This paper analyses the effects of job release policies, priority scheduling rules and setup times on the performance of a dynamic job shop in a sequence dependent setup time environment. Two job release policies namely, immediate job release and job release based on a specified work-in-process are investigated. A simulation model of a realistic manufacturing system is developed for detailed analysis. The dynamic total work content method is adopted to assign the due dates of jobs. Six priority rules are applied for prioritizing jobs for processing on machines. Several performance criteria are considered for analyzing the system performance. The simulation results are used to conduct statistical tests. Analytical models have been formulated to represent the simulation model for post-simulation studies. These models are found to yield a satisfactory estimation of the system outputs.
The wear is one of the main issues in friction-stir welding (FSW) of dissimilar aluminium alloys. Two dissimilar metals welded with FSW have different mechanical, metallurgical and chemical properties compared to the parent materials. Therefore, the wear resistance is an important property in an FSW process. In this research, AA7075-T6 and A384.0-T6 aluminium alloys with a thickness of 6.35 mm were selected and a design matrix was developed to produce FSW welds using the MINITAB-17 software. Friction-stir-welded pieces were made as test specimens and their ultimate tensile strengths (UTSs) were tested. In these experiments, the maximum-and minimum-UTS specimens were used for a wear-resistance analysis and its results were compared with the wear resistance of the parent metals. The results were obtained by keeping the revolutions per minute (min-1), time and sliding velocity as constants, while varying the load applied. The rpm, time and sliding velocity were 500, 300 s and 3.141 m/s, respectively. The loads applied were (20, 40 and 60) N. The conclusion is that, under all the applied loads, the wear of the friction-stir weld produced is lower than that of the A384.0-T6 aluminium base metal. On the other hand, the wear of the weld is higher than the AA7075-T6 aluminium base metal.
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