What causes poor weld strength during ultrasonic metal welding?

1. Material-Related Factors
Impurities or oil contamination on the material surface: These contaminants hinder contact and diffusion between metal atoms. For instance, oil residue on the metal surface can form an insulating layer during the welding process, preventing direct contact between the metals and resulting in a weak weld.

Poor material compatibility: If the differences in crystal lattice structures, thermophysical properties, and other characteristics between the two metal materials are too significant, it becomes difficult for their atoms to diffuse into and fuse with one another, thereby severely compromising the strength of the weld.

2. Equipment-Related Factors
Unstable equipment performance: Fluctuations in parameters-such as output power and frequency-within the ultrasonic equipment during the welding process can negatively impact weld quality. For example, if the equipment's transducer ages or malfunctions, it can lead to unstable ultrasonic energy output, resulting in a weak weld.

Worn or damaged welding horn: The welding horn is a critical component responsible for transmitting ultrasonic energy; if it becomes worn or damaged, it cannot effectively transfer energy to the metal materials, thereby compromising the welding outcome.

(1) Poor Ultrasonic Conversion Efficiency

When ultrasonic conversion efficiency is low, the input electrical energy is not effectively converted into the mechanical vibrational energy required for welding. This means that insufficient energy is delivered to the metal welding interface, preventing the metals from reaching the specific temperature and degree of plastic deformation necessary for a high-quality weld. For instance, when welding metals with high melting points or high tensile strength, sufficient energy is required to overcome the interatomic forces within the metals and to generate adequate friction and plastic flow at the surface, thereby facilitating atomic diffusion and bonding. If the conversion efficiency is poor and energy is insufficient, the resulting weld will be weak; the joint may consist merely of simple physical contact rather than a robust metallurgical bond.

(2) Poor Ultrasonic Welding Compatibility

Poor welding compatibility indicates a mismatch between the ultrasonic vibration characteristics output by the equipment and the specific metal materials being welded. During the welding process, this mismatch prevents the metal materials from fusing together effectively. When welding combinations of different materials-such as dissimilar metals-equipment with poor compatibility is unable to properly coordinate the fusion process between the two metals. For example, when welding aluminum and copper, their distinct physical and chemical properties require the equipment to provide specific vibration conditions that facilitate the diffusion and bonding of the atoms from both metals at the interface. If the equipment exhibits poor compatibility, the two metals may fuse unevenly; this results in reduced strength at the welded joint and leads to an insecure weld.