Al 4043 is commonly used as a filler material for welding of aluminium-based components. In the present research work, Al 4043/xTiB2 (titanium diboride) microcomposites (x = 2, 4, 6, 8 and 10 wt%) are fabricated by dilution of Al 4043/10TiB2 in situ master composites synthesized by salt-melt reaction. To synthesize the nanocomposites, in situ Al 4043/10TiB2 composites are re-melted, diluted and ultrasonic-treated for 5 min. The microstructural analysis revealed that ultrasonic treatment (UT) is beneficial on improving the dispersion and refinement of TiB2 particles. UT of Al 4043/2TiB2 composites reduced the size of TiB2 particles from ~ 1270 to ~ 25 nm. At the same time, the efficiency of UT on agglomerate breakage and particle size reduction decreases with the increase in amount of reinforcement. The hindrance generated by increased amount of reinforcements to ultrasonic wave propagation and allied impairment on cavitation implosion is proposed as the reason for the reduction in efficiency of UT. The mechanical property analysis revealed that UT-assisted size reduction and agglomerate breakage of TiB2 particles significantly increase both hardness and compressive strength of in situ composites. The results revealed that the compressive strength of TiB2-reinforced Al 4043/10TiB2 micro- and nanocomposite increased up to ~ 90 and ~ 150 MPa, respectively. Also, a significant improvement in hardness was observed in the reinforced Al 4043/10TiB2 micro- and nanocomposites by ~ 94% and ~ 98%, respectively, compared to the monolithic alloy. © 2021, King Fahd University of Petroleum & Minerals.