镀层视为多孔材料的前提假设

多孔材料可以广义地定义为由大量的几何尺寸基本属于同数量级的颗粒所构成的，存在孔洞或空院的固体材料，其物理性质介于固体和液体的中间状态。据此分析，机械镀锌形成镀层过程的锌粉吸附、沉积层未致密化之前属于多孔材料，致密化过程中也属于多孔材料，致密化完成后镀层中仍存在定的空院，但此时镀层的物理性质基本和固体锌接近，故致密化结束后的镀层不应该视为多孔材料。因此，锌粉吸附沉积后镀层的形成过程可以视为多孔材料的压缩紧实过程。

锌粉在镀层致密化之前或致密化过程是一种松散的非连续介质，锌粉颗粒之间存在空隙，其变形行为与块状致密体材料有着明显的区别，如多孔材料压制成形时，表现出一系列的流变特性:应变推迟、应力松弛、压制蠕变、弹性失效等;多孔材料在塑性变形时，体积与密度均发生较大的变化。因此，传统块体材料的塑性变形理论已不再适用于镀层的致密化过程。目前，对金属粉末冷压缩的试验和理论研究进行的相对较少]，人们虽提出了一些数学模型，但都难以拟合并指导试验和加工过程。

金属粉末在压实之前，其相对密度低于块体材料的64%，并且粉末的紧实过程一般包括两个过程:①其微观过程可离散成有颈连接的小颗粒，其相对密度可达90%;②具有更高的相对密度，微观组织包括许多界面和独立空位。而对于实际的粉体多孔材料，未紧实之前只有少部分或基本没有颗粒之间存在连接颈。文献[22, 23]中列举了多个涉及粉末多孔材料的压缩模型，这些模型都基本假设在偏应力作用下会产生椭球形屈服面。而金属粉末的实际压缩过程受力情况非常复杂，既有压应力，又有切应力;既有接触颗粒之间力的传递，又有空院位置对力的阻断。因此，现有的粉末压缩模型都不适于机械镀锌层的致密化分析。

Porous materials can be defined broadly as solid materials consisting of a large number of particles of the same order of magnitude with holes or cavities, whose physical properties lie in the intermediate state between solid and liquid. According to this analysis, the zinc powder adsorption in the process of mechanical zinc plating and the deposit layer before densification belong to porous material, and also belong to porous material in the process of densification. There are still some vacancies in the deposit after densification, but the physical properties of the deposit are basically close to solid zinc, so the deposit after densification should not be regarded as porous material. Therefore, the formation process of the coating after zinc powder adsorption and deposition can be regarded as the compression and compaction process of porous materials.
Zinc powder is a loose discontinuous medium before or during the densification process of coating. There are gaps between zinc powder particles. Its deformation behavior is obviously different from that of bulk compact materials. For example, when porous materials are compacted, they show a series of rheological characteristics: strain delay, stress relaxation, compression creep, elastic failure, etc. When porous materials are plastic deformation, volume. There were great changes in density and density. Therefore, the traditional plastic deformation theory of bulk materials is no longer applicable to the densification process of coatings. At present, there are relatively few experimental and theoretical studies on cold compression of metal powders]. Although some mathematical models have been proposed, they are difficult to fit and guide the experimental and processing process.
Before compaction, the relative density of metal powder is lower than 64% of that of bulk material, and the compaction process of metal powder generally includes two processes: (1) its micro-process can be dispersed into small particles with neck connection, and its relative density can reach 90%; (2) it has higher relative density, and the micro-structure includes many interfaces and independent vacancies. For the actual powder porous materials, there are only a few or almost no connecting necks between particles before compaction. Several compression models involving powder porous materials are listed in reference [22, 23]. These models basically assume that ellipsoidal yield surfaces will occur under biased stress. The actual compression process of metal powder is very complicated, including compressive stress and shear stress, force transfer between contacting particles and force blocking by empty space. Therefore, the existing powder compression models are not suitable for the densification analysis of mechanical galvanizing coatings.

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