Ultrasonic Assisted Sintering Using Heat Converted from Mechanical Energy

Abstract

A new sintering method, namely ultrasonic assisted sintering (UAS), has been proposed using mechanical heat converted from high frequency motion between particles. Pure aluminum specimens with diameter of 5 mm and thickness of ~2 mm have been successfully sintered in two seconds. Based on the thermodynamic analysis, the underlying heating mechanism is quantitatively interpreted, which involves high-frequency interparticle friction and plastic deformation driven by ultrasonic squeezing. Consequently, temperature rises rapidly at a speed of about 300 K/s, and the maximum temperature reaches up to 0.9 times of melting point of the aluminum during UAS. The sintered specimens have a high density of dislocations, under the combined effects of dislocations and undulating stress field, volume diffusion coefficient for sintering increases by several orders of magnitude, therefore, rapid densification can be accomplished in seconds. In addition, the sintered aluminum has ultrahigh nanohardness (~1.13 GPa), which can be attributed to the hierarchical structure formed during UAS process.

Thermally Activated Jamming in Ultrasonic Powder Compaction

“…Of course, higher values of this ratio should also drive junction growth and densification. This appears to have been the case in the work on ultrasonic compaction of Al powders by Liu et al, [ 68 ] where we estimate / H was 0.1 at the onset of junction growth.…”

“…2D finite element simulations on the ultrasonic compaction of a stack of wires revealed uniform heating within the charge, as well as thermal softening and resulting densification. [ 4 ] In separate work on ultrasonic compaction of Ti‐base metallic glass powder by Chen et al [ 3 ] and aluminum powder by Liu et al, [ 68 ] interparticle junction growth was attributed to thermal softening from transient flash heating events at particle contacts, as one particle is sheared past another. These authors used a simple frictional heating model to estimate flash heating temperature increments in the range 200–400 K for Ti powder and ≈100 K for pure aluminum, which they argued was sufficient to enable the growth of strong interparticle junctions.…”

“…However, while thermal softening clearly plays an important role in ultrasonic powder compaction, there are several problems with the flash heating mechanism proposed by Chen et al First, the expression that these authors used to estimate the flash heating temperature does not describe flash heating at contacting asperities, but instead gives the steady‐state temperature increment at the interface between a stationary semi‐infinite body and a flat pin moving with constant velocity. [ 69,70 ] Second, Chen et al predicted such high flash heating temperatures in part because they assumed unreasonably high interparticle contact stresses, on the order of 10 GPa, [ 3,68 ] which titanium particles could not physically support. Third, the timescale of the temperature spikes in Figure 3 is on the order of 0.1 s, many orders of magnitude longer than the expected lifetime of a sheared contact, which we estimate later.…”

Investigation of Formation Behaviour of Al–Cu Intermetallic Compounds in Al–50vol%Cu Composites Prepared by Spark Plasma Sintering under High Pressure

“…Yu et al reported the formation of interfacial Al–Cu ICs via cold rolling [ 18 ]. According to their report, the dislocations accumulated near the interfaces owing to compaction pressure weakened the inter-atomic bonding forces between the individual Al (or Cu), leading to their higher diffusivity [ 27 , 28 ]. Thus, it was concluded that in the present work, the ICs in the Al–Cu composites sintered at approximately 100 MPa were formed via thermal diffusion, but those in the Al–Cu composites sintered at 200–250 MPa were formed via deformation.…”

Ultrasound-Assisted Conventional Sintering of Silicate Ceramics

“…Ultrasonic techniques have widely been used for molten alloy material preparation and heat treatment [14][15][16][17][18][19]. However, ultrasonic-assisted sintering techniques are only available at a few hundred degrees Celsius in the current studies [20][21][22][23].…”