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Characteristic Analysis Of Powder Injected And Compacted Green Body By Using X-ray Tomography

Posted on:2016-05-23Degree:DoctorType:Dissertation
Country:ChinaCandidate:S D YangFull Text:PDF
GTID:1228330470959041Subject:Materials Science and Engineering
Abstract/Summary:PDF Full Text Request
X-ray tomography can characterize the spatial inner structure of materials nondestructively, and preparations of the samples (cutting or polishing) for scanning are not needed. This technology is applied to many fields of materials science. Compaction and powder injection moulding are important processes in powder metallurgy. The traditional observation tools such as scanning electron microscopy (SEM) and optical microscopy are not very proper for characterization of the green compacts obtained by the above two processes. Therefore, application of the X-ray tomography to compaction and injection process is significant.The most important data obtained by X-ray tomography is projection and reconstruction images. For accurate analysis of the inner structure in materials, image processing is needed. In this study, the programs of image processing are written by using Matlab. Firstly, Adapt median filter is written for noise removal. Then, based on wavelet transformation and’unsharp’mask, enhancement of the images was performed. Additionally, image morphology was introduced in this paper. By using open operation of this method, the defects in stainless steel green bodies were extracted. Finally, after the previous image process, image segmentation was operated. The different contents in image can be extacted and their volume fraction can be calculated. The accuracy of the image processing developed in this study is verified by the tested results (Archimeds’ principle and metallographs) of aluminum green compacts.The appearance of defects in green body is influenced by the injection parameters. The different kinds of defects in green bodies obtained by different injection parameters (temperature, pressure and speed) were observed by using X-ray tomography, and the formation reasons were also discussed. During mould filling, the serious defects mainly formed in the case of low injection temperature and high injection speed. By comparing the gray value distribution of projection images of green bodies with defects and fine ones, an evaluation standard was obtained to evaluate the quanity of the injection process. To verify the efficiency of the test method obtained by the previous analysis, in this study, the optimization process of the316L stainless steel injection parameters was performed.Powder-binder separation is an inevitable phenomenon in mould filling process. In the past, effective method for the quantitative observation of this phenomenon is very fcw. With the help of X-ray tomography, the linear relationship between the gray value and actual density of standard sample is established. The powder content distribution of SiC green body can be calculated by substituting the gray value into the linear equation. Powder-binder separation can be analyzed on the base of the powder content distribution. Through observation of the powder content distribution, it can be seen that different powder-binder separation phenomenons appear in the green bodies with different initial solid loadings. In the all analyzed samples, binder accumulation regions were mainly found near the gate and die wall. To analyze the effects of mould shape on the powder-binder separation, the powder content distribution in L-shaped green body was analyzed. Moreover, the effects of three injection parameters (injection speed, temperature and injection pressure) on the powder-binder separation during mould filling were analyzed.Density distribution in green compact is a significant problem in powder metallurgy. The linear relationship between gray value and actual density of green compacts were firstly established. Then, density distribution in the6061aluminum alloy green compacts was analyzed. Based on the density distribution, Heckel equation, one of mostly the used theoretical models, was improved to analyze the pressure distribution, and the compressive pressure transmitted ratio and friction force during compaction were also calculated. By the way of the image processing developed in this paper, the binary images were obtained to illustrate the pores in green compacts.3D rendering of green compact was obtained by using the corresponding stack of the binary images. With the help of Amira, statistical morphological analysis (pore size, shape and orientation) of the pores in the green compacts is performed and the results confirm that the morphological characteristics of the pores are different in the case of different compaction conditions.
Keywords/Search Tags:X-ray tomography, Powder injection moulding, Compaction, Powder-binder separation, Density distribution
PDF Full Text Request
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