting speed of 600 mm·min-1.A thickness of the solidifying shell of the billet was shortened with increasing the casting speed.     

Effect of vacuum on solidification process and microstructure of LFC magnesium alloy

Lost foam casting （LFC） is regarded as a cost-effective, environment-friendly vital option to the conventional casting process for production of near-net shape castings with high quality. Effect of vacuum on the solidification process and microstructure of LFC magnesium alloy were explored. The results indicate that vacuum plays a very important role in the heat transfer during mould filling and solidification periods, it increases the cooling rate of the filling melt, but greatly decreases the cooling rate of the casting during solidification period, and the solidification time of the casting is greater than that without vacuum. The microstructure of LFC magnesium alloy is rather coarse. Compared with that without vacuum, the microstructure of the LFC magnesium alloy under vacuum is more refined and has less precipitated β-phase, which is formed at the grain boundry and around the Al-Mn compound particle.     

Constitutive description of casting aluminum alloy based on cylindrical void-cell model

Casting aluminum alloys are highly heterogeneous materials with different types of voids that affect the mechanical properties of the material. Through the analysis of a cylindrical void-cell model the evolution equation of the voids was obtained. The evolution equation was embedded into a nonclassical elastoplastic constitutive relation, and an elastoplastic constitutive relation involving void evolution was obtained. A corresponding finite element procedure was developed and applied to the analyses of the distributions of the axial stress and porosity of notched cylindrical specimens of casting aluminum alloy A101. The computed results show good agreement with experimental data.     

基于模块化的快速工艺制定

针对零件的多样化和零件之间局部特征相似性,提出了工艺模块化和工艺模块的概念;研究了工艺模块的横向拓展、纵向拓展和复合拓展方法,可以提高工艺设计效率,实现工艺的快速制定。     

Effect of electromagnetic casting process on microstructure and properties of AZ31 magnesium alloy

ABSTRACT

In this paper, AZ31 magnesium alloy is cast by applying the semi-continuous casting process with a low-frequency electromagnetic field. By studying the influence of electromagnetic field frequency, excitation current intensity and casting velocity on the microstructure and mechanical properties, the optimum process under oil-slip electromagnetic casting conditions was determined to improve the degree of grain refinement, yield strength, elongation and tensile strength of AZ31 alloy. An improved microstructure refining effect and higher hardness can be obtained with a current intensity af 60 A. The microstructures and mechanical properties obtained for different casting velocities of V = 200 mm/min and V = 230 mm/min at processing parameters of f = 30 Hz and I = 120 A were compared. Our results suggest that a higher casting speed does not lead to grain refinement or improved mechanical properties. Frequency