Thermal Modeling of Metal Powder-Based Selective Laser Sintering

In order to get a better understanding of Selective Laser Sintering (SLS) process of the metal powders, three-dimensional modeling of laser sintering of a metal powder mixture that contains two kinds of metal powder with significantly different melting points under a moving Gaussian laser beam is investigated numerically. Laser induced melting and resolidification accompanied by shrinkage are modeled using a temperature transforming model. The liquid flow of the melted low melting point metal driven by capillary and gravity forces is also included in the physical model. Both complete and partial shrinkages are considered in the model. Simulations are performed for both single line laser scanning and multiple-line laser scanning. The numerical results are compared with experimental results and a detailed parametric study is performed. The effects of the moving heat source intensity, the scanning velocity, the thickness of the powder layer and the number of existing sintered layers underneath on the sintering depth, the shape of the heat affected zone (HAZ) and the temperature distribution are discussed. The optimized dimensionless moving heat source intensity increases with increasing scanning velocity in order to achieve the desired sintering depth and bond the newly sintered layer to the previously sintered layers.Mechanical Engineerin

Chinese Transnational Adolescents’ Responses to Multicultural Children’s Literature in Culture Circles

The purpose of this study is to examine how Chinese transnational adolescents (CTAs) negotiate their identity based on their cultural knowledge and experiences through book discussion in Freirean “culture circle” (Freire, 2000, p. 120). This study is an interpretivist qualitative study of community-based action research (Glesne, 2010). The participants were seven American-born Chinese, two current Chinese and Taiwanese, and one Chinese adopted adolescent. Within the culture circles, CTAs responded to seven selected multicultural children’s literature which represents Chinese immigrants’ stories in the United States. The topics of the books included (1) who am I, (2) relationships with extended family I, (3) relationships with extended family II, (4) Chinese immigrants’ histories in the U.S., (5) holiday celebrations in the U.S., and (6) dream pursuits. This study was followed by the interweaving of Rosenblatt’s reader response theory (Rosenblatt, 2005), Holland’s psychoanalytic response- Defense, Expect, Fantasy, Transformation (DEFT) model (Holland, 1975), and Bleich’s literacy community formation (Bleich, 1986) in the culture circle setting. A variety of literacy activities (i.e., literary responses, group discussion, graphic organizers, and videos, etc.) were also incorporated in order to enrich interactive dialogues and investigate how CTAs (re)construct their identity through a critical lens. The culture circle meeting took place for two hours on Saturday afternoons over the course of six weeks. Data sources included video records of culture circles, participants’ literary response journals, artifacts, and researcher’s notes. The data analysis proceeded with transcription, pattern coding, and triangulation. The data showed that two primary stages occurred over the six weeks of the culture circles. The first stage was an invitation, that is, CTAs and I made our life experience connections. The CTAs were involved in conversation through generative themes including shared communal experiences, building knowledge together, recognizing living in bi-cultural contexts, juxtaposing and negotiating Chinese immigrants’ history and stories of the literature, and reconnecting cultural heritage and roots. The second stage was the process of identity (re)construction: critical inquiries and responses to the texts, examination of social perception and parents\u27 expectations, and identity negotiation. In the continuous process of the culture circle meetings, CTAs formed their unique literacy community around these two stages in the culture circle over six weeks. The findings showed that the collective identities that the CTAs formed as Chinese American+, Taiwanese American, and Chinese resulted from their collective responses to the books and mutual feedback with their bi-cultural life experiences and cultural values. CTAs gained a deeper understanding of and appreciation for their intercultural fellows. Also, they spoke up for themselves and detached from the social stigma of the Asian model minority stereotype. Regarding CTAs’ personal stance (inner identity), the recurring data demonstrate that CTAs became more aware of their ontological existence and critically thought about the environment and context that they were situated in. They came to understand that as long as they continued to explore their identities, they will find who they are in different aspects of life. To conclude, this study may be of importance in explaining the inter-cultural group of Chinese transnational students’ identities, as well as in providing school teachers and the community with a better understanding of how Chinese transnational students’ beliefs and perspectives of living in a bi-cultural context (Chinese family and American society) affect their ever-changing identities and relationships with themselves and others

Two-Dimensional Modeling of Sintering of a Two-Component Metal Powder Layer on Top of Multiple Sintered Layers with a Moving Gaussian Heat Source

Selective Laser Sintering (SLS) of metal powder is modeled as a two-dimensional melting and resolidification of a loose powder layer on top of the sintered metal layers with a moving heat source. The shrinkage induced by melting is accounted for and the problem is modeled using a temperature-transforming model. The results indicate that both the moving heat source intensity and scanning velocity have significant effects on the sintering process. Since the thermal conductivity of the sintered layer is relatively high compared with that of the loose powder, higher heat source intensity and lower scanning velocity are needed to achieve complete melting of the loose powder and bond the current layer to the existing sintered layers. A parametric study is performed and the best combinations of the processing parameters are recommended.Support for this work by the Office of Naval Research under grant number N00014-02-1- 0356 is greatly acknowledged.Mechanical Engineerin

Single firm product diffusion model for single-function and fusion products

The prosperity of multifunction products (also referred to as fusion products) has changed the landscape of the marketplace for several electronics products. To illustrate, as fusion products gain popularity in cellular phones and office machines, we observe that single-function products (e.g., stand-alone PDAs and stand-alone scanners) gradually disappear from the market as they are supplanted by fusion products. This paper presents a product diffusion model that captures the diffusion transition from two distinct single-function products into one fusion product. We investigate the optimal launch time of the fusion product in various conditions and conduct a numerical analysis to demonstrate the dynamics among the three products. Similar to previous multi-generation single product diffusion models, we find that the planning horizon, the products\u27 relative profit margin, and substitution effects are important to the launch time decision. However, there are several unique factors that warrant special consideration when a firm introduces a fusion product to the market: the firm\u27s competitive role, buyer consolidation of purchases to a multi-function product, the fusion technology and the age of current single-function products

Contouring Accuracy Improvement Using an Adaptive Feedrate Planning Method for CNC Machine Tools

AbstractThe reduction of contour error plays an important role in achieving high accuracy machining. To reduce contour error, most of previous studies have focused on developing advanced control strategies. As an alternative strategy, contouring accuracy improvement using an adaptive feedrate planning method is proposed in this paper. First, a typical PID controller is adopted to build the contour error model, from which the feedrate can be scheduled in the contour error violated zones. Then, the relations between each constraint and the cutter tip feedrate are derived. After that, a linear programming model is applied to obtain the optimal feedrate profile on the sampling positions of the given tool path. Finally, illustrated examples are given to validate the feasibility and applicability of the proposed feedrate planning method. The comparison results show that the proposed method has a significant effect on improving contouring accuracy