Influence of core box vents distribution on flow dynamics of core shooting process based on experiment and numerical simulation

Core shooting process plays a decisive role in the quality of sand cores, and core box vents distribution is one of the most important factor determining the effectiveness of core shooting process. In this paper, the influence of core box vents distribution on the flow dynamics of core shooting process was investigated based on in situ experimental observations with transparent core box, high-speed camera and pressure measuring system. Attention was focused on the variation of both the flow behavior of sand and pressure curves due to different vents distribution. Taking both kinetic and frictional stress into account, a kinetic-frictional constitutive model was established to describe the internal momentum transfer in the solid phase. Two-fluid model(TFM) simulation was then performed and good agreement was achieved between the experimental and simulated results on both the flow behavior of sand and the pressure curves. It was found that vents distribution has direct effect on the pressure difference of different locations in the core box, which determines the buoyancy force exerting on the sand particles and significantly influences the filling process of core sand.     

铝、镁合金真空低压消失模壳型铸造的缺陷分析

分析了采用泡沫模用精密铸造制壳,并用真空低压浇注成形铝、镁合金铸件的典型缺陷(主要是浇不足和冷隔、孔洞及粘砂缺陷)的行貌特征和形成机理,并提出相应的防治措施。结果表明,浇不足和冷隔缺陷主要由金属液充型速度慢、充型动力不足、浇注温度低等原因造成;孔洞缺陷主要由金属液浇注速度过快、保压压力小或保压时间短,浇注温度高,浇注系统设计不合理及型壳透气性差等原因造成。粘砂缺陷主要由型壳强度低、充型压力和真空度过大、造型型砂粒径大和激冷效果差等原因所致。     

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.     

厚大球铁曲轴断裂原因分析

QT700-2大截面曲轴在浇涛后的拆箱过程中发生断裂现象,经分析主要是由于曲轴含Mn量偏高、含Si量偏低,浇铸后冷却不当形成晶界碳化物;并且铸件在砂箱中冷透,形成较大的内应力,拆箱时造成脆性断裂。经调整化学成分,工艺改进,最终避免了此类现象的再发生。     

冷芯盒树脂砂对湿型砂系统的影响

冷芯盒树脂砂进入湿型砂系统会使湿型砂性能和铸件表面质量下降。为此对不同粒度新砂、不同加入量对型砂粒度组成的影响进行了研究。结果发现：当粒度为50／100的旧砂中加入1．5％～2．5％的粒度规格为70／140的细砂时，就可以有效改变型砂的粒度组成，使湿型砂的各项性能以及铸件表面质量明显改善。     

冷芯盒造型工艺与制芯技术的研究

论述了目前使用较广泛的冷芯盒工艺的一些关键工序,参考德国铸造现状,着重从冷芯盒的树脂、原砂、混砂工艺、芯盒设计等方面进行了分析。对一些常见的铸造缺陷(例如脉纹),以及混制后砂型输送、射芯机制芯个数、型砂存放时间控制等作了简要的分析和建议。     

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.     

Development of efficient production routes based on strip casting for advanced high strength steels for crash-relevant parts

Twin roll strip casting can be an effective alternative to produce high manganese TWIP steel, which provides extraordinary mechanical properties. In the work presented, 1.5–3 mm thin hot strips with up to 30 wt% manganese were produced directly from the melt and further processed to cold strip. An adapted thermo-mechanical treatment, consisting of cold rolling with or without subsequent annealing, enables to adjust different material states, such as recrystallised or strengthened state, and thus to tailor the mechanical properties. As an example for the superior crash behaviour of high manganese TWIP steels, dynamic crash tests were carried out using cold rolled Fe–29Mn–0.3C steel in different material states.     

铝合金铸件冷芯用球形砂的选择与应用

为了在铝合金缸体铸件的生产中更好地应用陶瓷球形砂,对铸铝用球形砂的理化及制芯工艺性能进行了全面的分析检测.对球形砂的粒度、酸耗值和高温膨胀性能对冷芯质量的影响进行了实验室试验及生产验证,并和其他芯砂进行了对比.经试验,挑选出的陶瓷球形砂(或陶瓷砂)性能良好,在一些铝缸体的冷芯上得到了成功应用,解决了水套芯、油道芯易变形,粘砂和气孔等问题.     

Mechanical properties and microstructures of AlSi7 alloy cast in sand and in polystyrene-containing sand moulds

Experimental work has been undertaken to study the effects of polystyrene pattern material on the mechanical properties and microstructures of cast aluminium alloys. This paper reports results obtained using Al—Si 7% (LM25) alloy. Rectangular tensile test-pieces of various thickness were cast at different pouring temperatures into standard resin-bonded sand moulds. The experiments were then repeated with polystyrene patterns placed into the sand mould cavities. A direct comparison of the effects of polystyrene pattern material on the properties of the cast test-pieces with those obtained under identical conditions by a standard sand casting method has thus been obtained. Ultimate tensile strength, elongation, Vickers hardness, porosity volume, and dendrite arm spacing (DAS) values relating to both methods were evaluated for a range of casting section thickness (4–16 mm) and pouring temperature (690–780 °C). The microstructural differences observed between the test pieces obtained, with and without polystyrene patterns, were verified by the changes in the solidification cooling curves recorded simultaneously for both methods.

The results obtained show that an expanded polystyrene pattern contained within a sand mould, under the experimental conditions used, does not have an adverse effect on the as-cast mechanical properties of LM25 alloy. On the contrary, the presence of polystyrene in the sand moulds resulted in higher rather than lower tensile properties. These findings have been supported by microstructural observations which reveal finer microstructures and lower volumes of porosity in the test pieces produced with the use of polystyrene, rather than in the absence of it. These observations are further supported by the evidence obtained from the cooling curves which reveal that the presence of polystyrene in the sand mould results in a faster casting cooling rate compared with that when no polystyrene is present.     

精密组芯造型工艺的应用及展望

随着树脂自硬砂的发展,传统组芯造型工艺在大型、复杂铸件生产中得到大量应用。采用三乙胺冷芯盒法的自动化精密组芯造型生产线已成功应用于汽车零件铸造,如整铸式铸铝冷凝换热器,发动机缸体、缸盖等尺寸精度要求高、工艺复杂的中小铸件的批量生产。综述了三乙胺冷芯盒法的精密组芯造型工艺的发展及其应用,该工艺是砂型铸造实现绿色化、智能化生产的重要途径之一。进一步提高原材料和设备的国产化率,强化生产中的环保措施,无机粘结剂及相关设备的研制与工艺开发是今后发展的方向。     

The effect of various production parameters on the heat absorbing capacity of greensand

Greensand is a widely-used material to produce sand moulds in high production, mainly for iron casting applications. Several authors have studied the high temperature behavior of greensand regarding to the vapour condensation and transfer zones, and the effect of different production parameters on the thermal conductivity. This work focuses on the effect of the mixture composition on the heat absorption (cooling) capacity of the greensand. During the experimental work, various greensand mixtures with changing bentonite + water and coal dust contents were prepared and tested by a novel application of Fourier thermal analysis. The method provides new thermophysical knowledge about moulding sand, through applying real casting conditions by the immersion of spherical sand samples into liquid iron. The findings of the work direclty contribute to the area of thermal sciences and also brings novel understandings to foundry technology in general.     

气冲造型转移涂料用铸型脱膜剂的研究

在气冲造型条件下对粘土砂转移涂料的脱模剂进行了研究。研究表明，在实验模板加热条件下，以甲基硅油为主的液态复合脱模剂有较好的脱模效果；在模板不加热的情况下，复合型塑性薄膜脱模剂有较好的脱摸效果。     

新型动物胶粘结剂的制备与硬化工艺的研究

为了满足铸造生产中造型、制芯的要求,开发了一种无毒、无污染、高强度、高溃散性的铝合金铸件用的冷芯盒新型动物胶粘结剂。该粘结剂由动物胶、淀粉、醇在催化剂的作用下经过化学反应得到,合成参数：pH值5．5,合成温度80℃,反应时间90min。采用环保节能的CO2冷芯盒法研究其使用性能,砂型初始强度达0．55MPa,24h抗压强度达4．2MPa,性能稳定,满足生产要求。     

Spatio-temporal dynamics of the Portevin–Le Chatelier effect: experiment and modelling

The temporal and spatial features of the Portevin–Le Chatelier plastic instabilities in single and polycrystals of Al–Mg alloys were investigated systematically, with special emphasis being put on the character of the statistical distributions of the stress drops. The effect of strain rate, temperature and the microstructural state of the alloy was studied experimentally. It was shown that an experimentally accessible quantity, the flow stress, governs to a large extent the observed correlation between the variation of the type of serrations and of the stress drop distributions. Computer simulations of the Portevin–Le Chatelier effect were carried out using a simple spatial coupling model. It was demonstrated that the salient features of the complex spatio-temporal behaviour observed experimentally for different microstructural states are adequately reproduced by the model. A comparison between the experimental data and the simulation results suggests that the spatial coupling stems from plastic strain incompatibilities.     

Non-standard tests for process control in chemically bonded sands

Chemically bonded sand cores and molds are more commonly referred to as precision sand systems in the high production automotive powertrain sector. Their behavior in contact with molten metal can lead to casting defects. Consequently, the interaction is of great interest and an important part of metal casting technology. The American Foundry Society(AFS) sand testing is based on physical, mechanical, thermal and chemical properties of the sand system. Foundry engineers have long known that certain AFS sand tests provide limited information regarding control of molding and casting quality. The inadequacy is due to the fact that sand casting processes are inherently thermo-mechanical, thermo-chemical and thermo-physical. Non-standard foundry sand testing has proven useful for laboratory measurement of these characteristics in foundry sand using a disc-shaped specimen. Similarly, the equivalent disc-shaped specimens are used for casting trials. In order to accomplish near-net-shape casting with minimal defects, it is necessary to understand both the properties of the sand system, as well as the interface of molten metal when different binders, additives and/or refractory coatings are used. The methodology for the following non-standard chemically bonded sand tests is described:(1) disc transverse;(2) impact;(3) modified permeability;(4) abrasion;(5) thermal distortion;(6) quick loss on ignition. The data related to the non-standard sand tests were analyzed and interpreted. The test results indicate that there is relatively lower test-to-test variability with the disc-shaped specimens. The non-standard tests were able to discriminate between the chemically bonded polyurethane cold box sand specimens. Further studies should be conducted on various other sand and binder systems as well as on different specimen thicknesses.     

Experimental investigation and numerical analysis of compaction behaviour of moulding sand

Abstract

The compaction behaviour of green sand was studied to assess the applicability of the Cooper–Eaton equation under conditions close to those of real foundry operations. The results indicate that the Cooper–Eaton equation describes the behaviour reasonably well, allowing prediction of the pressure required to achieve sand moulds of reasonable density. A corresponding distinct element method numerical simulation was also attempted. The study contributes useful information to the understanding of green sand compaction. It is clear that spherical sand is preferable to improve filling behaviour and that the gap between mould wall and pattern must be sufficiently large to avoid prevent weak sand compaction.     

Prediction of burn-on and mould penetration in steel casting using simulation

Abstract

Burn-on and penetration defects in steel casting are principally caused by localised overheating of the sand mould or cores. Such overheating can cause liquid metal to compromise the mould surface and entrain onto the surface of the mould. A method has been developed to predict likely burn-on and penetration defect locations as part of a standard casting simulation. The method relies on determining, from simulation results, the locations where the mould is above a certain critical temperature. The critical temperature is generally above the temperature at which the steel is fully solidified. By measuring the time periods during which these locations in the mould are above the critical temperature, burn-on and penetration defects can be predicted. The method is validated through comparison with previous experimental data. Several parametric studies are conducted to investigate the sensitivity of the predictions to the choice of the critical temperature, the interfacial heat transfer coefficient between the steel and the mould, the pouring temperature, and the mould material. The results of one case study are presented where burn-on or penetration defects observed on a production steel casting are successfully predicted.     

Sensitivity analysis of history dependent material mechanical models for numerical simulation of welding process

Abstract

Yield stress of 6013-T6 aluminium alloy was tested on Gleeble 1500D thermal–mechanical system at predesigned temperatures during different typical thermal cycles, in order to accurately reflect the influence of weld thermal history on material properties. The typical thermal cycles were referred to the temperature field simulation results of real welding process. The changes of yield stress were obtained directly from the stress–strain curves generated by the tensile tests. The tests were more accurate than previous publications, where only the yield stresses at room temperature after thermal history were tested or calculated from microstructure evolution model. Experimental results showed that the changes of yield stress during the cooling stage of typical thermal cycles followed one set of curves. These yield stress–temperature curves were different from those during the heating stage. Temperature and temperature history dependent material model M2 and M3 were established based on the experimental results. M2 model was perfectly plastic model while work hardening effect was considered in M3 model. Compared with conventional temperature dependent material model M1, the distributions of longitudinal residual stress and strain obtained with temperature and temperature history dependent models fit better with published results. Yield stress of the material at the weld zone decreased a lot after having experienced weld thermal history and longitudinal compressive plastic strain at the weld zone recovered to some extent during the cooling stage in M2 and M3 models. These were the main causes for lower peak longitudinal residual tensile stress in M2 and M3 models.     

Influence of casting defects on fatigue behaviour of A356 aluminium alloy

Abstract

The fatigue behaviour of A356 cast aluminium alloy under 10⁴ cycles was investigated by a servo-hydraulic machine at the temperature of 150°C. The specimens for the fully reversible tension compression tests were cut from cylinder head castings of automobile engines. The testing data were treated by S–N curve and the staircase method and the mean values of fatigue property coincided with each other by these two methods, though the data scattered with different casting defects. The fracture surfaces of fatigue specimens were examined by OM, SEM and EDS. It was found that in most of the specimens two or more fatigue crack initiations appeared at a fracture surface. Compared with casting defects such as oxide film, inclusion and blowhole, shrinkage porosity is more detrimental to the fatigue behaviour of aluminium alloy castings.