To laser onto metal, your artwork needs to be 100% RGB black; R,G,B (0,0,0)
Sometimes when you have a CMYK graphic and you convert it RGB, it will convert the RGB code to something like 35, 31, 32 (slightly grayscale). The way most lasers interpret grayscale is it reduces the power or the laser starts to utilize the default dithering pattern (Stucki, Jarvis, etc). Basically, if you don’t have full black, you’re running at a lower power than what you think you’re running or the end image will look off-tone or pixelated. First, make sure you have your artwork set to full-black RGB.
The artwork doesn’t need to be inverted. I can’t think of any situations where you would need to invert. That would be more for transparent materials like glass or something like that.