大层厚下选区激光熔化成形316L不锈钢工艺的优化

在铺粉厚度为0.15 mm的大层厚条件下采用选区激光熔化技术制备316L不锈钢试样,研究了激光功率(750~850 W)、扫描速度(800~1000 mm·s^(-1))、扫描间距(0.06~0.10 mm)对其相对密度、显微组织与性能的影响,对选区激光熔化工艺进行了优化。结果表明:随着激光功率、扫描速度或扫描间距的增大,大层厚下成形试样的相对密度都呈先增大再减小的变化趋势。当激光功率为800 W、扫描速度为900 mm·s^(-1)、扫描间距为0.08 mm时,成形试样的拉伸性能最优,其抗拉强度和断后伸长率分别为682.7 MPa和33.4%,与小层厚成形试样相近,此时的显微硬度为224.77 HV,显微组织由尺寸为0.4~1.0μm的等轴晶组成,在质量分数3.5%NaCl溶液中的自腐蚀电位达到-0.69 V,表现出最优的耐腐蚀性能。

选区激光熔化316L大层厚成形工艺及性能研究

针对316L金属粉末进行选区激光熔化(SLM)成形工艺研究,以获得高致密度块体。为了提高SLM成形效率,对150μm大层厚成形过程进行了研究。通过改变曝光时间,块体试样致密度可达到99.99%,成形率可达到8.1mm3/s,约为以往研究中成形率的2~8倍。此外,还发现三种典型缺陷,熔池间未熔合缺陷,熔池内部微孔缺陷和无规律分布的球化缺陷。熔池间未熔合缺陷可通过调整工艺参数予以消除,微孔和球化缺陷对致密度的影响较小,但通过调整工艺参数,很难将其完全消除。平均屈服强度、抗拉强度和延伸率分别为525MPa、625MPa和40.1%,这与以往小层厚成形的拉伸性能没有明显差异,这是由于不同层厚都具有类似的冶金结合和微观结构。

选区激光熔化316L不锈钢大层厚成形质量研究

针对选区激光熔化成形150μm大层厚316L不锈钢进行工艺试验研究,通过对比不同的激光功率、能量密度、曝光时间、点距、线间距对316L不锈钢大层厚成形质量的影响,探索高致密度成形工艺。研究发现,高功率激光参数下成形大层厚试件效果更好,最佳激光能量密度范围是50～70J/mm^3。通过对比小层厚与大层厚成形效果,发现大层厚在成形过程中会产生一些缺陷,通过调整工艺参数可减少缺陷产生。大层厚成形316L不锈钢微观组织主要由等轴晶和少量的柱状晶组成,拉伸力学性能达到锻件标准。

Underground mining of thick coal seams

This paper reviews underground mining methods for total thickness of a thick coal seam in single lift （TI＇rCSSL）. Review shows the required engineering for extraction of thick seams needs to be fitted with thickness of the seam, behavior of rock-mass and surrounding stress conditions for efficient mining. Variants of TI＇rCSSL are able to extract a maximum 10-12 m thickness only. An improvement in bending moment of the overlying coal band in longwall top coal caving （LTCC） provides better under-winning opportunity for the roof coal band. An acceptable limit of 25 MPa compressive strength of coal for the success of LTCC may be increased under favorable geo-technical conditions. Bord and pillar in India adopted induced caving of roof coal band for single lift depillaring of total thickness （SLDTr） of a compe- tent thick coal seam developed along floor. Case studies are given to arrest the adverse effects of extrac- tion height on pillars.

Changes in Lake Ice Cover on the Morskie Oko Lake in Poland(1971-2007)

On the basis of data from the period 1971-2007, and by applying trend analysis, a study on formation, disappearance and duration of lake ice cover on the Morskie Oko Lake in the Tatra Mountains in southern Poland was carried out. The results show decreasing trends in the maximum thickness of winter lake ice cover and in duration of lake ice phenomena, while air temperature recorded at the same period at the foot of the Tatra Mountains shows increasing trend. There are strong relationships between the course of lake ice phenomena and air temperature.

Simulation Analysis of Coal Mining with Top-Coal Caving Under Hard-and-Thick Strata

The top-coal failing ability is a key factor to analyze for the application of coal mining with top-coal caving. Based on a hard-and-thick strata, which acts both as the floor of the upper coal seam and as the roof of the lower coal seam, nine mining projects wcrc put forward to examine the mining of upper and lower coal seams, and a numerical simulation was used to study in detail the corresponding top-coal compressed volume of the lower coal seam. By the simulation effects of different layouts of coalface, the rational mining method was determined to bc the staggered layout of coalface in the upper and the lower coal seam. This can ensure the successful use of fully-mechanized coalfacc with top--coal caving in the lower coal seam.

Back-and-forth mining for hard and thick coal seams—research about the mining technology for fully mechanized caving working face of Datong Mine

The article introduced the key technology, mining process, and back-and-forth mining method for the caving working face of hard-thick coal seams in Datong mine, and researched this innovations process, optimized the systemic design and working face out-play, tried to perfect the caving mining technology of hard-thick coal seams further.

An Assessment of the Groundwater Potential of Bayero University Kano Permanent Site Using Induced Polarization and Self-potential Methods

This paper centers on the investigation of the subsurface condition of Bayero University Kano Permanent Site with the aim of understanding the lithology and also mapping out the groundwater patterns within the area. To achieve this, time domain IP （induced polarization） and SP （self-potential） methods were adopted using VES （vertical electrical sounding） technique with 49 stations sounded. The result of the interpreted and analyzed measured data shows that the area is underlain by two to five subsurface layers. These layers are top soil, laterite, weathered basement complex rocks, fractured basement complex rocks and fresh basement complex rocks. The aquiferous zone of the study area occurs in the weathered and fractured basements and its thickness ranges from 1.44 m to 70.157 m while the overburden thickness lies between 1.6 m and 72.104 m. SP values were plotted against depths of investigation in order to identify areas with greater depth of flow in the study area. From the analysis of the overburden thickness, aquifer thickness and SP values, the most favorable regions for groundwater exploitation were found around VES 6, 11, 13, 19, 26, 38, 44 and 48. The investigation also provides information about the subsurface condition with regards to engineering construction and safe place for refuse dumping in order to avoid groundwater contamination.