Ultrasonic powder compaction is a rapid low-temperature consolidation technique that can fully densify metal powders in several seconds. Current understanding of the process is derived from qualitative interpretations of relative density measurements and postmortem analyses of compacts. Here, we elucidate the underlying densification mechanisms using in situ measurements of the instantaneous stress state during ultrasonic powder compaction, then infer transitions in the active mechanisms through comparisons with yield criteria for granular materials and porous solids. Our results reveal that by adjusting the process parameters (e.g., oscillation amplitude, normal load), the stress path can be manipulated to encourage certain densification mechanisms. For example, operating with higher applied stresses causes the initially fluidized powder to lock into a jammed network at lower relative densities, prioritizing particle deformation over particle rearrangement. In addition to this continuum-scale analysis, we probe the underlying particle-scale behavior using first-principles models of particle kinematics during the initial densification stages and heating mechanisms and plastic flow in the later stages. This analysis shows that local temperatures due to oscillatory microslip and strain localization are greater than flash heating temperatures, but do not offer significant improvements in densification beyond that due to bulk heating via plastic deformation.

超声波粉末压制是一种快速低温固结技术，可以在几秒钟内使金属粉末完全致密化。目前对该过程的理解来自对相对密度测量的定性解释和压块的死后分析。在这里，我们使用原位阐明了潜在的致密化机制测量超声粉末压实过程中的瞬时应力状态，然后通过与颗粒材料和多孔固体的屈服标准进行比较来推断活性机制的转变。我们的结果表明，通过调整工艺参数（例如，振荡幅度、法向载荷），可以操纵应力路径以促进某些致密化机制。例如，以较高的施加应力操作会导致最初流化的粉末以较低的相对密度锁定到堵塞的网络中，从而优先考虑粒子变形而不是粒子重排。除了这种连续尺度分析之外，我们还使用初始致密化阶段的粒子运动学第一性原理模型以及后期阶段的加热机制和塑性流动来探索潜在的粒子尺度行为。