Performance of digital patternless freeze-casting sand mould

Digital patternless freeze-casting technology is a new approach for obtaining frozen sand moulds using digital milling technology. The change law of tensile strength and air permeability of frozen sand moulds (100-mesh and 200-mesh silica sand, and zircon sand moulds) under different freezing temperatures and water contents was studied. Results show that with the decrease of freezing temperature and the increase of water contents, the tensile strength and air permeability of the sand moulds are gradually improved. Meanwhile, computed tomography technology was used to characterize the shape and size of the water film between the sand particles mixed with 4wt.% water. The results show that in silica sand moulds, the form of water film is lumpy, and 200-mesh silica sand moulds have more water films and higher proportion of small-sized water films than 100-mesh silica sand moulds, while in zircon sand moulds, the form of water film is membranous. At the same freezing temperature and water content, the tensile strength of zircon sand mould is the highest, and 100-mesh silica sand mould is the lowest. A comparative solidification experiment of A356 aluminum alloy was carried out in frozen sand mould and resin sand mould. The results show that the primary α-Al phase appears in the form of equiaxed and eutectic silicon phase is needle-like in freezing sand mould casting, but the primary α-Al phase grows in the form of dendrites, and the eutectic silicon phase is coarse needle-like in the resin sand mould casting. The difference of microstructure is caused by the different cooling rate. The cooling rate of A356 aluminum alloy in frozen sand mould is higher than that in resin sand mould.

Coating process of multi-material composite sand mold 3D printing

Sand mold 3 D printing technology is an advanced manufacturing technology which has great flexible manufacturing ability. A multi-material composite sand mold can control the temperature field of metallic parts during the pouring process, while the current sand mold 3 D printing technology can only fabricate a single material sand mold. The casting temperature field can not be adjusted by using single sand mold material with isotropous heat exchange ability during the pouring process. In this work, a kind of novel coating device was designed. Multi-material composite sand molds could be manufactured using the coating device according to the casting process demands of the final parts. The influences of curing agent content, coating velocity and scraper shape on compactness and surface roughness of the sand layer(silica sand and zircon sand) were studied. The shapes and sizes of transition intervals of two kinds of sand granules were also tested. The results show that, with the increase of the added volume of curing agent, the compactness of sand layer reduces and the surface roughness value rises. With the increase of the velocity of the coating device, the compactness of sand layer reduces and the surface roughness value rises similarly. In addition, the scraper with a dip angle of 72 degrees could increase the compactness value of the sand layer. The criteria of quality parmeters of the coating procedure are obtained. That is, the surface roughness(δ) of sand layer should be equal to or lesser than half of main size of the sand particles(Dm). The parameter H of the coating device which is the distance between the base of hopper and the surface of sand layer impacts the size of transition zone. The width of the transition zone is in direct proportion to the parameter H, qualitatively. Through the optimization of the coating device, high quality of multi-material sand layers can be obtained. This will provide a solution in manufacturing the multi-material composite sand mold.

High-quality manufacturing method ofcomplicated castings based on multi-materialhybrid moulding process

A multi-material hybrid patternless moulding process for complicated castings has been proposed. Moulding sands used in the hybrid moulding process include silica sand, ceramic sand, chromite sand, zircon sand, and steel shot sand. Experimental method was used to study the effects of moulding sands on the temperature field, mechanical properties, and dimensional precision of the iron castings. Under the condition that the wall thickness on different sides of the casting is the same, when the wall thickness is greater than 10 mm, the heat storage capacity of the moulding sands from strong to weak is steel shot sand, zircon sand, chromite sand, ceramic foundry sand, and silica sand. Tensile strength of the obtained castings from high to low is zircon sand, chromite sand, steel shot sand, ceramic sand, and silica sand. Contraction rate of the obtained castings from high to low is steel shot sand, zircon sand, chromite sand, silica sand, and ceramic sand. Therefore, steel shot sand and zircon sand can be used as chilled sand, and even can be used instead of cold iron when the casting wall thickness is greater than 10 mm. Zircon sand and chromite sand can be used to obtain high mechanical properties, and silica sand and ceramic sand can be selected to obtain high dimensional precision of the castings. Finally, a typical iron casting piece was tested by experiment using the hybrid moulding process. Excellent performances of iron castings confirm the feasibility of the hybrid moulding process.

A Model for Predicting Dynamic Cutting Forces in Sand Mould Milling with Orthogonal Cutting

Cutting force is one of the research hotspots in direct sand mould milling because the cutting force directly a ects the machining quality and tool wear. Unlike metals, sand mould is a heterogeneous discrete deposition material. There is still a lack of theoretical research on the cutting force. In order to realize the prediction and control of the cut?ting force in the sand mould milling process, an analytical model of cutting force is proposed based on the unequal division shear zone model of orthogonal cutting. The deformation velocity relations of the chip within the orthogonal cutting shear zone are analyzed first. According to the flow behavior of granular, the unequal division shear zone model of sand mould is presented, in which the governing equations of shear strain rate, strain and velocity are estab?lished. The constitutive relationship of quasi?solid–liquid transition is introduced to build the 2D constitutive equation and deduce the cutting stress in the mould shear zone. According to the cutting geometric relations of up milling with straight cutting edge and the transformation relationship between cutting stress and cutting force, the dynamic cutting forces are predicted for di erent milling conditions. Compared with the experimental results, the predicted results show good agreement, indicating that the predictive model of cutting force in milling sand mould is validated. Therefore, the proposed model can provide the theoretical guidance for cutting force control in high e ciency mill?ing sand mould.

Simulation and experimental research on extra-squeeze forming method during gradient sand molding

The flexible extrusion forming process (FEFP) is a sand mold patternless manufacturing technology that enables digital near-net shaping of complex sand molds. But, it is difficult to achieve the gradient sand molds with high surface strength and strong interior permeability by FEFP. To solve this problem, an extra-squeeze forming method based on FEFP for gradient sand mold was developed. To further reveal the extra-squeeze forming mechanism, based on the Johnson-Kendall-Roberts (JKR) theory and “gluing” notions, the single and double-sided squeeze models of gradient sand molds were established using the EDEM software. The squeezing processes of sand molds with different cavity depths of 60, 100, 140, 180, and 220 mm were systemically studied under single and double-sided squeeze conditions. The variation in the void fraction of sand mold as also investigated at a variety of extra-squeeze distances of 2, 3, 4, 5, and 6 mm, respectively. Simulation and test results show that a deeper cavity depth weakens the extrusion force transmission, which leads to a decrease in strength. The sand mold permeability and void fraction are identified to be positively correlated, while the tensile strength and void fraction appear to be negatively correlated. The void fraction of sand molds decreases with a longer extra-squeeze distance. A 6 mm extra-squeeze distance for the sand mold with 220 mm cavity depth results in a 26.8% increase in tensile strength with only a 5.7% reduction in the permeability. Hence, the extra- squeeze forming method can improve the quality of the sand mold by producing a gradient sand mold with high surface strength and strong interior permeability.

Method for near-net forming of a sand mold with digital flexible extrusion technology

In order to further improve the precision forming efficiency of a sand mold digital patternless casting and reduce the amount of sand mold cutting, a method for near-net forming of the sand mold with digital flexible extrusion technology was put forward. The theory, optimization algorithm and technology for sand mold nearnet forming were studied. Experimental results show that the sand mold forming efficiency can be increased by 34%, and the molding sand can be reduced by 44%. The method for near-net forming of a sand mold with digital flexible extrusion technology can effectively promote the application of digital patternless casting technology in the mass production of castings and thus greatly improves the efficiency and automation of sand mold manufacturing.

Control over Mechanical Properties and Microstructure of BR1500HS Hot-stamped Parts

The hot stamping processing parameters are of critical importance in transforming ultra-high-strength steel （UHSS） into high-quality parts, which were studied by mechanical properties tests, metallographic observations and calculation analysis method based on hot stamping experiments and numerical simulation technology, the mechanical properties, thickness, dimensional accuracy, and microstructure of the hot formed parts are analyzed to determine the influence of different processing parameters for UHSS parts formed from BR1500HS. The results indicate that the quenching time had the most significant impact on the mechanical properties of the parts, and longer quenching time resulted in better mechanical properties. In addition, the pressing speed had a significant influence on the thick ness of the formed parts, and the part-opening temperature had the most significant effect on the dimensional accura cy of the parts. And to get hot stamped parts with excellent quality, the optimum process conditions should be set as heating temperature of 930 ℃, soaking time of 4 min, stamping force of 7 MPa, pressing speed of 75 mm/s, quench ing time of 15 s, and water-flow rate of 1.1 m/s.