《声音是怎样产生的》教学谈

虽然我们仍用着《自然》教材,但如何转变教学观念,运用新课标的理念重新审视《自然》教材,赋予“旧教材”以“新生命”,尽快地和新教材接轨,是摆在每一个自然教师面前的一项艰巨的任务。尤其是如何尊重学生的个体差异,引导学生参与教学评价,促进每个学生的健康发展等方面更值得我们研究。我在教学《声音是怎样产生的》一课时作了一些有益的尝试。教学目标1.注重事实、敢于提出不同见解。2.能够提出问题、作出假设,积极和同学合作进行实验。3.了解声音产生的原因。教学片断(背景:前一课鼓励学生确定与声音有关的课题,在教师指导下创造性地设…     

Correlation between process parameters and primary dendrite arm spacing in laser welding of Cu and Al

A combined numerical model of thermal field and the primary dendrite arm spacing （PDAS） was proposed to correlate the process parameters and PDAS in laser welding of Cu and A1. The solidification parameters simulated by the finite volume method with commercial software ANASYS FLUENT were applied in the PDAS model to predict the dendrite arm spacing of fusion zone. Dendrite was also examined by the metallographic method to validate the model. Results indicate that the calculated PDAS agrees with metallographic measurements reasonably, especially the Hunt model. PDAS increases apparently with increasing laser power while decreases slightly with increasing welding speed. Increasing laser power increases the secondary dendrite and increasing welding speed increases the microporosity in dendrite.