Preparation, Characterization And Engineering Of Closed-cell Aluminum Foam Based On Novel Slow-release Foaming Agent

Abstract:Owing to its structure and function properties, closed-cell aluminum foam has a wide range of applications in automotive, construction and aerospace field, etc. At present, the melt foaming technology using TiH2powder as foaming agent is the main method of preparing closed-cell aluminum foam. However, because of the low initial decomposition temperature and the fast decomposition speed of TiH2foaming agent, there are many defects in the traditional melt foaming process, such as strict technical control conditions and non-uniform pore distribution in the products. In addition, the traditional melt foaming process can not be operated continuously, which restrict the fabrication of large-size product in industry scale and cause the performance of products from different batches with bad consistency. To solve the problems existing in the current melt foaming method, we first adopted heating oxidation and Al2O3coating process for the modification of TiH2foaming agent. Then a novel slow-release foaming agent substituting for TiH2was originally developed. Based on the new slow-release technology, industrial production line was designed and the industrialization was realized, with which large-size aluminum foam products were successfully manufactured. The mechanical properties and thermal properties of aluminum foam products were also studied. The main research results are as follows:(1) The heating oxidation treatment was effective to improve the initial decomposition temperature and delay the decomposition speed of TiH2foaming agent. The main reason was that the oxidation treatment could increase the oxide layer thickness on the surface of TiH2powders. The Al2O3coating process by heterogeneous nucleation method was also studied. The results showed that the main decomposition temperature range of TiH2powder was enlarged to nearly100℃. Despite the heating oxidation and the Al2O3coating process can improve the thermal decomposition characteristics of TiH2powder to some extent, it is still difficult to fundamentally solve the problems such as the strict technical control conditions in the melt foaming process.(2) A novel slow-release foaming agent was synthesized. The thermal decomposition testing results indicated that:①The initial decomposition temperature of the novel foaming agent was608.31℃, much higher than that of modified TiH2powder.②The decomposition rate began to increase rapidly after650℃. The rapid decomposition temperature scope was between700℃and750℃. When the temperature reached755.86℃, the decomposition reaction process was ended.③With the heating rate of5℃/min, the duration of decomposition reaction process can be lasted as long as29.5min.④The complete decomposition time of the novel foaming agent at680℃was more than40min, and the decomposition rate curve was close to a straight line. The thermal performance study results showed that the decomposition speed of the novel foaming agent was slow and steady near the aluminum melt foaming temperature, which was suitable for preparing aluninum foam with controllable process parameters.(3) Closed-cell aluminum foam was prepared by the melt foaming technology using the novel foaming powder as foaming agent. The increase of novel foaming agent could lead to the rise of porosity and foaming efficiency of aluminum foam. When the foaming temperature was below720℃, the porosity increased gradually with the raising of foaming temperature, while the apparent density decreased oppositely. When prolonging the soaked time, the porosity increased, while the foaming efficiency decreased. Closed-cell aluminum foam samples with porosity of75～85%, uniformity structure and average pore diameter of1～4mm could be produced when the novel foaming agent addition of1.2～1.6wt%, foaming temperature of680～720℃, stirring time of2.5～5.5min, stirring rate of1500～2500rpm and holding time of3.0～7.Omin.(4) According to the laboratory research results, a semi-continuous production line based on slow-release foaming technology for large-size aluminum foam was designed. The semi-continuous production line mainly consists of the composite mold and continuous preheating system, the melt fusion and distribution system, the melt foaming system and automatic cooling system. The automatic and semi-continuous industrialized production system based on such design idea was created, with which closed-cell aluminum foams of different melting point could be prepared successfully. Large-size aluminum foam materials with no obvious macro defects, uniformity structure, small free-bubble layer, overall dimensions of2600×800×X mm(X could be adjusted according to the need of cutting) and porosity of75%approximately were produced by this industrialized production system. By controlling the foaming agent, the density-controllable preparation of closed-cell aluminum foam can be realized. These results suggest that the as-developed industrialized preparation technology can effectively increase the yield of closed-cell aluminum foam.(5) The quasi-static compressive properties and bending performances of closed-cell aluminum foams were studied. It could be found that the stress-strain curves had the "three stage" feature, including linear elastic stage, yield platform stage and close-grained change stage. Compared with the product prepared by pure Al, the yield stress of product prepared by the ZLD102alloy was bigger, nearly20MPa; and the yield platform stage had a certain fluctuation, showing some brittle characteristics of porous material. The porosity and the pore size of closed-cell aluminum foams were the key factors for their compression performance. The smaller pore size of closed-cell aluminum foams, the bigger of the relative density and the lower of the pore degrees, the greater of their yield stress; However, the yield stress decreased gradually along with the increase of porosity. The bending capacity of material prepared by the ZLD102alloy(more than15MPa) was obviously higher than that of material prepared by pure Al. With the increase of porosity, the load capacity of material prepared by the ZLD102alloy decreased significantly, and the bending strength declined accordingly.(6) In the study of thermal performance, control volume method was used to derive the discrete equation of aluminum foam during heat conduction process, and the numerical algorithm program of the effective thermal conductivity was compiled. Through comparison of measured and calculated values, it could be found that when porosity was above 83.8%the calculated values matched the measured values well, while the calculated values were higher than the measured values when porosity was below83.8%.(7) Spherical bubble three-dimension model of aluminum foam was built by modeling software, and the relationship among pore wall thickness, porosity and average pore size of aluminum foam model was deduced. Non-uniform closed-cell foam aluminum model with different structural parameters and random pore distribution was established based on C language program using the relationship among pore wall thickness, porosity and average pore size. Effective thermal conductivity increased with the reducing of porosity. For the porous structure of aluminum foam with the same porosity, different pore distribution resulted in different effective thermal conductivity, which indicated that the pore distribution had much impact on thermal conductivity of closed-cell aluminum foam. Through analyzing the temperature field, it could be found that the temperature had mutations in the junction of the matrix and the pore and the temperature gradient in the pores became larger. The distribution temperature in the aluminum foam was non-uniform, which was closely related with the pore size and distribution. In the case of the same porosity, pore structure and distribution were dominating factors of thermal conductivity of closed-cell aluminum foam. The pores which were extended or distributed along the direction perpendicular to heat flow strengthened the obstructive capability for heat flow. When pores were connected along the direction perpendicular to heat flow, a "wall of high thermal resistance" was appeared to decline the thermal conductivity rapidly. This phenomenon showed that only porosity could not completely determine the effective thermal conductivity of closed-cell aluminum foam. Figures(81), tables(6),references(155).