I play a hexblade warlock with an Improved Pact Blade longsword. I like the idea of the eldritch tinkerer, who builds his patron's gifts into his gadgets, which fits in a warlock/artificer multiclass. The artificer's Enhanced Weapon infusion would be very nice with the warlock if it stacks with the Improved Pact Blade bonuses, but I can't tell if it does from the class descriptions. Do these features stack, or do I need to rethink this build?
It depends on if the DM rules that a warlock's improved pact weapon is a magical weapon.
First, this answer correctly argues that the artificer's enhanced weapon infusion cannot be used on magical weapons when it cites this rule:
"Whenever you finish a long rest, you can touch a non-magical object and imbue it with one of your artificer infusions, turning it into a magic item." (E:RftLW, pg. 57)
This answer argues that a warlock's improved pact weapon is not a magical weapon. If your DM goes with this ruling, then the artificer's infusion should stack with it.
In this answer I argue that that the warlock's improved pact weapon is magical. If the DM rules this way, then the artificer's infusion will not stack.
You should share the linked question concerning the magical nature of a warlock's improved pact weapon with your DM and allow them to make a ruling - that will tell you if you need to rethink this build.
No, Improved Pact Weapon does not work on magic items1 and Artificer Infusions make Magic Items.
As Thomas Markov notes the description of Infuse Item states (E:RftLW, pg. 57)
Whenever you finish a long rest, you can touch a non-magical object and imbue it with one of your artificer infusions, turning it into a magic item.
Whether Improved Pact Weapon makes the pact weapon a magic weapon is does not matter, however, because either way the invocation would not affect a weapon infused with Enhanced Weapon.
The Improved Pact Weapon invocation states (XGtE, 57)
the weapon gains a +1 bonus to its attack and damage rolls, unless it is a magic weapon that already has a bonus to those rolls.
Because Enhanced Weapon would make the Pact Weapon a magic weapon, with a bonus to hit and damage, Improved Pact Weapon would cease to function.
This does not prevent you from using the Enhanced Weapon infusion on your pact weapon while you have the Improved Pact Weapon invocation. It does mean that you would only benefit from Enhanced Weapon. The two features cannot stack.
1: to be exact Improved Pact Weapon does not give a bonus to magic weapons that give a bonus to attack or damage rolls