Defect formation is a critical challenge for powder-based metal additive manufacturing(AM).Current understanding on the three important issues including formation mechanism,influence and control method of metal AM def...Defect formation is a critical challenge for powder-based metal additive manufacturing(AM).Current understanding on the three important issues including formation mechanism,influence and control method of metal AM defects should be updated.In this review paper,multi-scale defects in AMed metals and alloys are identified and for the first time classified into three categories,including geometry related,surface integrity related and microstructural defects.In particular,the microstructural defects are further divided into internal cracks and pores,textured columnar grains,compositional defects and dislocation cells.The root causes of the multi-scale defects are discussed.The key factors that affect the defect formation are identified and analyzed.The detection methods and modeling of the multi-scale defects are briefly introduced.The effects of the multi-scale defects on the mechanical properties especially for tensile properties and fatigue performance of AMed metallic components are reviewed.Various control and mitigation methods for the corresponding defects,include process parameter control,post processing,alloy design and hybrid AM techniques,are summarized and discussed.From research aspect,current research gaps and future prospects from three important aspects of the multi-scale AM defects are identified and delineated.展开更多
This paper examines the performance of three families of percussive drilling methods,Churn(CD),Down-the-Hole(DTH),and Top-Hammer(TH),through the prism of time scales.These time scales characterize different aspects of...This paper examines the performance of three families of percussive drilling methods,Churn(CD),Down-the-Hole(DTH),and Top-Hammer(TH),through the prism of time scales.These time scales characterize different aspects of the dynamics of the drilling process.One time scale represents the travel time of an elastic perturbation in the hammer,while another one corresponds to the travel time in the drillstring and/or the bit assembly.The duration of the response of the bit/rock interface to an impulse load,and the duration of the pulse generated by the impact of the hammer in DTH and TH tools are two other time scales.Within the simplified modeling framework considered in this study,the dynamics of the percussion tools is at most controlled by three numbers,which are ratios of time scales.However,some of these numbers could be irrelevant depending on the design and class of the percussion drilling system,because they are either too small or too large and thus do not affect the dynamical response.For example,the energy transfer efficiency-the fraction of the impact energy effectively delivered to the rock-depends on one number when drilling with a TH tool,but on three numbers for a hydraulically powered DTH tool.This approach enables the identification of the point of maximum efficiency in the parametric space of the time scale ratios.The so-called sweet spot can thus be understood as representing an optimum match of the different timescales characterizing the drilling system.For instance,maximum performance is achieved with a TH tool if the two time scales controlling its dynamics are equal.展开更多
基金the funding support to this research via the projects of ZVMR,BBAT and ZE1W from The Hong Kong Polytechnic Universityproject#RNE-p2–21 of the Shun Hing Institute of Advanced EngineeringThe Chinese University of Hong Kong and the GRF projects(Nos.15223520 and 15228621)。
文摘Defect formation is a critical challenge for powder-based metal additive manufacturing(AM).Current understanding on the three important issues including formation mechanism,influence and control method of metal AM defects should be updated.In this review paper,multi-scale defects in AMed metals and alloys are identified and for the first time classified into three categories,including geometry related,surface integrity related and microstructural defects.In particular,the microstructural defects are further divided into internal cracks and pores,textured columnar grains,compositional defects and dislocation cells.The root causes of the multi-scale defects are discussed.The key factors that affect the defect formation are identified and analyzed.The detection methods and modeling of the multi-scale defects are briefly introduced.The effects of the multi-scale defects on the mechanical properties especially for tensile properties and fatigue performance of AMed metallic components are reviewed.Various control and mitigation methods for the corresponding defects,include process parameter control,post processing,alloy design and hybrid AM techniques,are summarized and discussed.From research aspect,current research gaps and future prospects from three important aspects of the multi-scale AM defects are identified and delineated.
基金This study was a part of the research project INNO-Drill(Technology platform for research-based innovations in deep geothermal drilling)funded by The Research Council of Norway(grant 254984)industry partners.
文摘This paper examines the performance of three families of percussive drilling methods,Churn(CD),Down-the-Hole(DTH),and Top-Hammer(TH),through the prism of time scales.These time scales characterize different aspects of the dynamics of the drilling process.One time scale represents the travel time of an elastic perturbation in the hammer,while another one corresponds to the travel time in the drillstring and/or the bit assembly.The duration of the response of the bit/rock interface to an impulse load,and the duration of the pulse generated by the impact of the hammer in DTH and TH tools are two other time scales.Within the simplified modeling framework considered in this study,the dynamics of the percussion tools is at most controlled by three numbers,which are ratios of time scales.However,some of these numbers could be irrelevant depending on the design and class of the percussion drilling system,because they are either too small or too large and thus do not affect the dynamical response.For example,the energy transfer efficiency-the fraction of the impact energy effectively delivered to the rock-depends on one number when drilling with a TH tool,but on three numbers for a hydraulically powered DTH tool.This approach enables the identification of the point of maximum efficiency in the parametric space of the time scale ratios.The so-called sweet spot can thus be understood as representing an optimum match of the different timescales characterizing the drilling system.For instance,maximum performance is achieved with a TH tool if the two time scales controlling its dynamics are equal.
