The influence of an energy adjustable annular spot laser on the formation and mechanical properties of intermetallic compounds in steel aluminum laser welded lap joints

When connecting steel to aluminum, the reaction between Fe and Al atoms during the connection process forms brittle intermetallic compounds (IMCs). The presence of these IMCs limits the mechanical strength of the connection, therefore it is necessary to control the quantity of these compounds. The reason for the formation of IMCs is that the solubility of Fe in Al is poor. If it exceeds a certain amount, it may affect the mechanical properties of the weld. IMCs have unique properties such as hardness, limited ductility and toughness, and morphological features. Research has found that compared to other IMCs, the Fe2Al5 IMC layer is widely considered the most brittle (11.8 ± 1.8 GPa) IMC phase, and is also the main reason for the decrease in mechanical properties due to welding failure. This paper investigates the remote laser welding process of IF steel and 1050 aluminum using an adjustable ring mode laser, and investigates in depth the influence of laser beam shape on the formation of intermetallic compounds and mechanical properties. By adjusting the core/ring power ratio, it was found that under conduction mode, a core/ring power ratio of 0.2 can achieve better weld interface bonding surface area and significantly reduce the thickness of Fe2Al5 IMC, thereby improving the shear strength of the joint.

This article introduces the influence of adjustable ring mode laser on the formation of intermetallic compounds and mechanical properties during remote laser welding of IF steel and 1050 aluminum. The research results indicate that under conduction mode, a core/ring power ratio of 0.2 provides a larger weld interface bonding surface area, which is reflected by a maximum shear strength of 97.6 N/mm2 (joint efficiency of 71%). In addition, compared to Gaussian beams with a power ratio greater than 1, this significantly reduces the thickness of Fe2Al5 intermetallic compound (IMC) by 62% and the total IMC thickness by 40%. In the perforation mode, cracks and lower shear strength were observed compared to the conduction mode. It is worth noting that significant grain refinement was observed in the weld seam when the core/ring power ratio was 0.5.

When r=0, only loop power is generated, while when r=1, only core power is generated.

 

Schematic diagram of power ratio r between Gaussian beam and annular beam

(a) Welding device; (b) The depth and width of the weld profile; (c) Schematic diagram of displaying sample and fixture settings

MC test: Only in the case of Gaussian beam, the weld seam is initially in shallow conduction mode (ID 1 and 2), and then transitions to partially penetrating lockhole mode (ID 3-5), with obvious cracks appearing. When the ring power increased from 0 to 1000 W, there were no obvious cracks at ID 7 and the depth of iron enrichment was relatively small. When the ring power increases to 2000 and 2500 W (IDs 9 and 10), the depth of the rich iron zone increases. Excessive cracking at 2500w ring power (ID 10).

MR test: When the core power is between 500 and 1000 W (ID 11 and 12), the weld seam is in conduction mode; Comparing ID 12 and ID 7, although the total power (6000w) is the same, ID 7 implements a lock hole mode. This is due to the significant decrease in power density at ID 12 due to the dominant loop characteristic (r=0.2). When the total power reaches 7500 W (ID 15), full penetration mode can be achieved, and compared to the 6000 W used in ID 7, the power of full penetration mode is significantly increased.

IC test: Conducted mode (ID 16 and 17) was achieved at 1500w core power and 3000w and 3500w ring power. When the core power is 3000w and the ring power is between 1500w and 2500w (ID 19-20), obvious cracks appear at the interface between rich iron and rich aluminum, forming a local penetrating small hole pattern. When the ring power is 3000 and 3500w (ID 21 and 22), achieve full penetration keyhole mode.

Representative cross-sectional images of each welding identification under an optical microscope

Figure 4. (a) The relationship between ultimate tensile strength (UTS) and power ratio in welding tests; (b) The total power of all welding tests